The SSReflect proof language¶
| Authors: | Georges Gonthier, Assia Mahboubi, Enrico Tassi |
|---|
Introduction¶
This chapter describes a set of tactics known as SSReflect originally designed to provide support for the so-called small scale reflection proof methodology. Despite the original purpose this set of tactic is of general interest and is available in Coq starting from version 8.7.
SSReflect was developed independently of the tactics described in Chapter Tactics. Indeed the scope of the tactics part of SSReflect largely overlaps with the standard set of tactics. Eventually the overlap will be reduced in future releases of Coq.
Proofs written in SSReflect typically look quite different from the ones written using only tactics as per Chapter Tactics. We try to summarise here the most “visible” ones in order to help the reader already accustomed to the tactics described in Chapter Tactics to read this chapter.
The first difference between the tactics described in this chapter and the
tactics described in Chapter Tactics is the way hypotheses are managed
(we call this bookkeeping). In Chapter Tactics the most common
approach is to avoid moving explicitly hypotheses back and forth between the
context and the conclusion of the goal. On the contrary in SSReflect all
bookkeeping is performed on the conclusion of the goal, using for that
purpose a couple of syntactic constructions behaving similar to tacticals
(and often named as such in this chapter). The : tactical moves hypotheses
from the context to the conclusion, while => moves hypotheses from the
conclusion to the context, and in moves back and forth a hypothesis from the
context to the conclusion for the time of applying an action to it.
While naming hypotheses is commonly done by means of an as clause in the
basic model of Chapter Tactics, it is here to => that this task is
devoted. Tactics frequently leave new assumptions in the conclusion, and are
often followed by => to explicitly name them. While generalizing the
goal is normally not explicitly needed in Chapter Tactics, it is an
explicit operation performed by :.
See also
Beside the difference of bookkeeping model, this chapter includes
specific tactics which have no explicit counterpart in Chapter Tactics
such as tactics to mix forward steps and generalizations as
generally have or without loss.
SSReflect adopts the point of view that rewriting, definition
expansion and partial evaluation participate all to a same concept of
rewriting a goal in a larger sense. As such, all these functionalities
are provided by the rewrite tactic.
SSReflect includes a little language of patterns to select subterms in
tactics or tacticals where it matters. Its most notable application is
in the rewrite tactic, where patterns are
used to specify where the rewriting step has to take place.
Finally, SSReflect supports so-called reflection steps, typically allowing to switch back and forth between the computational view and logical view of a concept.
To conclude it is worth mentioning that SSReflect tactics can be mixed with non SSReflect tactics in the same proof, or in the same Ltac expression. The few exceptions to this statement are described in section Compatibility issues.
Acknowledgments¶
The authors would like to thank Frédéric Blanqui, François Pottier and Laurence Rideau for their comments and suggestions.
Usage¶
Getting started¶
To be available, the tactics presented in this manual need the
following minimal set of libraries to be loaded: ssreflect.v,
ssrfun.v and ssrbool.v.
Moreover, these tactics come with a methodology
specific to the authors of SSReflect and which requires a few options
to be set in a different way than in their default way. All in all,
this corresponds to working in the following context:
- From Coq Require Import ssreflect ssrfun ssrbool.
- [Loading ML file ssrmatching_plugin.cmxs ... done] [Loading ML file ssreflect_plugin.cmxs ... done] Overwriting previous delimiting key bool in scope bool_scope Ambiguous paths: [pred_of_mem_pred; sort_of_simpl_pred] : mem_pred >-> pred_sort
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
Compatibility issues¶
Requiring the above modules creates an environment which is mostly compatible with the rest of Coq, up to a few discrepancies:
New keywords (
is) might clash with variable, constant, tactic or tactical names, or with quasi-keywords in tactic or vernacular notations.New tactic(al)s names (
last,done,have,suffices,suff,without loss,wlog,congr,unlock) might clash with user tactic names.Identifiers with both leading and trailing
_, such as_x_, are reserved by SSReflect and cannot appear in scripts.The extensions to the
rewritetactic are partly incompatible with those available in current versions of Coq; in particular:rewrite .. in (type of k)orrewrite .. in *or any other variant ofrewritewill not work, and the SSReflect syntax and semantics for occurrence selection and rule chaining is different. Use an explicit rewrite direction (rewrite <- …orrewrite -> …) to access the Coq rewrite tactic.New symbols (
//,/=,//=) might clash with adjacent existing symbols. This can be avoided by inserting white spaces.New constant and theorem names might clash with the user theory. This can be avoided by not importing all of SSReflect:
- From Coq Require ssreflect.
- Import ssreflect.SsrSyntax.
Note that the full syntax of SSReflect’s rewrite and reserved identifiers are enabled only if the ssreflect module has been required and if
SsrSyntaxhas been imported. Thus a file that requires (without importing)ssreflectand importsSsrSyntax, can be required and imported without automatically enabling SSReflect’s extended rewrite syntax and reserved identifiers.Some user notations (in particular, defining an infix
;) might interfere with the "open term", parenthesis free, syntax of tactics such as have, set and pose.The generalization of if statements to non-Boolean conditions is turned off by SSReflect, because it is mostly subsumed by Coercion to
boolof thesumXXXtypes (declared inssrfun.v) and theif term is pattern then term else termconstruct (see Pattern conditional). To use the generalized form, turn off the SSReflect Booleanifnotation using the command:Close Scope boolean_if_scope.The following two options can be unset to disable the incompatible rewrite syntax and allow reserved identifiers to appear in scripts.
- Unset SsrRewrite.
- Unset SsrIdents.
Gallina extensions¶
Small-scale reflection makes an extensive use of the programming subset of Gallina, Coq’s logical specification language. This subset is quite suited to the description of functions on representations, because it closely follows the well-established design of the ML programming language. The SSReflect extension provides three additions to Gallina, for pattern assignment, pattern testing, and polymorphism; these mitigate minor but annoying discrepancies between Gallina and ML.
Pattern assignment¶
The SSReflect extension provides the following construct for irrefutable pattern matching, that is, destructuring assignment:
Note the colon : after the let keyword, which avoids any ambiguity
with a function definition or Coq’s basic destructuring let. The let:
construct differs from the latter in that
- The pattern can be nested (deep pattern matching), in particular, this allows expression of the form:
The destructured constructor is explicitly given in the pattern, and is used for type inference.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Definition f u := let: (m, n) := u in m + n.
- f is defined
- Check f.
- f : nat * nat -> nat
Using
let:Coq infers a type forf, whereas with a usualletthe same term requires an extra type annotation in order to type check.- Fail Definition f u := let (m, n) := u in m + n.
- The command has indeed failed with message: Cannot infer a type for this expression.
The let: construct is just (more legible) notation for the primitive
Gallina expression match term with pattern => term end.
The SSReflect destructuring assignment supports all the dependent match annotations; the full syntax is
where the second pattern and the second term are types.
When the as and return keywords are both present, then ident is bound
in both the second pattern and the second term; variables
in the optional type pattern are bound only in the second term, and
other variables in the first pattern are bound only in the third
term, however.
Pattern conditional¶
The following construct can be used for a refutable pattern matching, that is, pattern testing:
Although this construct is not strictly ML (it does exist in variants such as the pattern calculus or the ρ-calculus), it turns out to be very convenient for writing functions on representations, because most such functions manipulate simple data types such as Peano integers, options, lists, or binary trees, and the pattern conditional above is almost always the right construct for analyzing such simple types. For example, the null and all list function(al)s can be defined as follows:
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Variable d: Set.
- d is declared
- Fixpoint null (s : list d) := if s is nil then true else false.
- null is defined null is recursively defined (decreasing on 1st argument)
- Variable a : d -> bool.
- a is declared
- Fixpoint all (s : list d) : bool := if s is cons x s' then a x && all s' else true.
- all is defined all is recursively defined (decreasing on 1st argument)
The pattern conditional also provides a notation for destructuring
assignment with a refutable pattern, adapted to the pure functional
setting of Gallina, which lacks a Match_Failure exception.
Like let: above, the if…is construct is just (more legible) notation
for the primitive Gallina expression
match term with pattern => term | _ => term end.
Similarly, it will always be displayed as the expansion of this form in terms of primitive match expressions (where the default expression may be replicated).
Explicit pattern testing also largely subsumes the generalization of
the if construct to all binary data types; compare
if term is inl _ then term else term and
if term then term else term.
The latter appears to be marginally shorter, but it is quite
ambiguous, and indeed often requires an explicit annotation
(term : {_} + {_}) to type check, which evens the character count.
Therefore, SSReflect restricts by default the condition of a plain if
construct to the standard bool type; this avoids spurious type
annotations.
Example
- Definition orb b1 b2 := if b1 then true else b2.
- orb is defined
As pointed out in section Compatibility issues, this restriction can be removed with the command:
Close Scope boolean_if_scope.
Like let: above, the if-is-then-else
construct supports
the dependent match annotations:
As in let: the variable ident (and those in the type pattern)
are bound in the second term; ident is also bound in the
third term (but not in the fourth term), while the
variables in the first pattern are bound only in the third
term.
Another variant allows to treat the else case first:
Note that pattern eventually binds variables in the third
term and not in the second term.
Parametric polymorphism¶
Unlike ML, polymorphism in core Gallina is explicit: the type parameters of polymorphic functions must be declared explicitly, and supplied at each point of use. However, Coq provides two features to suppress redundant parameters:
- Sections are used to provide (possibly implicit) parameters for a set of definitions.
- Implicit arguments declarations are used to tell Coq to use type inference to deduce some parameters from the context at each point of call.
The combination of these features provides a fairly good emulation of ML-style polymorphism, but unfortunately this emulation breaks down for higher-order programming. Implicit arguments are indeed not inferred at all points of use, but only at points of call, leading to expressions such as
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Variable T : Type.
- T is declared
- Variable null : forall T : Type, T -> bool.
- null is declared
- Variable all : (T -> bool) -> list T -> bool.
- all is declared
- Definition all_null (s : list T) := all (@null T) s.
- all_null is defined
Unfortunately, such higher-order expressions are quite frequent in
representation functions, especially those which use Coq's
Structures to emulate Haskell typeclasses.
Therefore, SSReflect provides a variant of Coq’s implicit argument declaration, which causes Coq to fill in some implicit parameters at each point of use, e.g., the above definition can be written:
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Variable T : Type.
- T is declared
- Variable null : forall T : Type, T -> bool.
- null is declared
- Variable all : (T -> bool) -> list T -> bool.
- all is declared
- Prenex Implicits null.
- Definition all_null (s : list T) := all null s.
- all_null is defined
Better yet, it can be omitted entirely, since all_null s isn’t much of
an improvement over all null s.
The syntax of the new declaration is
Let us denote \(c_1\) … \(c_n\) the list of identifiers given to a
Prenex Implicits command. The command checks that each ci is the name of
a functional constant, whose implicit arguments are prenex, i.e., the first
\(n_i > 0\) arguments of \(c_i\) are implicit; then it assigns
Maximal Implicit status to these arguments.
As these prenex implicit arguments are ubiquitous and have often large
display strings, it is strongly recommended to change the default
display settings of Coq so that they are not printed (except after
a Set Printing All command). All SSReflect library files thus start
with the incantation
Anonymous arguments¶
When in a definition, the type of a certain argument is mandatory, but
not its name, one usually uses “arrow” abstractions for prenex
arguments, or the (_ : term) syntax for inner arguments. In SSReflect,
the latter can be replaced by the open syntax of term or
(equivalently) & term, which are both syntactically equivalent to a
(_ : term) expression. This feature almost behaves as the
following extension of the binder syntax:
Caveat: & T and of T abbreviations have to appear at the end
of a binder list. For instance, the usual two-constructor polymorphic
type list, i.e. the one of the standard List library, can be
defined by the following declaration:
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Inductive list (A : Type) : Type := nil | cons of A & list A.
- list is defined list_rect is defined list_ind is defined list_rec is defined
Wildcards¶
The terms passed as arguments to SSReflect tactics can contain
holes, materialized by wildcards _. Since SSReflect allows a more
powerful form of type inference for these arguments, it enhances the
possibilities of using such wildcards. These holes are in particular
used as a convenient shorthand for abstractions, especially in local
definitions or type expressions.
Wildcards may be interpreted as abstractions (see for example sections
Definitions and ref:structure_ssr), or their content can be
inferred from the whole context of the goal (see for example section
Abbreviations).
Definitions¶
-
pose¶ This tactic allows to add a defined constant to a proof context. SSReflect generalizes this tactic in several ways. In particular, the SSReflect pose tactic supports open syntax: the body of the definition does not need surrounding parentheses. For instance:
is a valid tactic expression.
The pose tactic is also improved for the local definition of higher
order terms. Local definitions of functions can use the same syntax as
global ones.
For example, the tactic pose supoprts parameters:
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Lemma test : True.
- 1 subgoal ============================ True
- pose f x y := x + y.
- 1 subgoal f := fun x y : nat => x + y : nat -> nat -> nat ============================ True
The SSReflect pose tactic also supports (co)fixpoints, by providing
the local counterpart of the Fixpoint f := … and CoFixpoint f := …
constructs. For instance, the following tactic:
defines a local fixpoint f, which mimics the standard plus operation
on natural numbers.
Similarly, local cofixpoints can be defined by a tactic of the form:
The possibility to include wildcards in the body of the definitions offers a smooth way of defining local abstractions. The type of “holes” is guessed by type inference, and the holes are abstracted. For instance the tactic:
is shorthand for:
When the local definition of a function involves both arguments and holes, hole abstractions appear first. For instance, the tactic:
is shorthand for:
The interaction of the pose tactic with the interpretation of implicit arguments results in a powerful and concise syntax for local definitions involving dependent types. For instance, the tactic:
adds to the context the local definition:
The generalization of wildcards makes the use of the pose tactic resemble ML-like definitions of polymorphic functions.
Abbreviations¶
The SSReflect set tactic performs abbreviations: it introduces a defined constant for a subterm appearing in the goal and/or in the context.
SSReflect extends the set tactic by supplying:
- an open syntax, similarly to the pose tactic;
- a more aggressive matching algorithm;
- an improved interpretation of wildcards, taking advantage of the matching algorithm;
- an improved occurrence selection mechanism allowing to abstract only selected occurrences of a term.
The general syntax of this tactic is
-
set ident : term? := occ_switch? term¶
where:
identis a fresh identifier chosen by the user.- term 1 is an optional type annotation. The type annotation term 1
can be given in open syntax (no surrounding parentheses). If no
occ_switch(described hereafter) is present, it is also the case for the secondterm. On the other hand, in presence ofocc_switch, parentheses surrounding the secondtermare mandatory. - In the occurrence switch
occ_switch, if the first element of the list is a natural, this element should be a number, and not an Ltac variable. The empty list{}is not interpreted as a valid occurrence switch, it is rather used as a flag to signal the intent of the user to clear the name following it (see Occurrence switches and redex switches and Introduction in the context)
The tactic:
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Axiom f : nat -> nat.
- f is declared
- Lemma test x : f x + f x = f x.
- 1 subgoal x : nat ============================ f x + f x = f x
- set t := f _.
- 1 subgoal x : nat t := f x : nat ============================ t + t = t
- Undo.
- 1 subgoal x : nat ============================ f x + f x = f x
- set t := {2}(f _).
- 1 subgoal x : nat t := f x : nat ============================ f x + t = f x
The type annotation may contain wildcards, which will be filled with the appropriate value by the matching process.
The tactic first tries to find a subterm of the goal matching
the second term
(and its type), and stops at the first subterm it finds. Then
the occurrences of this subterm selected by the optional occ_switch
are replaced by ident and a definition ident := term
is added to the
context. If no occ_switch is present, then all the occurrences are
abstracted.
Matching¶
The matching algorithm compares a pattern term with a subterm of the
goal by comparing their heads and then pairwise unifying their
arguments (modulo conversion). Head symbols match under the following
conditions:
- If the head of
termis a constant, then it should be syntactically equal to the head symbol of the subterm. - If this head is a projection of a canonical structure, then canonical structure equations are used for the matching.
- If the head of term is not a constant, the subterm should have the same structure (λ abstraction,let…in structure …).
- If the head of
termis a hole, the subterm should have at least as many arguments asterm.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Lemma test (x y z : nat) : x + y = z.
- 1 subgoal x, y, z : nat ============================ x + y = z
- set t := _ x.
- 1 subgoal x, y, z : nat t := Nat.add x : nat -> nat ============================ t y = z
In the special case where
termis of the form(let f := t0 in f) t1 … tn, then the patterntermis treated as(_ t1 … tn). For each subterm in the goal having the form(A u1 … um)with m ≥ n, the matching algorithm successively tries to find the largest partial application(A u1 … uj)convertible to the headt0ofterm.Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Lemma test : (let f x y z := x + y + z in f 1) 2 3 = 6.
- 1 subgoal ============================ (let f := fun x y z : nat => x + y + z in f 1) 2 3 = 6
- set t := (let g y z := S y + z in g) 2.
- 1 subgoal t := (unkeyed (fun y z : nat => S y + z)) 2 : nat -> nat ============================ t 3 = 6
The notation
unkeyeddefined inssreflect.vis a shorthand for the degenerate termlet x := … in x.
Moreover:
Multiple holes in
termare treated as independent placeholders.Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Lemma test x y z : x + y = z.
- 1 subgoal x, y, z : nat ============================ x + y = z
- set t := _ + _.
- 1 subgoal x, y, z : nat t := x + y : nat ============================ t = z
The type of the subterm matched should fit the type (possibly casted by some type annotations) of the pattern
term.The replacement of the subterm found by the instantiated pattern should not capture variables. In the example above
xis bound and should not be captured.Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Lemma test : forall x : nat, x + 1 = 0.
- 1 subgoal ============================ forall x : nat, x + 1 = 0
- Fail set t := _ + 1.
- The command has indeed failed with message: Ltac call to "set (ssrfwdid) (ssrsetfwd) (ssrclauses)" failed. The pattern (_ + 1) did not match and has holes. Did you mean pose?
Typeclass inference should fill in any residual hole, but matching should never assign a value to a global existential variable.
Occurrence selection¶
SSReflect provides a generic syntax for the selection of occurrences by their position indexes. These occurrence switches are shared by all SSReflect tactics which require control on subterm selection like rewriting, generalization, …
An occurrence switch can be:
A list natural numbers
{+ n1 … nm}of occurrences affected by the tactic.Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Axiom f : nat -> nat.
- f is declared
- Lemma test : f 2 + f 8 = f 2 + f 2.
- 1 subgoal ============================ f 2 + f 8 = f 2 + f 2
- set x := {+1 3}(f 2).
- 1 subgoal x := f 2 : nat ============================ x + f 8 = f 2 + x
Notice that some occurrences of a given term may be hidden to the user, for example because of a notation. The vernacular
Set Printing Allcommand displays all these hidden occurrences and should be used to find the correct coding of the occurrences to be selected [10].Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Notation "a < b":= (le (S a) b).
- Lemma test x y : x < y -> S x < S y.
- 1 subgoal x, y : nat ============================ x < y -> S x < S y
- set t := S x.
- 1 subgoal x, y : nat t := S x : nat ============================ t <= y -> t < S y
A list of natural numbers between
{n1 … nm}. This is equivalent to the previous{+ n1 … nm}but the list should start with a number, and not with an Ltac variable.A list
{- n1 … nm}of occurrences not to be affected by the tactic.Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Axiom f : nat -> nat.
- f is declared
- Lemma test : f 2 + f 8 = f 2 + f 2.
- 1 subgoal ============================ f 2 + f 8 = f 2 + f 2
- set x := {-2}(f 2).
- 1 subgoal x := f 2 : nat ============================ x + f 8 = f 2 + x
Note that, in this goal, it behaves like
set x := {1 3}(f 2).In particular, the switch
{+}selects all the occurrences. This switch is useful to turn off the default behavior of a tactic which automatically clears some assumptions (see section Discharge for instance).The switch
{-}imposes that no occurrences of the term should be affected by the tactic. The tactic:set x := {-}(f 2).leaves the goal unchanged and adds the definitionx := f 2to the context. This kind of tactic may be used to take advantage of the power of the matching algorithm in a local definition, instead of copying large terms by hand.
It is important to remember that matching preceeds occurrence selection.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Lemma test x y z : x + y = x + y + z.
- 1 subgoal x, y, z : nat ============================ x + y = x + y + z
- set a := {2}(_ + _).
- 1 subgoal x, y, z : nat a := x + y : nat ============================ x + y = a + z
Hence, in the following goal, the same tactic fails since there is only one occurrence of the selected term.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Lemma test x y z : (x + y) + (z + z) = z + z.
- 1 subgoal x, y, z : nat ============================ x + y + (z + z) = z + z
- Fail set a := {2}(_ + _).
- The command has indeed failed with message: Ltac call to "set (ssrfwdid) (ssrsetfwd) (ssrclauses)" failed. Only 1 < 2 occurrence of (x + y + (z + z))
Basic localization¶
It is possible to define an abbreviation for a term appearing in the context of a goal thanks to the in tactical.
A tactic of the form:
introduces a defined constant called x in the context, and folds it in
the context entries mentioned on the right hand side of in.
The body of x is the first subterm matching these context entries
(taken in the given order).
A tactic of the form:
matches term and then folds x similarly in all the given context entries
but also folds x in the goal.
Example
- From Coq Require Import ssreflect.
- Lemma test x t (Hx : x = 3) : x + t = 4.
- 1 subgoal x, t : nat Hx : x = 3 ============================ x + t = 4
- set z := 3 in Hx.
- 1 subgoal x, t : nat z := 3 : nat Hx : x = z ============================ x + t = 4
If the localization also mentions the goal, then the result is the following one:
Example
- From Coq Require Import ssreflect.
- Lemma test x t (Hx : x = 3) : x + t = 4.
- 1 subgoal x, t : nat Hx : x = 3 ============================ x + t = 4
- set z := 3 in Hx * .
- 1 subgoal x, t : nat z := 3 : nat Hx : x = z ============================ x + t = S z
Indeed, remember that 4 is just a notation for (S 3).
The use of the in tactical is not limited to the localization of
abbreviations: for a complete description of the in tactical, see
section Bookkeeping and Localization.
Basic tactics¶
A sizable fraction of proof scripts consists of steps that do not "prove" anything new, but instead perform menial bookkeeping tasks such as selecting the names of constants and assumptions or splitting conjuncts. Although they are logically trivial, bookkeeping steps are extremely important because they define the structure of the data-flow of a proof script. This is especially true for reflection-based proofs, which often involve large numbers of constants and assumptions. Good bookkeeping consists in always explicitly declaring (i.e., naming) all new constants and assumptions in the script, and systematically pruning irrelevant constants and assumptions in the context. This is essential in the context of an interactive development environment (IDE), because it facilitates navigating the proof, allowing to instantly "jump back" to the point at which a questionable assumption was added, and to find relevant assumptions by browsing the pruned context. While novice or casual Coq users may find the automatic name selection feature convenient, the usage of such a feature severely undermines the readability and maintainability of proof scripts, much like automatic variable declaration in programming languages. The SSReflect tactics are therefore designed to support precise bookkeeping and to eliminate name generation heuristics. The bookkeeping features of SSReflect are implemented as tacticals (or pseudo-tacticals), shared across most SSReflect tactics, and thus form the foundation of the SSReflect proof language.
Bookkeeping¶
During the course of a proof Coq always present the user with a sequent whose general form is:
ci : Ti
…
dj := ej : Tj
…
Fk : Pk
…
=================
forall (xl : Tl ) …,
let ym := bm in … in
Pn -> … -> C
The goal to be proved appears below the double line; above the line
is the context of the sequent, a set of declarations of constants
ci , defined constants d i , and facts Fk that can be used to
prove the goal (usually, Ti , Tj : Type and Pk : Prop).
The various
kinds of declarations can come in any order. The top part of the
context consists of declarations produced by the Section
commands Variable, Let, and Hypothesis.
This section context is never
affected by the SSReflect tactics: they only operate on the lower part
— the proof context. As in the figure above, the goal often
decomposes into a series of (universally) quantified variables
(xl : Tl), local definitions
let ym := bm in, and assumptions
P n ->,
and a conclusion C (as in the context, variables, definitions, and
assumptions can appear in any order). The conclusion is what actually
needs to be proved — the rest of the goal can be seen as a part of the
proof context that happens to be “below the line”.
However, although they are logically equivalent, there are fundamental differences between constants and facts on the one hand, and variables and assumptions on the others. Constants and facts are unordered, but named explicitly in the proof text; variables and assumptions are ordered, but unnamed: the display names of variables may change at any time because of α-conversion.
Similarly, basic deductive steps such as apply can only operate on the
goal because the Gallina terms that control their action (e.g., the
type of the lemma used by apply) only provide unnamed bound variables.
[11] Since the proof script can only refer directly to the context, it
must constantly shift declarations from the goal to the context and
conversely in between deductive steps.
In SSReflect these moves are performed by two tacticals => and
:, so that the bookkeeping required by a deductive step can be
directly associated to that step, and that tactics in an SSReflect
script correspond to actual logical steps in the proof rather than
merely shuffle facts. Still, some isolated bookkeeping is unavoidable,
such as naming variables and assumptions at the beginning of a
proof. SSReflect provides a specific move tactic for this purpose.
Now move does essentially nothing: it is mostly a placeholder for
=> and :. The => tactical moves variables, local definitions,
and assumptions to the context, while the : tactical moves facts and
constants to the goal.
Example
For example, the proof of [12]
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Lemma subnK : forall m n, n <= m -> m - n + n = m.
- 1 subgoal ============================ forall m n : nat, n <= m -> m - n + n = m
might start with
- move=> m n le_n_m.
- 1 subgoal m, n : nat le_n_m : n <= m ============================ m - n + n = m
where move does nothing, but => m n le_m_n changes
the variables and assumption of the goal in the constants
m n : nat and the fact le_n_m : n <= m, thus exposing the
conclusion m - n + n = m.
The : tactical is the converse of =>, indeed it removes facts and
constants from the context by turning them into variables and
assumptions.
- move: m le_n_m.
- 1 subgoal n : nat ============================ forall m : nat, n <= m -> m - n + n = m
turns back m and le_m_n into a variable and an assumption,
removing them from the proof context, and changing the goal to
forall m, n <= m -> m - n + n = m
which can be proved by induction on n using elim: n.
Because they are tacticals, : and => can be combined, as in
simultaneously renames m and le_m_n into p and le_n_p,
respectively, by first turning them into unnamed variables, then
turning these variables back into constants and facts.
Furthermore, SSReflect redefines the basic Coq tactics case, elim,
and apply so that they can take better advantage of
: and =>. In there
SSReflect variants, these tactic operate on the first variable or
constant of the goal and they do not use or change the proof context.
The : tactical is used to operate on an element in the context.
Example
For instance the proof ofsubnKcould continue withelim: n. Instead ofelim n(note, no colon), this has the advantage of removing n from the context. Better yet, thiselimcan be combined with previous move and with the branching version of the=>tactical (described in Introduction in the context), to encapsulate the inductive step in a single command:
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Lemma subnK : forall m n, n <= m -> m - n + n = m.
- 1 subgoal ============================ forall m n : nat, n <= m -> m - n + n = m
- move=> m n le_n_m.
- 1 subgoal m, n : nat le_n_m : n <= m ============================ m - n + n = m
- elim: n m le_n_m => [|n IHn] m => [_ | lt_n_m].
- 2 subgoals m : nat ============================ m - 0 + 0 = m subgoal 2 is: m - S n + S n = m
which breaks down the proof into two subgoals, the second one
having in its context
lt_n_m : S n <= m and
IHn : forall m, n <= m -> m - n + n = m.
The : and => tacticals can be explained very simply if one views
the goal as a stack of variables and assumptions piled on a conclusion:
tactic : a b cpushes the context constantsa,b,cas goal variables before performing tactic.tactic => a b cpops the top three goal variables as context constantsa,b,c, after tactic has been performed.
These pushes and pops do not need to balance out as in the examples
above, so move: m le_n_m => p
would rename m into p, but leave an extra assumption n <= p
in the goal.
Basic tactics like apply and elim can also be used without the ’:’
tactical: for example we can directly start a proof of subnK by
induction on the top variable m with
The general form of the localization tactical in is also best explained in terms of the goal stack:
tactic in a H1 H2 *.
is basically equivalent to
with two differences: the in tactical will preserve the body of a ifa
is a defined constant, and if the * is omitted it will use a
temporary abbreviation to hide the statement of the goal from
tactic.
The general form of the in tactical can be used directly with the
move, case and elim tactics, so that one can write
instead of
This is quite useful for inductive proofs that involve many facts.
See section Localization for the general syntax and presentation of the in tactical.
The defective tactics¶
In this section we briefly present the three basic tactics performing context manipulations and the main backward chaining tool.
The move tactic.¶
The move tactic, in its defective form, behaves like the primitive hnf
Coq tactic. For example, such a defective:
-
move¶
exposes the first assumption in the goal, i.e. its changes the
goal not False into False -> False.
More precisely, the move tactic inspects the goal and does nothing
(idtac) if an introduction step is possible, i.e. if the goal is a
product or a let…in, and performs hnf otherwise.
Of course this tactic is most often used in combination with the
bookkeeping tacticals (see section Introduction in the context and Discharge). These
combinations mostly subsume the intros, generalize, revert, rename,
clear and pattern tactics.
The case tactic¶
The case tactic performs primitive case analysis on (co)inductive
types; specifically, it destructs the top variable or assumption of
the goal, exposing its constructor(s) and its arguments, as well as
setting the value of its type family indices if it belongs to a type
family (see section Type families).
The SSReflect case tactic has a special behavior on equalities. If the
top assumption of the goal is an equality, the case tactic “destructs”
it as a set of equalities between the constructor arguments of its
left and right hand sides, as per the tactic injection. For example,
case changes the goal:
(x, y) = (1, 2) -> G.
into:
x = 1 -> y = 2 -> G.
Note also that the case of SSReflect performs False elimination, even
if no branch is generated by this case operation. Hence the command:
case. on a goal of the form False -> G will succeed and
prove the goal.
The elim tactic¶
The elim tactic performs inductive elimination on inductive types. The
defective:
-
elim¶
tactic performs inductive elimination on a goal whose top assumption has an inductive type.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Lemma test m : forall n : nat, m <= n.
- 1 subgoal m : nat ============================ forall n : nat, m <= n
- elim.
- 2 subgoals m : nat ============================ m <= 0 subgoal 2 is: forall n : nat, m <= n -> m <= S n
The apply tactic¶
The apply tactic is the main backward chaining tactic of the proof
system. It takes as argument any term and applies it to the goal.
Assumptions in the type of term that don’t directly match the goal
may generate one or more subgoals.
In fact the SSReflect tactic:
-
apply¶
is a synonym for:
intro top; first [refine top | refine (top _) | refine (top _ _) | …]; clear top.
where top is a fresh name, and the sequence of refine tactics tries to
catch the appropriate number of wildcards to be inserted. Note that
this use of the refine tactic implies that the tactic tries to match
the goal up to expansion of constants and evaluation of subterms.
SSReflect’s apply has a special behavior on goals containing existential metavariables of sort Prop.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Axiom lt_trans : forall a b c, a < b -> b < c -> a < c.
- lt_trans is declared
- Lemma test : forall y, 1 < y -> y < 2 -> exists x : { n | n < 3 }, 0 < proj1_sig x.
- 1 subgoal ============================ forall y : nat, 1 < y -> y < 2 -> exists x : {n : nat | n < 3}, 0 < proj1_sig x
- move=> y y_gt1 y_lt2; apply: (ex_intro _ (exist _ y _)).
- 2 focused subgoals (shelved: 2) y : nat y_gt1 : 1 < y y_lt2 : y < 2 ============================ y < 3 subgoal 2 is: forall Hyp0 : y < 3, 0 < proj1_sig (exist (fun n : nat => n < 3) y Hyp0)
- by apply: lt_trans y_lt2 _.
- 1 focused subgoal (shelved: 1) y : nat y_gt1 : 1 < y y_lt2 : y < 2 ============================ forall Hyp0 : y < 3, 0 < proj1_sig (exist (fun n : nat => n < 3) y Hyp0)
- by move=> y_lt3; apply: lt_trans y_gt1.
- No more subgoals.
Note that the last _ of the tactic
apply: (ex_intro _ (exist _ y _))
represents a proof that y < 3. Instead of generating the goal:
0 < proj1_sig (exist (fun n : nat => n < 3) y ?Goal).
the system tries to prove y < 3 calling the trivial tactic.
If it succeeds, let’s say because the context contains
H : y < 3, then the
system generates the following goal:
0 < proj1_sig (exist (fun n => n < 3) y H).
Otherwise the missing proof is considered to be irrelevant, and is thus discharged generating the two goals shown above.
Last, the user can replace the trivial tactic by defining an Ltac
expression named ssrautoprop.
Discharge¶
The general syntax of the discharging tactical : is:
-
tactic ident? : d_item+ clear_switch?¶
-
d_item ::=occ_switch clear_switch? term
with the following requirements:
tacticmust be one of the four basic tactics described in The defective tactics, i.e.,move,case,elimorapply, theexacttactic (section Terminators), thecongrtactic (section Congruence), or the application of the view tactical ‘/’ (section Interpreting assumptions) to one of move, case, or elim.- The optional
identspecifies equation generation (section Generation of equations), and is only allowed if tactic ismove,caseorelim, or the application of the view tactical ‘/’ (section Interpreting assumptions) tocaseorelim. - An
occ_switchselects occurrences ofterm, as in Abbreviations;occ_switchis not allowed iftacticisapplyorexact. - A clear item
clear_switchspecifies facts and constants to be deleted from the proof context (as per the clear tactic).
The : tactical first discharges all the d_item, right to left,
and then performs tactic, i.e., for each d_item, starting with the last one :
- The SSReflect matching algorithm described in section Abbreviations is used to find occurrences of term in the goal, after filling any holes ‘_’ in term; however if tactic is apply or exact a different matching algorithm, described below, is used [13].
- These occurrences are replaced by a new variable; in particular, if term is a fact, this adds an assumption to the goal.
- If term is exactly the name of a constant or fact in the proof
context, it is deleted from the context, unless there is an
occ_switch.
Finally, tactic is performed just after the first d_item
has been generalized
— that is, between steps 2 and 3. The names listed in
the final clear_switch (if it is present) are cleared first, before
d_item n is discharged.
Switches affect the discharging of a d_item as follows:
- An
occ_switchrestricts generalization (step 2) to a specific subset of the occurrences of term, as per section Abbreviations, and prevents clearing (step 3). - All the names specified by a
clear_switchare deleted from the context in step 3, possibly in addition to term.
For example, the tactic:
- first generalizes
(refl_equal n : n = n); - then generalizes the second occurrence of
n. - finally generalizes all the other occurrences of
n, and clearsnfrom the proof context (assuming n is a proof constant).
Therefore this tactic changes any goal G into
where the name n0 is picked by the Coq display function, and assuming
n appeared only in G.
Finally, note that a discharge operation generalizes defined constants
as variables, and not as local definitions. To override this behavior,
prefix the name of the local definition with a @, like in move: @n.
This is in contrast with the behavior of the in tactical (see section Localization), which preserves local definitions by default.
Clear rules¶
The clear step will fail if term is a proof constant that appears in
other facts; in that case either the facts should be cleared
explicitly with a clear_switch, or the clear step should be disabled.
The latter can be done by adding an occ_switch or simply by putting
parentheses around term: both
move: (n).
and
move: {+}n.
generalize n without clearing n from the proof context.
The clear step will also fail if the clear_switch contains a ident that
is not in the proof context. Note that SSReflect never clears a
section constant.
If tactic is move or case and an equation ident is given, then clear
(step 3) for d_item is suppressed (see section Generation of equations).
Matching for apply and exact¶
The matching algorithm for d_item of the SSReflect
apply and exact
tactics exploits the type of the first d_item to interpret
wildcards in the
other d_item and to determine which occurrences of these should be
generalized. Therefore, occur switches are not needed for apply and
exact.
Indeed, the SSReflect tactic apply: H x is equivalent to
refine (@H _ … _ x); clear H x
with an appropriate number of wildcards between H and x.
Note that this means that matching for apply and exact has much more
context to interpret wildcards; in particular it can accommodate the
_ d_item, which would always be rejected after move:.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Axiom f : nat -> nat.
- f is declared
- Axiom g : nat -> nat.
- g is declared
- Lemma test (Hfg : forall x, f x = g x) a b : f a = g b.
- 1 subgoal Hfg : forall x : nat, f x = g x a, b : nat ============================ f a = g b
- apply: trans_equal (Hfg _) _.
- 1 focused subgoal (shelved: 1) Hfg : forall x : nat, f x = g x a, b : nat ============================ g a = g b
This tactic is equivalent (see section
Bookkeeping) to:
refine (trans_equal (Hfg _) _).
and this is a common idiom for applying transitivity on the left hand
side of an equation.
The abstract tactic¶
-
abstract: d_item+¶
This tactic assigns an abstract constant previously introduced with the [:
name ] intro pattern (see section Introduction in the context).
In a goal like the following:
m : nat
abs : <hidden>
n : nat
=============
m < 5 + n
The tactic abstract: abs n first generalizes the goal with respect ton
(that is not visible to the abstract constant abs) and then assigns
abs. The resulting goal is:
m : nat
n : nat
=============
m < 5 + n
Once this subgoal is closed, all other goals having abs in their
context see the type assigned to abs. In this case:
m : nat
abs : forall n, m < 5 + n
=============
…
For a more detailed example the reader should refer to section Structure.
Introduction in the context¶
The application of a tactic to a given goal can generate (quantified) variables, assumptions, or definitions, which the user may want to introduce as new facts, constants or defined constants, respectively. If the tactic splits the goal into several subgoals, each of them may require the introduction of different constants and facts. Furthermore it is very common to immediately decompose or rewrite with an assumption instead of adding it to the context, as the goal can often be simplified and even proved after this.
All these operations are performed by the introduction tactical =>,
whose general syntax is
-
i_item ::=i_pattern s_item clear_switch {}? /term
-
s_item ::=/= // //=
-
i_pattern ::=ident _ ? * occ_switch? -> occ_switch?<- [ i_item?| ] - [: ident+ ]
The => tactical first executes tactic, then the i_item s,
left to right. An s_item specifies a
simplification operation; a clear_switch
h specifies context pruning as in Discharge.
The i_pattern s can be seen as a variant of intro patterns
Tactics: each performs an introduction operation, i.e., pops some
variables or assumptions from the goal.
An s_item can simplify the set of subgoals or the subgoals themselves:
//removes all the “trivial” subgoals that can be resolved by the SSReflect tacticdonedescribed in Terminators, i.e., it executestry done./=simplifies the goal by performing partial evaluation, as per the tacticsimpl[14].//=combines both kinds of simplification; it is equivalent to/= //, i.e.,simpl; try done.
When an s_item bears a clear_switch, then the
clear_switch is executed
after the s_item, e.g., {IHn}// will solve some subgoals,
possibly using the fact IHn, and will erase IHn from the context
of the remaining subgoals.
The last entry in the i_item grammar rule, /term,
represents a view (see section Views and reflection).
If the next i_item is a view, then the view is
applied to the assumption in top position once all the
previous i_item have been performed.
The view is applied to the top assumption.
SSReflect supports the following i_pattern s:
ident- pops the top variable, assumption, or local definition into
a new constant, fact, or defined constant
ident, respectively. Note that defined constants cannot be introduced when δ-expansion is required to expose the top variable or assumption. ?- pops the top variable into an anonymous constant or fact, whose name is picked by the tactic interpreter. SSReflect only generates names that cannot appear later in the user script [15].
_- pops the top variable into an anonymous constant that will be deleted
from the proof context of all the subgoals produced by the
=>tactical. They should thus never be displayed, except in an error message if the constant is still actually used in the goal or context after the lasti_itemhas been executed (s_itemcan erase goals or terms where the constant appears). *pops all the remaining apparent variables/assumptions as anonymous constants/facts. Unlike
?andmovethe*i_itemdoes not expand definitions in the goal to expose quantifiers, so it may be useful to repeat amove=> *tactic, e.g., on the goal:forall a b : bool, a <> b
a first
move=> *adds only_a_ : booland_b_ : boolto the context; it takes a secondmove=> *to add_Hyp_ : _a_ = _b_.occ_switch->- (resp.
occ_switch<-) pops the top assumption (which should be a rewritable proposition) into an anonymous fact, rewrites (resp. rewrites right to left) the goal with this fact (using the SSReflectrewritetactic described in section Rewriting, and honoring the optional occurrence selector), and finally deletes the anonymous fact from the context. [i_item*| … |i_item*]- when it is the
very first
i_patternafter tactic=>tactical and tactic is not a move, is a branchingi_pattern. It executes the sequencei_item\(_i\) on the i-th subgoal produced by tactic. The execution of tactic should thus generate exactly m subgoals, unless the[…]i_patterncomes after an initial//or//=s_itemthat closes some of the goals produced bytactic, in which case exactly m subgoals should remain after thes- item, or we have the trivial branchingi_pattern[], which always does nothing, regardless of the number of remaining subgoals. [i_item*| … |i_item*]- when it is not
the first
i_patternor when tactic is amove, is a destructingi_pattern. It starts by destructing the top variable, using the SSReflectcasetactic described in The defective tactics. It then behaves as the corresponding branchingi_pattern, executing the sequence:token:i_item\(_i\) in the i-th subgoal generated by the case analysis; unless we have the trivial destructingi_pattern[], the latter should generate exactly m subgoals, i.e., the top variable should have an inductive type with exactly m constructors [16]. While it is good style to use thei_itemi * to pop the variables and assumptions corresponding to each constructor, this is not enforced by SSReflect. /term- Interprets the top of the stack with the view
term. It is equivalent tomove/term. The optional flag{}can be used to signal that theterm, when it is a context entry, has to be cleared. -- does nothing, but counts as an intro pattern. It can also be used to
force the interpretation of
[i_item*| … |i_item*]as a case analysis like inmove=> -[H1 H2]. It can also be used to indicate explicitly the link between a view and a name like inmove=> /eqP-H1. Last, it can serve as a separator between views. Section Views and reflection [18] explains in which respect the tacticmove=> /v1/v2differs from the tacticmove=> /v1-/v2. [:ident…]- introduces in the context an abstract constant
for each
ident. Its type has to be fixed later on by using theabstracttactic. Before then the type displayed is<hidden>.
Note that SSReflect does not support the syntax (ipat, …, ipat) for
destructing intro-patterns.
Clears are deferred until the end of the intro pattern.
Example
- From Coq Require Import ssreflect ssrbool.
- Overwriting previous delimiting key bool in scope bool_scope Ambiguous paths: [pred_of_mem_pred; sort_of_simpl_pred] : mem_pred >-> pred_sort
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Lemma test x y : Nat.leb 0 x = true -> (Nat.leb 0 x) && (Nat.leb y 2) = true.
- 1 subgoal x, y : nat ============================ Nat.leb 0 x = true -> Nat.leb 0 x && Nat.leb y 2 = true
- move=> {x} ->.
- 1 subgoal y : nat ============================ true && Nat.leb y 2 = true
If the cleared names are reused in the same intro pattern, a renaming is performed behind the scenes.
Facts mentioned in a clear switch must be valid names in the proof context (excluding the section context).
The rules for interpreting branching and destructing i_pattern are
motivated by the fact that it would be pointless to have a branching
pattern if tactic is a move, and in most of the remaining cases
tactic is case or elim, which implies destruction.
The rules above imply that:
move=> [a b].case=> [a b].case=> a b.
are all equivalent, so which one to use is a matter of style; move should
be used for casual decomposition, such as splitting a pair, and case
should be used for actual decompositions, in particular for type families
(see Type families) and proof by contradiction.
The trivial branching i_pattern can be used to force the branching
interpretation, e.g.:
case=> [] [a b] c.move=> [[a b] c].case; case=> a b c.
are all equivalent.
Generation of equations¶
The generation of named equations option stores the definition of a new constant as an equation. The tactic:
where l is a list, replaces size l by n in the goal and
adds the fact En : size l = n to the context.
This is quite different from:
which generates a definition n := (size l). It is not possible to
generalize or rewrite such a definition; on the other hand, it is
automatically expanded during computation, whereas expanding the
equation En requires explicit rewriting.
The use of this equation name generation option with a case or an
elim tactic changes the status of the first i_item, in order to
deal with the possible parameters of the constants introduced.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Lemma test (a b :nat) : a <> b.
- 1 subgoal a, b : nat ============================ a <> b
- case E : a => [|n].
- 2 subgoals a, b : nat E : a = 0 ============================ 0 <> b subgoal 2 is: S n <> b
If the user does not provide a branching i_item as first
i_item, or if the i_item does not provide enough names for
the arguments of a constructor, then the constants generated are introduced
under fresh SSReflect names.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Lemma test (a b :nat) : a <> b.
- 1 subgoal a, b : nat ============================ a <> b
- case E : a => H.
- 2 subgoals a, b : nat E : a = 0 H : 0 = b ============================ False subgoal 2 is: False
- Show 2.
- subgoal 2 is: a, b, _n_ : nat E : a = S _n_ H : S _n_ = b ============================ False
Combining the generation of named equations mechanism with thecase tactic strengthens the power of a case analysis. On the other hand, when combined with the elim tactic, this feature is mostly useful for debug purposes, to trace the values of decomposed parameters and pinpoint failing branches.
Type families¶
When the top assumption of a goal has an inductive type, two specific
operations are possible: the case analysis performed by the case
tactic, and the application of an induction principle, performed by
the elim tactic. When this top assumption has an inductive type, which
is moreover an instance of a type family, Coq may need help from the
user to specify which occurrences of the parameters of the type should
be substituted.
A specific / switch indicates the type family parameters of the type
of a d_item immediately following this / switch,
using the syntax:
The d_item on the right side of the / switch are discharged as
described in section Discharge. The case analysis or elimination
will be done on the type of the top assumption after these discharge
operations.
Every d_item preceding the / is interpreted as arguments of this
type, which should be an instance of an inductive type family. These terms
are not actually generalized, but rather selected for substitution.
Occurrence switches can be used to restrict the substitution. If a term is
left completely implicit (e.g. writing just _), then a pattern is
inferred looking at the type of the top assumption. This allows for the
compact syntax:
- case: {2}_ / eqP.
- Toplevel input, characters 13-16: > case: {2}_ / eqP. > ^^^ Error: Ltac call to "case (ssrcasearg) (ssrclauses)" failed. The reference eqP was not found in the current environment.
where _ is interpreted as (_ == _) since
eqP T a b : reflect (a = b) (a == b) and reflect is a type family with
one index.
Moreover if the d_item list is too short, it is padded with an
initial sequence of _ of the right length.
Example
Here is a small example on lists. We define first a function which adds an element at the end of a given list.
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Require Import List.
- Section LastCases.
- Variable A : Type.
- A is declared
- Implicit Type l : list A.
- Fixpoint add_last a l : list A := match l with | nil => a :: nil | hd :: tl => hd :: (add_last a tl) end.
- add_last is defined add_last is recursively defined (decreasing on 2nd argument)
Then we define an inductive predicate for case analysis on lists according to their last element:
- Inductive last_spec : list A -> Type := | LastSeq0 : last_spec nil | LastAdd s x : last_spec (add_last x s).
- last_spec is defined last_spec_rect is defined last_spec_ind is defined last_spec_rec is defined
- Theorem lastP : forall l : list A, last_spec l.
- 1 subgoal A : Type ============================ forall l : list A, last_spec l
- Admitted.
- lastP is declared
We are now ready to use lastP in conjunction with case.
- Lemma test l : (length l) * 2 = length (l ++ l).
- 1 subgoal A : Type l : list A ============================ length l * 2 = length (l ++ l)
- case: (lastP l).
- 2 subgoals A : Type l : list A ============================ length nil * 2 = length (nil ++ nil) subgoal 2 is: forall (s : list A) (x : A), length (add_last x s) * 2 = length (add_last x s ++ add_last x s)
Applied to the same goal, the command:
case: l / (lastP l).
generates the same subgoals but l has been cleared from both contexts.
Again applied to the same goal, the command.
- Abort.
- Lemma test l : (length l) * 2 = length (l ++ l).
- 1 subgoal A : Type l : list A ============================ length l * 2 = length (l ++ l)
- case: {1 3}l / (lastP l).
- 2 subgoals A : Type l : list A ============================ length nil * 2 = length (l ++ nil) subgoal 2 is: forall (s : list A) (x : A), length (add_last x s) * 2 = length (l ++ add_last x s)
Note that selected occurrences on the left of the /
switch have been substituted with l instead of being affected by
the case analysis.
The equation name generation feature combined with a type family /
switch generates an equation for the first dependent d_item
specified by the user. Again starting with the above goal, the
command:
Example
- Abort.
- Lemma test l : (length l) * 2 = length (l ++ l).
- 1 subgoal A : Type l : list A ============================ length l * 2 = length (l ++ l)
- case E: {1 3}l / (lastP l) => [|s x].
- 2 subgoals A : Type l : list A E : l = nil ============================ length nil * 2 = length (l ++ nil) subgoal 2 is: length (add_last x s) * 2 = length (l ++ add_last x s)
- Show 2.
- subgoal 2 is: A : Type l, s : list A x : A E : l = add_last x s ============================ length (add_last x s) * 2 = length (l ++ add_last x s)
There must be at least one d_item to the left of the / switch; this
prevents any confusion with the view feature. However, the d_item
to the right of the / are optional, and if they are omitted the first
assumption provides the instance of the type family.
The equation always refers to the first d_item in the actual tactic
call, before any padding with initial _. Thus, if an inductive type
has two family parameters, it is possible to have|SSR| generate an
equation for the second one by omitting the pattern for the first;
note however that this will fail if the type of the second parameter
depends on the value of the first parameter.
Control flow¶
Indentation and bullets¶
A linear development of Coq scripts gives little information on the structure of the proof. In addition, replaying a proof after some changes in the statement to be proved will usually not display information to distinguish between the various branches of case analysis for instance.
To help the user in this organization of the proof script at development
time, SSReflect provides some bullets to highlight the structure of branching
proofs. The available bullets are -, + and *. Combined with
tabulation, this lets us highlight four nested levels of branching; the most
we have ever needed is three. Indeed, the use of “simpl and closing”
switches, of terminators (see above section Terminators) and
selectors (see section Selectors) is powerful enough to avoid most
of the time more than two levels of indentation.
Here is a fragment of such a structured script:
case E1: (abezoutn _ _) => [[| k1] [| k2]].
- rewrite !muln0 !gexpn0 mulg1 => H1.
move/eqP: (sym_equal F0); rewrite -H1 orderg1 eqn_mul1.
by case/andP; move/eqP.
- rewrite muln0 gexpn0 mulg1 => H1.
have F1: t %| t * S k2.+1 - 1.
apply: (@dvdn_trans (orderg x)); first by rewrite F0; exact: dvdn_mull.
rewrite orderg_dvd; apply/eqP; apply: (mulgI x).
rewrite -{1}(gexpn1 x) mulg1 gexpn_add leq_add_sub //.
by move: P1; case t.
rewrite dvdn_subr in F1; last by exact: dvdn_mulr.
+ rewrite H1 F0 -{2}(muln1 (p ^ l)); congr (_ * _).
by apply/eqP; rewrite -dvdn1.
+ by move: P1; case: (t) => [| [| s1]].
- rewrite muln0 gexpn0 mul1g => H1.
...
Terminators¶
To further structure scripts, SSReflect supplies terminating tacticals to explicitly close off tactics. When replaying scripts, we then have the nice property that an error immediately occurs when a closed tactic fails to prove its subgoal.
It is hence recommended practice that the proof of any subgoal should end with a tactic which fails if it does not solve the current goal, like discriminate, contradiction or assumption.
In fact, SSReflect provides a generic tactical which turns any tactic into a closing one (similar to now). Its general syntax is:
-
by tactic¶
The Ltac expression by [tactic | tactic | …] is equivalent to
do [done | by tactic | by tactic | …], which corresponds to the
standard Ltac expression first [done | tactic; done | tactic; done | …].
In the script provided as example in section Indentation and bullets, the
paragraph corresponding to each sub-case ends with a tactic line prefixed
with a by, like in:
-
done¶
The by tactical is implemented using the user-defined, and extensible
done tactic. This done tactic tries to solve the current goal by some
trivial means and fails if it doesn’t succeed. Indeed, the tactic
expression by tactic is equivalent to tactic; done.
Conversely, the tactic
- by [ ].
- Toplevel input, characters 0-6: > by [ ]. > ^^^^^^ Error: In nested Ltac calls to "by (ssrhintarg)" and "done", last call failed. No applicable tactic.
is equivalent to:
- done.
- Toplevel input, characters 0-4: > done. > ^^^^ Error: Ltac call to "done" failed. No applicable tactic.
The default implementation of the done tactic, in the ssreflect.v
file, is:
- Ltac done := trivial; hnf; intros; solve [ do ![solve [trivial | apply: sym_equal; trivial] | discriminate | contradiction | split] | case not_locked_false_eq_true; assumption | match goal with H : ~ _ |- _ => solve [case H; trivial] end ].
- done is defined
The lemma not_locked_false_eq_true is needed to discriminate
locked boolean predicates (see section Locking, unlocking). The iterator
tactical do is presented in section Iteration. This tactic can be
customized by the user, for instance to include an auto tactic.
A natural and common way of closing a goal is to apply a lemma which is the exact one needed for the goal to be solved. The defective form of the tactic:
is equivalent to:
where top is a fresh name assigned to the top assumption of the goal.
This applied form is supported by the : discharge tactical, and the
tactic:
is equivalent to:
(see section Discharge for the documentation of the apply: combination).
Warning
The list of tactics (possibly chained by semicolons) that
follows the by keyword is considered to be a parenthesized block applied to
the current goal. Hence for example if the tactic:
succeeds, then the tactic:
usually fails since it is equivalent to:
Selectors¶
When composing tactics, the two tacticals first and last let the user
restrict the application of a tactic to only one of the subgoals
generated by the previous tactic. This covers the frequent cases where
a tactic generates two subgoals one of which can be easily disposed
of.
This is another powerful way of linearization of scripts, since it happens very often that a trivial subgoal can be solved in a less than one line tactic. For instance, the tactic:
-
tactic ; last by tactic¶
tries to solve the last subgoal generated by the first tactic using the given second tactic, and fails if it does not succeed. Its analogue
-
tactic ; first by tactic¶
tries to solve the first subgoal generated by the first tactic using the second given tactic, and fails if it does not succeed.
SSReflect also offers an extension of this facility, by supplying tactics to permute the subgoals generated by a tactic. The tactic:
-
Variant
tactic; last first
inverts the order of the subgoals generated by tactic. It is equivalent to:
-
Variant
tactic; first last
More generally, the tactic:
where num is a Coq numeral, or an Ltac variable
denoting a Coq
numeral, having the value k. It rotates the n subgoals G1 , …, Gn
generated by tactic. The first subgoal becomes Gn + 1 − k and the
circular order of subgoals remains unchanged.
Conversely, the tactic:
rotates the n subgoals G1 , …, Gn generated by tactic in order that the first subgoal becomes Gk .
Finally, the tactics last and first combine with the branching syntax
of Ltac: if the tactic generates n subgoals on a given goal,
then the tactic
where natural denotes the integer k as above, applies tactic1 to the n −k + 1-th goal, … tacticm to the n −k + 2 − m-th goal and tactic n to the others.
Example
Here is a small example on lists. We define first a function which adds an element at the end of a given list.
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Inductive test : nat -> Prop := | C1 n of n = 1 : test n | C2 n of n = 2 : test n | C3 n of n = 3 : test n | C4 n of n = 4 : test n.
- test is defined test_ind is defined
- Lemma example n (t : test n) : True.
- 1 subgoal n : nat t : test n ============================ True
- case: t; last 2 [move=> k| move=> l]; idtac.
- 4 subgoals n : nat ============================ forall n0 : nat, n0 = 1 -> True subgoal 2 is: k = 2 -> True subgoal 3 is: l = 3 -> True subgoal 4 is: forall n0 : nat, n0 = 4 -> True
Iteration¶
SSReflect offers an accurate control on the repetition of tactics, thanks to the do tactical, whose general syntax is:
where mult is a multiplier.
Brackets can only be omitted if a single tactic is given and a multiplier is present.
A tactic of the form:
is equivalent to the standard Ltac expression:
The optional multiplier mult specifies how many times the action of
tactic should be repeated on the current subgoal.
There are four kinds of multipliers:
Their meaning is:
n!the step tactic is repeated exactly n times (where n is a positive integer argument).!the step tactic is repeated as many times as possible, and done at least once.?the step tactic is repeated as many times as possible, optionally.n?the step tactic is repeated up to n times, optionally.
For instance, the tactic:
rewrites at most one time the lemma mult_comm in all the subgoals
generated by tactic , whereas the tactic:
rewrites exactly two times the lemma mult_comm in all the subgoals
generated by tactic, and fails if this rewrite is not possible in some
subgoal.
Note that the combination of multipliers and rewrite is so often used that multipliers are in fact integrated to the syntax of the SSReflect rewrite tactic, see section Rewriting.
Localization¶
In sections Basic localization and Bookkeeping, we have already presented the localization tactical in, whose general syntax is:
where ident is a name in the
context. On the left side of in,
tactic can be move, case, elim, rewrite, set,
or any tactic formed with the general iteration tactical do (see section
Iteration).
The operation described by tactic is performed in the facts listed after
in and in the goal if a * ends the list of names.
The in tactical successively:
- generalizes the selected hypotheses, possibly “protecting” the goal
if
*is not present, - performs
tactic, on the obtained goal, - reintroduces the generalized facts, under the same names.
This defective form of the do tactical is useful to avoid clashes
between standard Ltac in and the SSReflect tactical in.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Ltac mytac H := rewrite H.
- mytac is defined
- Lemma test x y (H1 : x = y) (H2 : y = 3) : x + y = 6.
- 1 subgoal x, y : nat H1 : x = y H2 : y = 3 ============================ x + y = 6
- do [mytac H2] in H1 *.
- 1 subgoal x, y : nat H2 : y = 3 H1 : x = 3 ============================ x + 3 = 6
the last tactic rewrites the hypothesis H2 : y = 3 both in
H1 : x = y and in the goal x + y = 6.
By default in keeps the body of local definitions. To erase the body
of a local definition during the generalization phase, the name of the
local definition must be written between parentheses, like in
rewrite H in H1 (def_n) H2.
From SSReflect 1.5 the grammar for the in tactical has been extended to the following one:
-
Variant
tactic in clear_switch | @? ident | ( ident ) | ( @? ident := c_pattern )+ *?
In its simplest form the last option lets one rename hypotheses that
can’t be cleared (like section variables). For example, (y := x)
generalizes over x and reintroduces the generalized variable under the
name y (and does not clear x).
For a more precise description of this form of localization refer
to Advanced generalization.
Structure¶
Forward reasoning structures the script by explicitly specifying some assumptions to be added to the proof context. It is closely associated with the declarative style of proof, since an extensive use of these highlighted statements make the script closer to a (very detailed) textbook proof.
Forward chaining tactics allow to state an intermediate lemma and start a piece of script dedicated to the proof of this statement. The use of closing tactics (see section Terminators) and of indentation makes syntactically explicit the portion of the script building the proof of the intermediate statement.
The have tactic.¶
The main SSReflect forward reasoning tactic is the have tactic. It can
be use in two modes: one starts a new (sub)proof for an intermediate
result in the main proof, and the other provides explicitly a proof
term for this intermediate step.
In the first mode, the syntax of have in its defective form is:
This tactic supports open syntax for term. Applied to a goal G, it
generates a first subgoal requiring a proof of term in the context of
G. The second generated subgoal is of the form term -> G, where term
becomes the new top assumption, instead of being introduced with a
fresh name. At the proof-term level, the have tactic creates a β
redex, and introduces the lemma under a fresh name, automatically
chosen.
Like in the case of the pose tactic (see section Definitions), the types of
the holes are abstracted in term.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Lemma test : True.
- 1 subgoal ============================ True
- have: _ * 0 = 0.
- 2 subgoals ============================ forall n : nat, n * 0 = 0 subgoal 2 is: (forall n : nat, n * 0 = 0) -> True
The invokation of have is equivalent to:
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Lemma test : True.
- 1 subgoal ============================ True
- have: forall n : nat, n * 0 = 0.
- 2 subgoals ============================ forall n : nat, n * 0 = 0 subgoal 2 is: (forall n : nat, n * 0 = 0) -> True
The have tactic also enjoys the same abstraction mechanism as the pose
tactic for the non-inferred implicit arguments. For instance, the
tactic:
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Lemma test : True.
- 1 subgoal ============================ True
- have: forall x y, (x, y) = (x, y + 0).
- 2 subgoals ============================ forall (T : Type) (x : T) (y : nat), (x, y) = (x, y + 0) subgoal 2 is: (forall (T : Type) (x : T) (y : nat), (x, y) = (x, y + 0)) -> True
opens a new subgoal where the type of x is quantified.
The behavior of the defective have tactic makes it possible to generalize it in the following general construction:
Open syntax is supported for both term. For the description
of i_item and s_item see section
Introduction in the context. The first mode of the
have tactic, which opens a sub-proof for an intermediate result, uses
tactics of the form:
-
Variant
have clear_switch i_item : term by tactic
which behave like:
Note that the clear_switch precedes the:token:i_item, which
allows to reuse
a name of the context, possibly used by the proof of the assumption,
to introduce the new assumption itself.
The``by`` feature is especially convenient when the proof script of the statement is very short, basically when it fits in one line like in:
The possibility of using i_item supplies a very concise syntax for
the further use of the intermediate step. For instance,
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Lemma test a : 3 * a - 1 = a.
- 1 subgoal a : nat ============================ 3 * a - 1 = a
- have -> : forall x, x * a = a.
- 2 subgoals a : nat ============================ forall x : nat, x * a = a subgoal 2 is: a - 1 = a
Note how the second goal was rewritten using the stated equality. Also note that in this last subgoal, the intermediate result does not appear in the context.
Thanks to the deferred execution of clears, the following idiom is also supported (assuming x occurs in the goal only):
Another frequent use of the intro patterns combined with have is the
destruction of existential assumptions like in the tactic:
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Lemma test : True.
- 1 subgoal ============================ True
- have [x Px]: exists x : nat, x > 0; last first.
- 2 subgoals x : nat Px : x > 0 ============================ True subgoal 2 is: exists x : nat, x > 0
An alternative use of the have tactic is to provide the explicit proof
term for the intermediate lemma, using tactics of the form:
-
Variant
have ident? := term
This tactic creates a new assumption of type the type of term.
If the
optional ident is present, this assumption is introduced under the
name ident. Note that the body of the constant is lost for the user.
Again, non inferred implicit arguments and explicit holes are abstracted.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Lemma test : True.
- 1 subgoal ============================ True
- have H := forall x, (x, x) = (x, x).
- 1 subgoal H : Type -> Prop ============================ True
adds to the context H : Type -> Prop. This is a schematic example but
the feature is specially useful when the proof term to give involves
for instance a lemma with some hidden implicit arguments.
After the i_pattern, a list of binders is allowed.
Example
- From Coq Require Import ssreflect.
- From Coq Require Import Omega.
- [Loading ML file quote_plugin.cmxs ... done] [Loading ML file newring_plugin.cmxs ... done] [Loading ML file omega_plugin.cmxs ... done]
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Lemma test : True.
- 1 subgoal ============================ True
- have H x (y : nat) : 2 * x + y = x + x + y by omega.
- 1 subgoal H : forall x y : nat, 2 * x + y = x + x + y ============================ True
A proof term provided after := can mention these bound variables
(that are automatically introduced with the given names).
Since the i_pattern can be omitted, to avoid ambiguity,
bound variables can be surrounded
with parentheses even if no type is specified:
The i_item and s_item can be used to interpret the asserted
hypothesis with views (see section Views and reflection) or simplify the resulting
goals.
The have tactic also supports a suff modifier which allows for
asserting that a given statement implies the current goal without
copying the goal itself.
Example
- Abort All.
- Lemma test : True.
- 1 subgoal ============================ True
- have suff H : 2 + 2 = 3; last first.
- 2 subgoals H : 2 + 2 = 3 -> True ============================ True subgoal 2 is: 2 + 2 = 3 -> True
Note that H is introduced in the second goal.
The suff modifier is not
compatible with the presence of a list of binders.
Generating let in context entries with have¶
Since SSReflect 1.5 the have tactic supports a “transparent” modifier
to generate let in context entries: the @ symbol in front of the
context entry name.
Example
- Abort All.
- Set Printing Depth 15.
- Inductive Ord n := Sub x of x < n.
- Ord is defined Ord_rect is defined Ord_ind is defined Ord_rec is defined
- Notation "'I_ n" := (Ord n) (at level 8, n at level 2, format "''I_' n").
- Arguments Sub {_} _ _.
- Lemma test n m (H : m + 1 < n) : True.
- 1 subgoal n, m : nat H : m + 1 < n ============================ True
- have @i : 'I_n by apply: (Sub m); omega.
- 1 subgoal n, m : nat H : m + 1 < n i := Sub m (Decidable.dec_not_not (m < n) (dec_lt m n) (fun ... => ... ...)) : 'I_n ============================ True
Note that the sub-term produced by omega is in general huge and
uninteresting, and hence one may want to hide it.
For this purpose the [: name ] intro pattern and the tactic
abstract (see The abstract tactic) are provided.
Example
- Abort All.
- Lemma test n m (H : m + 1 < n) : True.
- 1 subgoal n, m : nat H : m + 1 < n ============================ True
- have [:pm] @i : 'I_n by apply: (Sub m); abstract: pm; omega.
- 1 subgoal n, m : nat H : m + 1 < n pm : m < n (*1*) i := Sub m pm : 'I_n ============================ True
The type of pm can be cleaned up by its annotation (*1*) by just
simplifying it. The annotations are there for technical reasons only.
When intro patterns for abstract constants are used in conjunction with have and an explicit term, they must be used as follows:
Example
- Abort All.
- Lemma test n m (H : m + 1 < n) : True.
- 1 subgoal n, m : nat H : m + 1 < n ============================ True
- have [:pm] @i : 'I_n := Sub m pm.
- 2 subgoals n, m : nat H : m + 1 < n ============================ S m <= n subgoal 2 is: True
- by omega.
- 1 subgoal n, m : nat H : m + 1 < n pm : S m <= n (*1*) i := Sub m pm : 'I_n : 'I_n ============================ True
In this case the abstract constant pm is assigned by using it in
the term that follows := and its corresponding goal is left to be
solved. Goals corresponding to intro patterns for abstract constants
are opened in the order in which the abstract constants are declared
(not in the “order” in which they are used in the term).
Note that abstract constants do respect scopes. Hence, if a variable is declared after their introduction, it has to be properly generalized (i.e. explicitly passed to the abstract constant when one makes use of it).
Example
- Abort All.
- Lemma test n m (H : m + 1 < n) : True.
- 1 subgoal n, m : nat H : m + 1 < n ============================ True
- have [:pm] @i k : 'I_(n+k) by apply: (Sub m); abstract: pm k; omega.
- 1 subgoal n, m : nat H : m + 1 < n pm : (forall k : nat, m < n + k) (*1*) i := fun k : nat => Sub m (pm k) : forall k : nat, 'I_(n + k) ============================ True
Last, notice that the use of intro patterns for abstract constants is
orthogonal to the transparent flag @ for have.
The have tactic and typeclass resolution¶
Since SSReflect 1.5 the have tactic behaves as follows with respect to
typeclass inference.
- Abort All.
- Axiom ty : Type.
- ty is declared
- Axiom t : ty.
- t is declared
- Goal True.
- 1 subgoal ============================ True
have foo : ty.Full inference for
ty. The first subgoal demands a proof of such instantiated statement.have foo : ty := .No inference for
ty. Unresolved instances are quantified inty. The first subgoal demands a proof of such quantified statement. Note that no proof term follows:=, hence two subgoals are generated.have foo : ty := t.No inference for
tyandt.have foo := t.No inference for
t. Unresolved instances are quantified in the (inferred) type oftand abstracted int.
-
Flag
SsrHave NoTCResolution¶ This option restores the behavior of SSReflect 1.4 and below (never resolve typeclasses).
Variants: the suff and wlog tactics¶
As it is often the case in mathematical textbooks, forward reasoning may be used in slightly different variants. One of these variants is to show that the intermediate step L easily implies the initial goal G. By easily we mean here that the proof of L ⇒ G is shorter than the one of L itself. This kind of reasoning step usually starts with: “It suffices to show that …”.
This is such a frequent way of reasoning that SSReflect has a variant
of the have tactic called suffices (whose abridged name is suff).
The
have and suff tactics are equivalent and have the same syntax but:
the order of the generated subgoals is inversed
but the optional clear item is still performed in the second branch. This means that the tactic:
suff {H} H : forall x : nat, x >= 0.fails if the context of the current goal indeed contains an assumption named
H.
The rationale of this clearing policy is to make possible “trivial” refinements of an assumption, without changing its name in the main branch of the reasoning.
The have modifier can follow the suff tactic.
Example
- Abort All.
- Axioms G P : Prop.
- G is declared P is declared
- Lemma test : G.
- 1 subgoal ============================ G
- suff have H : P.
- 2 subgoals H : P ============================ G subgoal 2 is: (P -> G) -> G
Note that, in contrast with have suff, the name H has been introduced
in the first goal.
Another useful construct is reduction, showing that a particular case
is in fact general enough to prove a general property. This kind of
reasoning step usually starts with: “Without loss of generality, we
can suppose that …”. Formally, this corresponds to the proof of a goal
G by introducing a cut wlog_statement -> G. Hence the user shall
provide a proof for both (wlog_statement -> G) -> G and
wlog_statement -> G. However, such cuts are usually rather
painful to perform by
hand, because the statement wlog_statement is tedious to write by hand,
and sometimes even to read.
SSReflect implements this kind of reasoning step through the without loss
tactic, whose short name is wlog. It offers support to describe
the shape of the cut statements, by providing the simplifying
hypothesis and by pointing at the elements of the initial goals which
should be generalized. The general syntax of without loss is:
-
wlog suff? clear_switch? i_item? : ident* / term¶
-
Variant
without loss suff? clear_switch? i_item? : ident* / term¶
where each ident is a constant in the context
of the goal. Open syntax is supported for term.
In its defective form:
-
Variant
wlog: / term
-
Variant
without loss: / term
on a goal G, it creates two subgoals: a first one to prove the formula (term -> G) -> G and a second one to prove the formula term -> G.
If the optional list of ident is present
on the left side of /, these constants are generalized in the
premise (term -> G) of the first subgoal. By default bodies of local
definitions are erased. This behavior can be inhibited by prefixing the
name of the local definition with the @ character.
In the second subgoal, the tactic:
is performed if at least one of these optional switches is present in
the wlog tactic.
The wlog tactic is specially useful when a symmetry argument
simplifies a proof. Here is an example showing the beginning of the
proof that quotient and reminder of natural number euclidean division
are unique.
Example
- Abort All.
- Lemma quo_rem_unicity d q1 q2 r1 r2 : q1*d + r1 = q2*d + r2 -> r1 < d -> r2 < d -> (q1, r1) = (q2, r2).
- 1 subgoal d, q1, q2, r1, r2 : nat ============================ q1 * d + r1 = q2 * d + r2 -> r1 < d -> r2 < d -> (q1, r1) = (q2, r2)
- wlog: q1 q2 r1 r2 / q1 <= q2.
- 2 subgoals d, q1, q2, r1, r2 : nat ============================ (forall q3 q4 r3 r4 : nat, q3 <= q4 -> q3 * d + r3 = q4 * d + r4 -> r3 < d -> r4 < d -> (q3, r3) = (q4, r4)) -> q1 * d + r1 = q2 * d + r2 -> r1 < d -> r2 < d -> (q1, r1) = (q2, r2) subgoal 2 is: q1 <= q2 -> q1 * d + r1 = q2 * d + r2 -> r1 < d -> r2 < d -> (q1, r1) = (q2, r2)
- by case (le_gt_dec q1 q2)=> H; last symmetry; eauto with arith.
- 1 subgoal d, q1, q2, r1, r2 : nat ============================ q1 <= q2 -> q1 * d + r1 = q2 * d + r2 -> r1 < d -> r2 < d -> (q1, r1) = (q2, r2)
The wlog suff variant is simpler, since it cuts wlog_statement instead
of wlog_statement -> G. It thus opens the goals
wlog_statement -> G
and wlog_statement.
In its simplest form the generally have : … tactic is equivalent to
wlog suff : … followed by last first. When the have tactic is used
with the generally (or gen) modifier it accepts an extra identifier
followed by a comma before the usual intro pattern. The identifier
will name the new hypothesis in its more general form, while the intro
pattern will be used to process its instance.
Example
- From Coq Require Import ssreflect ssrfun ssrbool.
- Overwriting previous delimiting key bool in scope bool_scope Ambiguous paths: [pred_of_mem_pred; sort_of_simpl_pred] : mem_pred >-> pred_sort
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Axiom P : nat -> Prop.
- P is declared
- Axioms eqn leqn : nat -> nat -> bool.
- eqn is declared leqn is declared
- Notation "a != b" := (eqn a b) (at level 70) : this_scope.
- Notation "a <= b" := (leqn a b) (at level 70) : this_scope.
- Open Scope this_scope.
- Lemma simple n (ngt0 : 0 < n ) : P n.
- 1 subgoal n : nat ngt0 : 0 < n ============================ P n
- gen have ltnV, /andP[nge0 neq0] : n ngt0 / (0 <= n) && (n != 0); last first.
- 2 subgoals n : nat ngt0 : 0 < n ltnV : forall n : nat, 0 < n -> (0 <= n) && (n != 0) nge0 : 0 <= n neq0 : n != 0 ============================ P n subgoal 2 is: (0 <= n) && (n != 0)
Advanced generalization¶
The complete syntax for the items on the left hand side of the /
separator is the following one:
-
Variant
wlog … : clear_switch | @? ident | ( @? ident := c_pattern)? / term
Clear operations are intertwined with generalization operations. This helps in particular avoiding dependency issues while generalizing some facts.
If an ident is prefixed with the @ mark, then a let-in redex is
created, which keeps track if its body (if any). The syntax
( ident := c_pattern) allows to generalize an arbitrary term using a
given name. Note that its simplest form (x := y) is just a renaming of
y into x. In particular, this can be useful in order to simulate the
generalization of a section variable, otherwise not allowed. Indeed
renaming does not require the original variable to be cleared.
The syntax (@x := y) generates a let-in abstraction but with the
following caveat: x will not bind y, but its body, whenever y can be
unfolded. This covers the case of both local and global definitions, as
illustrated in the following example.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Variable x : nat.
- x is declared
- Definition addx z := z + x.
- addx is defined
- Lemma test : x <= addx x.
- 1 subgoal x : nat ============================ x <= addx x
- wlog H : (y := x) (@twoy := addx x) / twoy = 2 * y.
- 2 subgoals x : nat ============================ (forall y : nat, let twoy := y + y in twoy = 2 * y -> y <= twoy) -> x <= addx x subgoal 2 is: y <= twoy
To avoid unfolding the term captured by the pattern add x one can use
the pattern id (addx x), that would produce the following first
subgoal
- From Coq Require Import ssreflect Omega.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Variable x : nat.
- x is declared
- Definition addx z := z + x.
- addx is defined
- Lemma test : x <= addx x.
- 1 subgoal x : nat ============================ x <= addx x
- wlog H : (y := x) (@twoy := id (addx x)) / twoy = 2 * y.
- 2 subgoals x : nat ============================ (forall y : nat, let twoy := addx y in twoy = 2 * y -> y <= addx y) -> x <= addx x subgoal 2 is: y <= addx y
Rewriting¶
The generalized use of reflection implies that most of the
intermediate results handled are properties of effectively computable
functions. The most efficient mean of establishing such results are
computation and simplification of expressions involving such
functions, i.e., rewriting. SSReflect therefore includes an
extended rewrite tactic, that unifies and combines most of the
rewriting functionalities.
An extended rewrite tactic¶
The main features of the rewrite tactic are:
- It can perform an entire series of such operations in any subset of the goal and/or context;
- It allows to perform rewriting, simplifications, folding/unfolding of definitions, closing of goals;
- Several rewriting operations can be chained in a single tactic;
- Control over the occurrence at which rewriting is to be performed is significantly enhanced.
The general form of an SSReflect rewrite tactic is:
The combination of a rewrite tactic with the in tactical (see section Localization) performs rewriting in both the context and the goal.
A rewrite step rstep has the general form:
-
r_prefix ::=-? mult? occ_switch clear_switch? [ r_pattern ]?
An r_prefix contains annotations to qualify where and how the rewrite
operation should be performed:
- The optional initial
-indicates the direction of the rewriting ofr_item: if present the direction is right-to-left and it is left-to-right otherwise. - The multiplier
mult(see section Iteration) specifies if and how the rewrite operation should be repeated. - A rewrite operation matches the occurrences of a rewrite pattern,
and replaces these occurrences by another term, according to the
given
r_item. The optional redex switch[r_pattern], which should always be surrounded by brackets, gives explicitly this rewrite pattern. In its simplest form, it is a regular term. If no explicit redex switch is present the rewrite pattern to be matched is inferred from ther_item. - This optional term, or the
r_item, may be preceded by an occurrence switch (see section Selectors) or a clear item (see section Discharge), these two possibilities being exclusive. An occurrence switch selects the occurrences of the rewrite pattern which should be affected by the rewrite operation.
An r_item can be:
A simplification
r_item, represented by as_item(see section Introduction in the context). Simplification operations are intertwined with the possible other rewrite operations specified by the list ofr_item.A folding/unfolding
r_item. The tactic:rewrite /termunfolds the head constant of term in every occurrence of the first matching of term in the goal. In particular, ifmy_defis a (local or global) defined constant, the tactic:rewrite /my_def.is analogous to:unfold my_def. Conversely:rewrite -/my_def.is equivalent to:fold my_def. When an unfoldr_itemis combined with a redex pattern, a conversion operation is performed. A tactic of the form:rewrite -[term1]/term2.is equivalent to:change term1 with term2.Ifterm2is a single constant andterm1head symbol is notterm2, then the head symbol ofterm1is repeatedly unfolded untilterm2appears.- A
term, which can be: - A term whose type has the form:
forall (x1 : A1 )…(xn : An ), eq term1 term2whereeqis the Leibniz equality or a registered setoid equality. - A list of terms
(t1 ,…,tn), eachtihaving a type above. The tactic:rewrite r_prefix (t1 ,…,tn ).is equivalent to:do [rewrite r_prefix t1 | … | rewrite r_prefix tn ]. - An anonymous rewrite lemma
(_ : term), where term has a type as above. tactic:rewrite (_ : term)is in fact synonym of:cutrewrite (term)..
- A term whose type has the form:
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Definition double x := x + x.
- double is defined
- Definition ddouble x := double (double x).
- ddouble is defined
- Lemma test x : ddouble x = 4 * x.
- 1 subgoal x : nat ============================ ddouble x = 4 * x
- rewrite [ddouble _]/double.
- 1 subgoal x : nat ============================ double x + double x = 4 * x
Warning
The SSReflect terms containing holes are not typed as abstractions in this context. Hence the following script fails.
- Abort.
- Definition f := fun x y => x + y.
- f is defined
- Lemma test x y : x + y = f y x.
- 1 subgoal x, y : nat ============================ x + y = f y x
- Fail rewrite -[f y]/(y + _).
- The command has indeed failed with message: Ltac call to "rewrite (ssrrwargs) (ssrclauses)" failed. fold pattern (y + _) does not match redex (f y)
but the following script succeeds
- Restart.
- rewrite -[f y x]/(y + _).
- 1 subgoal x, y : nat ============================ x + y = y + x
- A
Remarks and examples¶
Rewrite redex selection¶
The general strategy of SSReflect is to grasp as many redexes as possible and to let the user select the ones to be rewritten thanks to the improved syntax for the control of rewriting.
This may be a source of incompatibilities between the two rewrite tactics.
In a rewrite tactic of the form:
term1 is the explicit rewrite redex and term2 is the rewrite rule.
This execution of this tactic unfolds as follows:
- First
term1andterm2are βι normalized. Thenterm2is put in head normal form if the Leibniz equality constructoreqis not the head symbol. This may involve ζ reductions. - Then, the matching algorithm (see section Abbreviations)
determines the
first subterm of the goal matching the rewrite pattern. The rewrite
pattern is given by
term1, if an explicit redex pattern switch is provided, or by the type ofterm2otherwise. However, matching skips over matches that would lead to trivial rewrites. All the occurrences of this subterm in the goal are candidates for rewriting. - Then only the occurrences coded by
occ_switch(see again section Abbreviations) are finally selected for rewriting. - The left hand side of
term2is unified with the subterm found by the matching algorithm, and if this succeeds, all the selected occurrences in the goal are replaced by the right hand side ofterm2. - Finally the goal is βι normalized.
In the case term2 is a list of terms, the first top-down (in the
goal) left-to-right (in the list) matching rule gets selected.
Chained rewrite steps¶
The possibility to chain rewrite operations in a single tactic makes scripts more compact and gathers in a single command line a bunch of surgical operations which would be described by a one sentence in a pen and paper proof.
Performing rewrite and simplification operations in a single tactic enhances significantly the concision of scripts. For instance the tactic:
unfolds my_def in the goal, simplifies the second occurrence of the
first subterm matching pattern [f _], rewrites my_eq, simplifies the
goals and closes trivial goals.
Here are some concrete examples of chained rewrite operations, in the proof of basic results on natural numbers arithmetic.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Axiom addn0 : forall m, m + 0 = m.
- addn0 is declared
- Axiom addnS : forall m n, m + S n = S (m + n).
- addnS is declared
- Axiom addSnnS : forall m n, S m + n = m + S n.
- addSnnS is declared
- Lemma addnCA m n p : m + (n + p) = n + (m + p).
- 1 subgoal m, n, p : nat ============================ m + (n + p) = n + (m + p)
- by elim: m p => [ | m Hrec] p; rewrite ?addSnnS -?addnS.
- No more subgoals.
- Qed.
- addnCA is defined
- Lemma addnC n m : m + n = n + m.
- 1 subgoal n, m : nat ============================ m + n = n + m
- by rewrite -{1}[n]addn0 addnCA addn0.
- No more subgoals.
- Qed.
- addnC is defined
Note the use of the ? switch for parallel rewrite operations in the
proof of addnCA.
Explicit redex switches are matched first¶
If an r_prefix involves a redex switch, the first step is to find a
subterm matching this redex pattern, independently from the left hand
side of the equality the user wants to rewrite.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Lemma test (H : forall t u, t + u = u + t) x y : x + y = y + x.
- 1 subgoal H : forall t u : nat, t + u = u + t x, y : nat ============================ x + y = y + x
- rewrite [y + _]H.
- 1 subgoal H : forall t u : nat, t + u = u + t x, y : nat ============================ x + y = x + y
Note that if this first pattern matching is not compatible with the
r_item, the rewrite fails, even if the goal contains a
correct redex matching both the redex switch and the left hand side of
the equality.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Lemma test (H : forall t u, t + u * 0 = t) x y : x + y * 4 + 2 * 0 = x + 2 * 0.
- 1 subgoal H : forall t u : nat, t + u * 0 = t x, y : nat ============================ x + y * 4 + 2 * 0 = x + 2 * 0
- Fail rewrite [x + _]H.
- The command has indeed failed with message: Ltac call to "rewrite (ssrrwargs) (ssrclauses)" failed. pattern (x + y * 4) does not match LHS of H
Indeed the left hand side of H does not match
the redex identified by the pattern x + y * 4.
Occurrence switches and redex switches¶
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Lemma test x y : x + y + 0 = x + y + y + 0 + 0 + (x + y + 0).
- 1 subgoal x, y : nat ============================ x + y + 0 = x + y + y + 0 + 0 + (x + y + 0)
- rewrite {2}[_ + y + 0](_: forall z, z + 0 = z).
- 2 subgoals x, y : nat ============================ forall z : nat, z + 0 = z subgoal 2 is: x + y + 0 = x + y + y + 0 + 0 + (x + y)
The second subgoal is generated by the use of an anonymous lemma in
the rewrite tactic. The effect of the tactic on the initial goal is to
rewrite this lemma at the second occurrence of the first matching
x + y + 0 of the explicit rewrite redex _ + y + 0.
An empty occurrence switch {} is not interpreted as a valid occurrence
switch. It has the effect of clearing the r_item (when it is the name
of a context entry).
Occurrence selection and repetition¶
Occurrence selection has priority over repetition switches. This means the repetition of a rewrite tactic specified by a multiplier will perform matching each time an elementary rewrite operation is performed. Repeated rewrite tactics apply to every subgoal generated by the previous tactic, including the previous instances of the repetition.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Lemma test x y (z : nat) : x + 1 = x + y + 1.
- 1 subgoal x, y, z : nat ============================ x + 1 = x + y + 1
- rewrite 2!(_ : _ + 1 = z).
- 4 subgoals x, y, z : nat ============================ x + 1 = z subgoal 2 is: z = z subgoal 3 is: x + y + 1 = z subgoal 4 is: z = z
This last tactic generates three subgoals because
the second rewrite operation specified with the 2! multiplier
applies to the two subgoals generated by the first rewrite.
Multi-rule rewriting¶
The rewrite tactic can be provided a tuple of rewrite rules, or more generally a tree of such rules, since this tuple can feature arbitrary inner parentheses. We call multirule such a generalized rewrite rule. This feature is of special interest when it is combined with multiplier switches, which makes the rewrite tactic iterate the rewrite operations prescribed by the rules on the current goal.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Variables (a b c : nat).
- a is declared b is declared c is declared
- Hypothesis eqab : a = b.
- eqab is declared
- Hypothesis eqac : a = c.
- eqac is declared
- Lemma test : a = a.
- 1 subgoal a, b, c : nat eqab : a = b eqac : a = c ============================ a = a
- rewrite (eqab, eqac).
- 1 subgoal a, b, c : nat eqab : a = b eqac : a = c ============================ b = b
Indeed rule eqab is the first to apply among the ones
gathered in the tuple passed to the rewrite tactic. This multirule
(eqab, eqac) is actually a Coq term and we can name it with a
definition:
- Abort.
- Definition multi1 := (eqab, eqac).
- multi1 is defined
In this case, the tactic rewrite multi1 is a synonym for
rewrite (eqab, eqac).
More precisely, a multirule rewrites the first subterm to which one of the rules applies in a left-to-right traversal of the goal, with the first rule from the multirule tree in left-to-right order. Matching is performed according to the algorithm described in Section Abbreviations, but literal matches have priority.
Example
- Definition d := a.
- d is defined
- Hypotheses eqd0 : d = 0.
- eqd0 is declared
- Definition multi2 := (eqab, eqd0).
- multi2 is defined
- Lemma test : d = b.
- 1 subgoal a, b, c : nat eqab : a = b eqac : a = c eqd0 : d = 0 ============================ d = b
- rewrite multi2.
- 1 subgoal a, b, c : nat eqab : a = b eqac : a = c eqd0 : d = 0 ============================ 0 = b
Indeed rule eqd0 applies without unfolding the
definition of d.
For repeated rewrites the selection process is repeated anew.
Example
- Abort.
- Hypothesis eq_adda_b : forall x, x + a = b.
- eq_adda_b is declared
- Hypothesis eq_adda_c : forall x, x + a = c.
- eq_adda_c is declared
- Hypothesis eqb0 : b = 0.
- eqb0 is declared
- Definition multi3 := (eq_adda_b, eq_adda_c, eqb0).
- multi3 is defined
- Lemma test : 1 + a = 12 + a.
- 1 subgoal a, b, c : nat eqab : a = b eqac : a = c eqd0 : d = 0 eq_adda_b : forall x : nat, x + a = b eq_adda_c : forall x : nat, x + a = c eqb0 : b = 0 ============================ 1 + a = 12 + a
- rewrite 2!multi3.
- 1 subgoal a, b, c : nat eqab : a = b eqac : a = c eqd0 : d = 0 eq_adda_b : forall x : nat, x + a = b eq_adda_c : forall x : nat, x + a = c eqb0 : b = 0 ============================ 0 = 12 + a
It uses eq_adda_b then eqb0 on the left-hand
side only. Without the bound 2 one would obtain 0 = 0.
The grouping of rules inside a multirule does not affect the selection
strategy but can make it easier to include one rule set in another or
to (universally) quantify over the parameters of a subset of rules (as
there is special code that will omit unnecessary quantifiers for rules
that can be syntactically extracted). It is also possible to reverse
the direction of a rule subset, using a special dedicated syntax: the
tactic rewrite (=~ multi1) is equivalent to rewrite multi1_rev.
Example
- Abort.
- Hypothesis eqba : b = a.
- eqba is declared
- Hypothesis eqca : c = a.
- eqca is declared
- Definition multi1_rev := (eqba, eqca).
- multi1_rev is defined
except that the constants eqba, eqab, mult1_rev
have not been created.
Rewriting with multirules is useful to implement simplification or
transformation procedures, to be applied on terms of small to medium
size. For instance the library ssrnat (Mathematical Components library)
provides two implementations
for arithmetic operations on natural numbers: an elementary one and a
tail recursive version, less inefficient but also less convenient for
reasoning purposes. The library also provides one lemma per such
operation, stating that both versions return the same values when
applied to the same arguments:
The operation on the left hand side of each lemma is the efficient
version, and the corresponding naive implementation is on the right
hand side. In order to reason conveniently on expressions involving
the efficient operations, we gather all these rules in the definition
trecE:
The tactic: rewrite !trecE.
restores the naive versions of each operation in a goal involving the
efficient ones, e.g. for the purpose of a correctness proof.
Wildcards vs abstractions¶
The rewrite tactic supports r_items containing holes. For example, in
the tactic rewrite (_ : _ * 0 = 0).
the term _ * 0 = 0 is interpreted as forall n : nat, n * 0 = 0.
Anyway this tactic is not equivalent to
rewrite (_ : forall x, x * 0 = 0)..
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Lemma test y z : y * 0 + y * (z * 0) = 0.
- 1 subgoal y, z : nat ============================ y * 0 + y * (z * 0) = 0
- rewrite (_ : _ * 0 = 0).
- 2 subgoals y, z : nat ============================ y * 0 = 0 subgoal 2 is: 0 + y * (z * 0) = 0
while the other tactic results in
- Undo.
- 1 subgoal y, z : nat ============================ y * 0 + y * (z * 0) = 0
- rewrite (_ : forall x, x * 0 = 0).
- 2 subgoals y, z : nat ============================ forall x : nat, x * 0 = 0 subgoal 2 is: 0 + y * (z * 0) = 0
The first tactic requires you to prove the instance of the (missing) lemma that was used, while the latter requires you prove the quantified form.
When SSReflect rewrite fails on standard Coq licit rewrite¶
In a few cases, the SSReflect rewrite tactic fails rewriting some redexes which standard Coq successfully rewrites. There are two main cases:
SSReflect never accepts to rewrite indeterminate patterns like:
Lemma foo (x : unit) : x = tt.SSReflect will however accept the ηζ expansion of this rule:
Lemma fubar (x : unit) : (let u := x in u) = tt.The standard rewrite tactic provided by Coq uses a different algorithm to find instances of the rewrite rule.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Variable g : nat -> nat.
- g is declared
- Definition f := g.
- f is defined
- Axiom H : forall x, g x = 0.
- H is declared
- Lemma test : f 3 + f 3 = f 6.
- 1 subgoal g : nat -> nat ============================ f 3 + f 3 = f 6
- rewrite -> H.
- 1 subgoal g : nat -> nat ============================ 0 + 0 = f 6
This rewriting is not possible in SSReflect because there is no occurrence of the head symbol
fof the rewrite rule in the goal.- Undo.
- 1 subgoal g : nat -> nat ============================ f 3 + f 3 = f 6
- Fail rewrite H.
- The command has indeed failed with message: Ltac call to "rewrite (ssrrwargs) (ssrclauses)" failed. The LHS of H (g _) does not match any subterm of the goal
Rewriting with
Hfirst requires unfolding the occurrences offwhere the substitution is to be performed (here there is a single such occurrence), using tacticrewrite /f(for a global replacement of f by g) orrewrite pattern/f, for a finer selection.- Undo.
- rewrite /f H.
- 1 subgoal g : nat -> nat ============================ 0 + 0 = g 6
alternatively one can override the pattern inferred from
H- Undo.
- 1 subgoal g : nat -> nat ============================ f 3 + f 3 = f 6
- rewrite [f _]H.
- 1 subgoal g : nat -> nat ============================ 0 + 0 = f 6
Existential metavariables and rewriting¶
The rewrite tactic will not instantiate existing existential metavariables when matching a redex pattern.
If a rewrite rule generates a goal with new existential metavariables
in the Prop sort, these will be generalized as for apply
(see The apply tactic) and
corresponding new goals will be generated.
Example
- From Coq Require Import ssreflect ssrfun ssrbool.
- Overwriting previous delimiting key bool in scope bool_scope Ambiguous paths: [pred_of_mem_pred; sort_of_simpl_pred] : mem_pred >-> pred_sort
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Set Warnings "-notation-overridden".
- Axiom leq : nat -> nat -> bool.
- leq is declared
- Notation "m <= n" := (leq m n) : nat_scope.
- Notation "m < n" := (S m <= n) : nat_scope.
- Inductive Ord n := Sub x of x < n.
- Ord is defined Ord_rect is defined Ord_ind is defined Ord_rec is defined
- Notation "'I_ n" := (Ord n) (at level 8, n at level 2, format "''I_' n").
- Arguments Sub {_} _ _.
- Definition val n (i : 'I_n) := let: Sub a _ := i in a.
- val is defined
- Definition insub n x := if @idP (x < n) is ReflectT _ Px then Some (Sub x Px) else None.
- insub is defined
- Axiom insubT : forall n x Px, insub n x = Some (Sub x Px).
- insubT is declared
- Lemma test (x : 'I_2) y : Some x = insub 2 y.
- 1 subgoal x : 'I_2 y : nat ============================ Some x = insub 2 y
- rewrite insubT.
- 2 subgoals x : 'I_2 y : nat ============================ forall Hyp0 : y < 2, Some x = Some (Sub y Hyp0) subgoal 2 is: y < 2
Since the argument corresponding to Px is not supplied by the user, the
resulting goal should be Some x = Some (Sub y ?Goal).
Instead, SSReflect rewrite tactic hides the existential variable.
As in The apply tactic, the ssrautoprop tactic is used to try to
solve the existential variable.
- Abort.
- Lemma test (x : 'I_2) y (H : y < 2) : Some x = insub 2 y.
- 1 subgoal x : 'I_2 y : nat H : y < 2 ============================ Some x = insub 2 y
- rewrite insubT.
- 1 subgoal x : 'I_2 y : nat H : y < 2 ============================ Some x = Some (Sub y H)
As a temporary limitation, this behavior is available only if the rewriting rule is stated using Leibniz equality (as opposed to setoid relations). It will be extended to other rewriting relations in the future.
Locking, unlocking¶
As program proofs tend to generate large goals, it is important to be
able to control the partial evaluation performed by the simplification
operations that are performed by the tactics. These evaluations can
for example come from a /= simplification switch, or from rewrite
steps which may expand large terms while performing conversion. We
definitely want to avoid repeating large subterms of the goal in the
proof script. We do this by “clamping down” selected function symbols
in the goal, which prevents them from being considered in
simplification or rewriting steps. This clamping is accomplished by
using the occurrence switches (see section:ref:abbreviations_ssr)
together with “term tagging” operations.
SSReflect provides two levels of tagging.
The first one uses auxiliary definitions to introduce a provably equal copy of any term t. However this copy is (on purpose) not convertible to t in the Coq system [17]. The job is done by the following construction:
Note that the definition of master_key is explicitly opaque. The
equation t = locked t given by the lock lemma can be used for
selective rewriting, blocking on the fly the reduction in the term t.
Example
- From Coq Require Import ssreflect ssrfun ssrbool.
- Overwriting previous delimiting key bool in scope bool_scope Ambiguous paths: [pred_of_mem_pred; sort_of_simpl_pred] : mem_pred >-> pred_sort
- From Coq Require Import List.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Variable A : Type.
- A is declared
- Fixpoint has (p : A -> bool) (l : list A) : bool := if l is cons x l then p x || (has p l) else false.
- has is defined has is recursively defined (decreasing on 2nd argument)
- Lemma test p x y l (H : p x = true) : has p ( x :: y :: l) = true.
- 1 subgoal A : Type p : A -> bool x, y : A l : list A H : p x = true ============================ has p (x :: y :: l) = true
- rewrite {2}[cons]lock /= -lock.
- 1 subgoal A : Type p : A -> bool x, y : A l : list A H : p x = true ============================ p x || has p (y :: l) = true
It is sometimes desirable to globally prevent a definition from being expanded by simplification; this is done by adding locked in the definition.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Definition lid := locked (fun x : nat => x).
- lid is defined
- Lemma test : lid 3 = 3.
- 1 subgoal ============================ lid 3 = 3
- rewrite /=.
- 1 subgoal ============================ lid 3 = 3
- unlock lid.
- 1 subgoal ============================ 3 = 3
We provide a special tactic unlock for unfolding such definitions while removing “locks”, e.g., the tactic:
-
unlock occ_switch? ident¶
replaces the occurrence(s) of ident coded by the
occ_switch with the corresponding body.
We found that it was usually preferable to prevent the expansion of
some functions by the partial evaluation switch /=, unless this
allowed the evaluation of a condition. This is possible thanks to another
mechanism of term tagging, resting on the following Notation:
The term (nosimpl t) simplifies to t except in a definition.
More precisely, given:
the term foo (or (foo t’)) will not be expanded by the simpl
tactic unless it is in a forcing context (e.g., in match foo t’ with …
end, foo t’ will be reduced if this allows match to be reduced).
Note that nosimpl bar is simply notation for a term that reduces to
bar; hence unfold foo will replace foo by bar, and
fold foo will replace bar by foo.
Warning
The nosimpl trick only works if no reduction is apparent in
t; in particular, the declaration:
will usually not work. Anyway, the common practice is to tag only the function, and to use the following definition, which blocks the reduction as expected:
A standard example making this technique shine is the case of arithmetic operations. We define for instance:
The operation addn behaves exactly like plus, except that
(addn (S n) m) will not simplify spontaneously to
(S (addn n m)) (the two terms, however, are convertible).
In addition, the unfolding step: rewrite /addn
will replace addn directly with plus, so the nosimpl form is
essentially invisible.
Congruence¶
Because of the way matching interferes with parameters of type families, the tactic:
will generally fail to perform congruence simplification, even on rather simple cases. We therefore provide a more robust alternative in which the function is supplied:
This tactic:
- checks that the goal is a Leibniz equality;
- matches both sides of this equality with “term applied to some arguments”, inferring the right number of arguments from the goal and the type of term. This may expand some definitions or fixpoints;
- generates the subgoals corresponding to pairwise equalities of the arguments present in the goal.
The goal can be a non dependent product P -> Q. In that case, the
system asserts the equation P = Q, uses it to solve the goal, and
calls the congr tactic on the remaining goal P = Q. This can be useful
for instance to perform a transitivity step, like in the following
situation.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Lemma test (x y z : nat) (H : x = y) : x = z.
- 1 subgoal x, y, z : nat H : x = y ============================ x = z
- congr (_ = _) : H.
- 1 focused subgoal (shelved: 1) x, y, z : nat ============================ y = z
- Abort.
- Lemma test (x y z : nat) : x = y -> x = z.
- 1 subgoal x, y, z : nat ============================ x = y -> x = z
- congr (_ = _).
- 1 focused subgoal (shelved: 1) x, y, z : nat ============================ y = z
The optional num forces the number of arguments for which the
tactic should generate equality proof obligations.
This tactic supports equalities between applications with dependent arguments. Yet dependent arguments should have exactly the same parameters on both sides, and these parameters should appear as first arguments.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Definition f n := if n is 0 then plus else mult.
- f is defined
- Definition g (n m : nat) := plus.
- g is defined
- Lemma test x y : f 0 x y = g 1 1 x y.
- 1 subgoal x, y : nat ============================ f 0 x y = g 1 1 x y
- congr plus.
- No more subgoals.
This script shows that the congr tactic matches plus
with f 0 on the left hand side and g 1 1 on the right hand
side, and solves the goal.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Lemma test n m (Hnm : m <= n) : S m + (S n - S m) = S n.
- 1 subgoal n, m : nat Hnm : m <= n ============================ S m + (S n - S m) = S n
- congr S; rewrite -/plus.
- 1 subgoal n, m : nat Hnm : m <= n ============================ m + (S n - S m) = n
The tactic rewrite -/plus folds back the expansion of plus
which was necessary for matching both sides of the equality with
an application of S.
Like most SSReflect arguments, term can contain wildcards.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Lemma test x y : x + (y * (y + x - x)) = x * 1 + (y + 0) * y.
- 1 subgoal x, y : nat ============================ x + y * (y + x - x) = x * 1 + (y + 0) * y
- congr ( _ + (_ * _)).
- 3 focused subgoals (shelved: 3) x, y : nat ============================ x = x * 1 subgoal 2 is: y = y + 0 subgoal 3 is: y + x - x = y
Contextual patterns¶
The simple form of patterns used so far, terms possibly containing
wild cards, often require an additional occ_switch to be specified.
While this may work pretty fine for small goals, the use of
polymorphic functions and dependent types may lead to an invisible
duplication of function arguments. These copies usually end up in
types hidden by the implicit arguments machinery or by user-defined
notations. In these situations computing the right occurrence numbers
is very tedious because they must be counted on the goal as printed
after setting the Printing All flag. Moreover the resulting script is
not really informative for the reader, since it refers to occurrence
numbers he cannot easily see.
Contextual patterns mitigate these issues allowing to specify occurrences according to the context they occur in.
Syntax¶
The following table summarizes the full syntax of c_pattern and the
corresponding subterm(s) identified by the pattern. In the third
column we use s.m.r. for “the subterms matching the redex” specified
in the second column.
c_pattern |
redex | subterms affected |
|---|---|---|
term |
term |
all occurrences of term |
ident in term |
subterm of term selected by ident |
all the subterms identified by ident in all the
occurrences of term |
term1 in ident in term2 |
term1 in all s.m.r. |
in all the subterms identified by
ident in all the occurrences of term2 |
term1 as ident in term2 |
term 1 |
in all the subterms identified by ident`
in all the occurrences of ``term2[term 1 /ident] |
The rewrite tactic supports two more patterns obtained prefixing the
first two with in. The intended meaning is that the pattern identifies
all subterms of the specified context. The rewrite tactic will infer a
pattern for the redex looking at the rule used for rewriting.
r_pattern |
redex | subterms affected |
|---|---|---|
in term |
inferred from rule | in all s.m.r. in all occurrences of term |
in ident in term |
inferred from rule | in all s.m.r. in all the subterms identified by ident
in all the occurrences of term |
The first c_pattern is the simplest form matching any context but
selecting a specific redex and has been described in the previous
sections. We have seen so far that the possibility of selecting a
redex using a term with holes is already a powerful means of redex
selection. Similarly, any terms provided by the user in the more
complex forms of c_patterns
presented in the tables above can contain
holes.
For a quick glance at what can be expressed with the last
r_pattern
consider the goal a = b and the tactic
It rewrites all occurrences of the left hand side of rule
inside b only (a, and the hidden type of the equality, are ignored). Note that the
variant rewrite [X in _ = X]rule would have rewritten b
exactly (i.e., it would only work if b and the left hand side
of rule can be unified).
Matching contextual patterns¶
The c_pattern and r_pattern involving terms
with holes are matched
against the goal in order to find a closed instantiation. This
matching proceeds as follows:
c_pattern |
instantiation order and place for term_i and redex |
|---|---|
term |
term is matched against the goal, redex is unified with
the instantiation of term |
ident in term |
term is matched against the goal, redex is unified with the
subterm of the instantiation of term identified by
ident |
term1 in ident in term2 |
term2 is matched against the goal, term1
is matched against the subterm of the instantiation of
term1 identified by ident, redex is unified with
the instantiation of term1 |
term1 as ident in term2 |
term2[term1/ident] is matched against
the goal, redex is unified with the instantiation of term1 |
In the following patterns, the redex is intended to be inferred from the rewrite rule.
r_pattern |
instantiation order and place for term_i and redex |
|---|---|
in ident in term |
term is matched against the goal, the redex is matched against
the subterm of the instantiation of term identified by
ident |
in term |
term is matched against the goal, redex is matched against the
instantiation of term |
Examples¶
Contextual pattern in set and the : tactical¶
As already mentioned in section Abbreviations the set
tactic takes as an
argument a term in open syntax. This term is interpreted as the
simplest form of c_pattern. To avoid confusion in the grammar, open
syntax is supported only for the simplest form of patterns, while
parentheses are required around more complex patterns.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Lemma test a b : a + b + 1 = b + (a + 1).
- 1 subgoal a, b : nat ============================ a + b + 1 = b + (a + 1)
- set t := (X in _ = X).
- 1 subgoal a, b : nat t := b + (a + 1) : nat ============================ a + b + 1 = t
- rewrite {}/t.
- 1 subgoal a, b : nat ============================ a + b + 1 = b + (a + 1)
- set t := (a + _ in X in _ = X).
- 1 subgoal a, b : nat t := a + 1 : nat ============================ a + b + 1 = b + t
Since the user may define an infix notation for in the result of the former
tactic may be ambiguous. The disambiguation rule implemented is to prefer
patterns over simple terms, but to interpret a pattern with double
parentheses as a simple term. For example, the following tactic would
capture any occurrence of the term a in A.
Contextual patterns can also be used as arguments of the : tactical.
For example:
Contextual patterns in rewrite¶
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Notation "n .+1" := (Datatypes.S n) (at level 2, left associativity, format "n .+1") : nat_scope.
- Axiom addSn : forall m n, m.+1 + n = (m + n).+1.
- addSn is declared
- Axiom addn0 : forall m, m + 0 = m.
- addn0 is declared
- Axiom addnC : forall m n, m + n = n + m.
- addnC is declared
- Lemma test x y z f : (x.+1 + y) + f (x.+1 + y) (z + (x + y).+1) = 0.
- 1 subgoal x, y, z : nat f : nat -> nat -> nat ============================ x.+1 + y + f (x.+1 + y) (z + (x + y).+1) = 0
- rewrite [in f _ _]addSn.
- 1 subgoal x, y, z : nat f : nat -> nat -> nat ============================ x.+1 + y + f (x + y).+1 (z + (x + y).+1) = 0
Note: the simplification rule addSn is applied only under the f
symbol.
Then we simplify also the first addition and expand 0 into 0+0.
- rewrite addSn -[X in _ = X]addn0.
- 1 subgoal x, y, z : nat f : nat -> nat -> nat ============================ (x + y).+1 + f (x + y).+1 (z + (x + y).+1) = 0 + 0
Note that the right hand side of addn0 is undetermined, but the
rewrite pattern specifies the redex explicitly. The right hand side
of addn0 is unified with the term identified by X, here 0.
The following pattern does not specify a redex, since it identifies an entire region, hence the rewrite rule has to be instantiated explicitly. Thus the tactic:
- rewrite -{2}[in X in _ = X](addn0 0).
- 1 subgoal x, y, z : nat f : nat -> nat -> nat ============================ (x + y).+1 + f (x + y).+1 (z + (x + y).+1) = 0 + (0 + 0)
The following tactic is quite tricky:
- rewrite [_.+1 in X in f _ X](addnC x.+1).
- 1 subgoal x, y, z : nat f : nat -> nat -> nat ============================ (x + y).+1 + f (x + y).+1 (z + (y + x.+1)) = 0 + (0 + 0)
The explicit redex _.+1 is important since its head constant S
differs from the head constant inferred from
(addnC x.+1) (that is +).
Moreover, the pattern f _ X is important to rule out
the first occurrence of (x + y).+1.
Last, only the subterms of f _ X
identified by X are rewritten, thus the first argument of
f is skipped too.
Also note the pattern _.+1 is interpreted in the context
identified by X, thus it gets instantiated to
(y + x).+1 and not (x + y).+1.
The last rewrite pattern allows to specify exactly the shape of the term identified by X, that is thus unified with the left hand side of the rewrite rule.
- rewrite [x.+1 + y as X in f X _]addnC.
- 1 subgoal x, y, z : nat f : nat -> nat -> nat ============================ (x + y).+1 + f (y + x.+1) (z + (y + x.+1)) = 0 + (0 + 0)
Patterns for recurrent contexts¶
The user can define shortcuts for recurrent contexts corresponding to
the ident in term part. The notation scope identified with
%pattern
provides a special notation (X in t) the user must adopt
in order to define
context shortcuts.
The following example is taken from ssreflect.v where the
LHS and RHS shortcuts are defined.
Shortcuts defined this way can be freely used in place of the trailing
ident in term part of any contextual pattern. Some examples follow:
Views and reflection¶
The bookkeeping facilities presented in section Basic tactics are crafted to ease simultaneous introductions and generalizations of facts and operations of casing, naming etc. It also a common practice to make a stack operation immediately followed by an interpretation of the fact being pushed, that is, to apply a lemma to this fact before passing it to a tactic for decomposition, application and so on.
SSReflect provides a convenient, unified syntax to combine these
interpretation operations with the proof stack operations. This view
mechanism relies on the combination of the / view switch with
bookkeeping tactics and tacticals.
Interpreting eliminations¶
The view syntax combined with the elim tactic specifies an elimination
scheme to be used instead of the default, generated, one. Hence the
SSReflect tactic:
is a synonym for:
where top is a fresh name and V any second-order lemma.
Since an elimination view supports the two bookkeeping tacticals of discharge and introduction (see section Basic tactics), the SSReflect tactic:
is a synonym for:
where x is a variable in the context, y a fresh name and V
any second order lemma; SSReflect relaxes the syntactic restrictions of the Coq
elim. The first pattern following : can be a _ wildcard if the
conclusion of the view V specifies a pattern for its last argument
(e.g., if V is a functional induction lemma generated by the
Function command).
The elimination view mechanism is compatible with the equation name generation (see section Generation of equations).
Example
The following script illustrates a toy example of this feature. Let us define a function adding an element at the end of a list:
- From Coq Require Import ssreflect List.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Variable d : Type.
- d is declared
- Fixpoint add_last (s : list d) (z : d) {struct s} : list d := if s is cons x s' then cons x (add_last s' z) else z :: nil.
- add_last is defined add_last is recursively defined (decreasing on 1st argument)
One can define an alternative, reversed, induction principle on inductively defined lists, by proving the following lemma:
- Axiom last_ind_list : forall P : list d -> Prop, P nil -> (forall s (x : d), P s -> P (add_last s x)) -> forall s : list d, P s.
- last_ind_list is declared
Then the combination of elimination views with equation names result in a concise syntax for reasoning inductively using the user-defined elimination scheme.
- Lemma test (x : d) (l : list d): l = l.
- 1 subgoal d : Type x : d l : list d ============================ l = l
- elim/last_ind_list E : l=> [| u v]; last first.
- 2 subgoals d : Type x : d u : list d v : d l : list d E : l = add_last u v ============================ u = u -> add_last u v = add_last u v subgoal 2 is: nil = nil
User-provided eliminators (potentially generated with Coq’s Function
command) can be combined with the type family switches described
in section Type families.
Consider an eliminator foo_ind of type:
and consider the tactic:
The elim/ tactic distinguishes two cases:
| truncated eliminator: | |
|---|---|
when x does not occur in P p1 … pm and the
type of en unifies with T and en is not _.
In that case, en is
passed to the eliminator as the last argument (x in foo_ind) and
en−1 … e1 are used as patterns to select in the goal the occurrences that
will be bound by the predicate P, thus it must be possible to unify
the sub-term of the goal matched by en−1 with pm , the one matched
by en−2 with pm−1 and so on. |
|
| regular eliminator: | |
in all the other cases. Here it must be possible
to unify the term matched by en with pm , the one matched by
en−1
with pm−1 and so on. Note that standard eliminators have the shape
…forall x, P … x, thus en is the pattern identifying the
eliminated term, as expected. |
|
As explained in section Type families, the initial prefix of
ei can be omitted.
Here is an example of a regular, but nontrivial, eliminator.
Example
Here is a toy example illustrating this feature.
- From Coq Require Import ssreflect FunInd.
- [Loading ML file extraction_plugin.cmxs ... done] [Loading ML file recdef_plugin.cmxs ... done]
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Function plus (m n : nat) {struct n} : nat := if n is S p then S (plus m p) else m.
- plus is defined plus is recursively defined (decreasing on 2nd argument) plus_equation is defined plus_ind is defined plus_rec is defined plus_rect is defined R_plus_correct is defined R_plus_complete is defined
- About plus_ind.
- plus_ind : forall (m : nat) (P : nat -> nat -> Prop), (forall n p : nat, n = S p -> P p (plus m p) -> P (S p) (S (plus m p))) -> (forall n _x : nat, n = _x -> match _x with | 0 => True | S _ => False end -> P _x m) -> forall n : nat, P n (plus m n) Arguments m, P are implicit Argument scopes are [nat_scope function_scope function_scope function_scope nat_scope] plus_ind is transparent Expands to: Constant Top.Test.plus_ind
- Lemma test x y z : plus (plus x y) z = plus x (plus y z).
- 1 subgoal x, y, z : nat ============================ plus (plus x y) z = plus x (plus y z)
The following tactics are all valid and perform the same elimination on this goal.
- From Coq Require Import ssreflect FunInd.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Function plus (m n : nat) {struct n} : nat := if n is S p then S (plus m p) else m.
- plus is defined plus is recursively defined (decreasing on 2nd argument) plus_equation is defined plus_ind is defined plus_rec is defined plus_rect is defined R_plus_correct is defined R_plus_complete is defined
- About plus_ind.
- plus_ind : forall (m : nat) (P : nat -> nat -> Prop), (forall n p : nat, n = S p -> P p (plus m p) -> P (S p) (S (plus m p))) -> (forall n _x : nat, n = _x -> match _x with | 0 => True | S _ => False end -> P _x m) -> forall n : nat, P n (plus m n) Arguments m, P are implicit Argument scopes are [nat_scope function_scope function_scope function_scope nat_scope] plus_ind is transparent Expands to: Constant Top.Test.plus_ind
- Lemma test x y z : plus (plus x y) z = plus x (plus y z).
- 1 subgoal x, y, z : nat ============================ plus (plus x y) z = plus x (plus y z)
- elim/plus_ind: z / _.
- 2 subgoals x, y : nat ============================ forall n p : nat, n = S p -> plus (plus x y) p = plus x (plus y p) -> S (plus (plus x y) p) = plus x (plus y (S p)) subgoal 2 is: forall n _x : nat, n = _x -> match _x with | 0 => True | S _ => False end -> plus x y = plus x (plus y _x)
The two latter examples feature a wildcard pattern: in this case,
the resulting pattern is inferred from the type of the eliminator.
In both these examples, it is (plus _ _), which matches the subterm
plus (plus x y) z thus instantiating the last _ with z.
Note that the tactic:
- From Coq Require Import ssreflect FunInd.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Function plus (m n : nat) {struct n} : nat := if n is S p then S (plus m p) else m.
- plus is defined plus is recursively defined (decreasing on 2nd argument) plus_equation is defined plus_ind is defined plus_rec is defined plus_rect is defined R_plus_correct is defined R_plus_complete is defined
- About plus_ind.
- plus_ind : forall (m : nat) (P : nat -> nat -> Prop), (forall n p : nat, n = S p -> P p (plus m p) -> P (S p) (S (plus m p))) -> (forall n _x : nat, n = _x -> match _x with | 0 => True | S _ => False end -> P _x m) -> forall n : nat, P n (plus m n) Arguments m, P are implicit Argument scopes are [nat_scope function_scope function_scope function_scope nat_scope] plus_ind is transparent Expands to: Constant Top.Test.plus_ind
- Lemma test x y z : plus (plus x y) z = plus x (plus y z).
- 1 subgoal x, y, z : nat ============================ plus (plus x y) z = plus x (plus y z)
- Fail elim/plus_ind: y / _.
- The command has indeed failed with message: Ltac call to "elim (ssrarg) (ssrclauses)" failed. The given pattern matches the term y while the inferred pattern z doesn't
triggers an error: in the conclusion
of the plus_ind eliminator, the first argument of the predicate
P should be the same as the second argument of plus, in the
second argument of P, but y and z do no unify.
Here is an example of a truncated eliminator:
Example
Consider the goal:
- From Coq Require Import ssreflect FunInd.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
where the type of the big_prop eliminator is
Since the pattern for the argument of Pb is not specified, the
inferred one is used instead: big[_/_]_(i <- _ | _ i) _ i,
and after the introductions, the following goals are generated:
Note that the pattern matching algorithm instantiated all the variables occurring in the pattern.
Interpreting assumptions¶
Interpreting an assumption in the context of a proof consists in
applying to it a lemma before generalizing, and/or decomposing this
assumption. For instance, with the extensive use of boolean reflection
(see section Views and reflection.4), it is quite frequent
to need to decompose the logical interpretation of (the boolean
expression of) a fact, rather than the fact itself. This can be
achieved by a combination of move : _ => _ switches, like in the
following example, where || is a notation for the boolean
disjunction.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Variables P Q : bool -> Prop.
- P is declared Q is declared
- Hypothesis P2Q : forall a b, P (a || b) -> Q a.
- P2Q is declared
- Lemma test a : P (a || a) -> True.
- 1 subgoal P, Q : bool -> Prop P2Q : forall a b : bool, P (a || b) -> Q a a : bool ============================ P (a || a) -> True
- move=> HPa; move: {HPa}(P2Q HPa) => HQa.
- 1 subgoal P, Q : bool -> Prop P2Q : forall a b : bool, P (a || b) -> Q a a : bool HQa : Q a ============================ True
which transforms the hypothesis HPa : P a which has been introduced
from the initial statement into HQa : Q a.
This operation is so common that the tactic shell has specific
syntax for it. The following scripts:
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Variables P Q : bool -> Prop.
- P is declared Q is declared
- Hypothesis P2Q : forall a b, P (a || b) -> Q a.
- P2Q is declared
- Lemma test a : P (a || a) -> True.
- 1 subgoal P, Q : bool -> Prop P2Q : forall a b : bool, P (a || b) -> Q a a : bool ============================ P (a || a) -> True
- move=> HPa; move/P2Q: HPa => HQa.
- 1 subgoal P, Q : bool -> Prop P2Q : forall a b : bool, P (a || b) -> Q a a : bool HQa : Q a ============================ True
or more directly:
- Abort.
- Lemma test a : P (a || a) -> True.
- 1 subgoal P, Q : bool -> Prop P2Q : forall a b : bool, P (a || b) -> Q a a : bool ============================ P (a || a) -> True
- move/P2Q=> HQa.
- 1 subgoal P, Q : bool -> Prop P2Q : forall a b : bool, P (a || b) -> Q a a : bool HQa : Q a ============================ True
are equivalent to the former one. The former script shows how to interpret a fact (already in the context), thanks to the discharge tactical (see section Discharge) and the latter, how to interpret the top assumption of a goal. Note that the number of wildcards to be inserted to find the correct application of the view lemma to the hypothesis has been automatically inferred.
The view mechanism is compatible with the case tactic and with the
equation name generation mechanism (see section Generation of equations):
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Variables P Q: bool -> Prop.
- P is declared Q is declared
- Hypothesis Q2P : forall a b, Q (a || b) -> P a \/ P b.
- Q2P is declared
- Lemma test a b : Q (a || b) -> True.
- 1 subgoal P, Q : bool -> Prop Q2P : forall a b : bool, Q (a || b) -> P a \/ P b a, b : bool ============================ Q (a || b) -> True
- case/Q2P=> [HPa | HPb].
- 2 subgoals P, Q : bool -> Prop Q2P : forall a b : bool, Q (a || b) -> P a \/ P b a, b : bool HPa : P a ============================ True subgoal 2 is: True
This view tactic performs:
The term on the right of the / view switch is called a view lemma.
Any SSReflect term coercing to a product type can be used as a view
lemma.
The examples we have given so far explicitly provide the direction of the translation to be performed. In fact, view lemmas need not to be oriented. The view mechanism is able to detect which application is relevant for the current goal.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Variables P Q: bool -> Prop.
- P is declared Q is declared
- Hypothesis PQequiv : forall a b, P (a || b) <-> Q a.
- PQequiv is declared
- Lemma test a b : P (a || b) -> True.
- 1 subgoal P, Q : bool -> Prop PQequiv : forall a b : bool, P (a || b) <-> Q a a, b : bool ============================ P (a || b) -> True
- move/PQequiv=> HQab.
- 1 subgoal P, Q : bool -> Prop PQequiv : forall a b : bool, P (a || b) <-> Q a a, b : bool HQab : Q a ============================ True
has the same behavior as the first example above.
The view mechanism can insert automatically a view hint to transform the double implication into the expected simple implication. The last script is in fact equivalent to:
where:
Specializing assumptions¶
The special case when the head symbol of the view lemma is a wildcard is used to interpret an assumption by specializing it. The view mechanism hence offers the possibility to apply a higher-order assumption to some given arguments.
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Lemma test z : (forall x y, x + y = z -> z = x) -> z = 0.
- 1 subgoal z : nat ============================ (forall x y : nat, x + y = z -> z = x) -> z = 0
- move/(_ 0 z).
- 1 subgoal z : nat ============================ (0 + z = z -> z = 0) -> z = 0
Interpreting goals¶
In a similar way, it is also often convenient to
changing a goal by turning it into an equivalent proposition. The view
mechanism of SSReflect has a special syntax apply/ for combining in a
single tactic simultaneous goal interpretation operations and
bookkeeping steps.
Example
The following example use the~~prenex notation for boolean negation:
- From Coq Require Import ssreflect ssrbool.
- Overwriting previous delimiting key bool in scope bool_scope Ambiguous paths: [pred_of_mem_pred; sort_of_simpl_pred] : mem_pred >-> pred_sort
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Variables P Q: bool -> Prop.
- P is declared Q is declared
- Hypothesis PQequiv : forall a b, P (a || b) <-> Q a.
- PQequiv is declared
- Lemma test a : P ((~~ a) || a).
- 1 subgoal P, Q : bool -> Prop PQequiv : forall a b : bool, P (a || b) <-> Q a a : bool ============================ P (~~ a || a)
- apply/PQequiv.
- 1 focused subgoal (shelved: 1) P, Q : bool -> Prop PQequiv : forall a b : bool, P (a || b) <-> Q a a : bool ============================ Q (~~ a)
thus in this case, the tactic apply/PQequiv is equivalent to
apply: (iffRL (PQequiv _ _)), where iffRL is tha analogue of
iffRL for the converse implication.
Any SSReflect term whose type coerces to a double implication can be used as a view for goal interpretation.
Note that the goal interpretation view mechanism supports both apply
and exact tactics. As expected, a goal interpretation view command
exact/term should solve the current goal or it will fail.
Warning
Goal interpretation view tactics are not compatible with
the bookkeeping tactical => since this would be redundant with the
apply: term => _ construction.
Boolean reflection¶
In the Calculus of Inductive Constructions, there is an obvious
distinction between logical propositions and boolean values. On the
one hand, logical propositions are objects of sort Prop which is
the carrier of intuitionistic reasoning. Logical connectives in
Prop are types, which give precise information on the structure
of their proofs; this information is automatically exploited by Coq
tactics. For example, Coq knows that a proof of A \/ B is
either a proof of A or a proof of B. The tactics left and
right change the goal A \/ B to A and B, respectively;
dually, the tactic case reduces the goal A \/ B => G to two
subgoals A => G and B => G.
On the other hand, bool is an inductive datatype with two constructors true and false. Logical connectives on bool are computable functions, defined by their truth tables, using case analysis:
Example
- From Coq Require Import ssreflect.
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Definition orb (b1 b2 : bool) := if b1 then true else b2.
- orb is defined
Properties of such connectives are also established using case analysis
Example
- From Coq Require Import ssreflect ssrbool.
- Overwriting previous delimiting key bool in scope bool_scope Ambiguous paths: [pred_of_mem_pred; sort_of_simpl_pred] : mem_pred >-> pred_sort
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Lemma test b : b || ~~ b = true.
- 1 subgoal b : bool ============================ b || ~~ b = true
- by case: b.
- No more subgoals.
Once b is replaced by true in the first goal and by false in the
second one, the goals reduce by computations to the trivial true = true.
Thus, Prop and bool are truly complementary: the former supports
robust natural deduction, the latter allows brute-force
evaluation. SSReflect supplies a generic mechanism to have the best of
the two worlds and move freely from a propositional version of a
decidable predicate to its boolean version.
First, booleans are injected into propositions using the coercion mechanism:
This allows any boolean formula b to be used in a context where Coq
would expect a proposition, e.g., after Lemma … : ``. It is then
interpreted as ``(is_true b), i.e., the proposition b = true. Coercions
are elided by the pretty-printer, so they are essentially transparent
to the user.
The reflect predicate¶
To get all the benefits of the boolean reflection, it is in fact
convenient to introduce the following inductive predicate reflect to
relate propositions and booleans:
The statement (reflect P b) asserts that (is_true b) and P are
logically equivalent propositions.
For instance, the following lemma:
relates the boolean conjunction to the logical one /\. Note that in
andP, b1 and b2 are two boolean variables and the
proposition b1 /\ b2 hides two coercions. The conjunction of
b1 and b2 can then be viewed as b1 /\ b2 or as b1 && b2.
Expressing logical equivalences through this family of inductive types makes possible to take benefit from rewritable equations associated to the case analysis of Coq’s inductive types.
Since the equivalence predicate is defined in Coq as:
where /\ is a notation for and:
This make case analysis very different according to the way an equivalence property has been defined.
Let us compare the respective behaviors of andE and andP.
Example
- From Coq Require Import ssreflect ssrbool.
- Overwriting previous delimiting key bool in scope bool_scope Ambiguous paths: [pred_of_mem_pred; sort_of_simpl_pred] : mem_pred >-> pred_sort
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Axiom andE : forall (b1 b2 : bool), (b1 /\ b2) <-> (b1 && b2).
- andE is declared
- Lemma test (b1 b2 : bool) : if (b1 && b2) then b1 else ~~(b1||b2).
- 1 subgoal b1, b2 : bool ============================ if b1 && b2 then b1 else ~~ (b1 || b2)
- case: (@andE b1 b2).
- 1 subgoal b1, b2 : bool ============================ (b1 /\ b2 -> b1 && b2) -> (b1 && b2 -> b1 /\ b2) -> if b1 && b2 then b1 else ~~ (b1 || b2)
- Lemma test (b1 b2 : bool) : if (b1 && b2) then b1 else ~~(b1||b2).
- Toplevel input, characters 0-66: > Lemma test (b1 b2 : bool) : if (b1 && b2) then b1 else ~~(b1||b2). > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Error: Nested proofs are not allowed unless you turn option Nested Proofs Allowed on.
- case: (@andP b1 b2).
- 2 subgoals b1, b2 : bool ============================ b1 /\ b2 -> (b1 /\ b2 -> true) -> (true -> b1 /\ b2) -> b1 subgoal 2 is: ~ (b1 /\ b2) -> (b1 /\ b2 -> false) -> (false -> b1 /\ b2) -> ~~ (b1 || b2)
Expressing reflection relation through the reflect predicate is hence
a very convenient way to deal with classical reasoning, by case
analysis. Using the reflect predicate allows moreover to program rich
specifications inside its two constructors, which will be
automatically taken into account during destruction. This
formalisation style gives far more efficient specifications than
quantified (double) implications.
A naming convention in SSReflect is to postfix the name of view lemmas
with P. For example, orP relates || and \/,
negP relates ~~ and ~.
The view mechanism is compatible with reflect predicates.
Example
- From Coq Require Import ssreflect ssrbool.
- Overwriting previous delimiting key bool in scope bool_scope Ambiguous paths: [pred_of_mem_pred; sort_of_simpl_pred] : mem_pred >-> pred_sort
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Lemma test (a b : bool) (Ha : a) (Hb : b) : a /\ b.
- 1 subgoal a, b : bool Ha : a Hb : b ============================ a /\ b
- apply/andP.
- 1 focused subgoal (shelved: 1) a, b : bool Ha : a Hb : b ============================ a && b
Conversely
- Abort.
- Lemma test (a b : bool) : a /\ b -> a.
- 1 subgoal a, b : bool ============================ a /\ b -> a
- move/andP.
- 1 subgoal a, b : bool ============================ a && b -> a
The same tactics can also be used to perform the converse operation, changing a boolean conjunction into a logical one. The view mechanism guesses the direction of the transformation to be used i.e., the constructor of the reflect predicate which should be chosen.
General mechanism for interpreting goals and assumptions¶
Specializing assumptions¶
The SSReflect tactic:
is equivalent to the tactic:
where top is a fresh name for introducing the top assumption of the
current goal.
Interpreting assumptions¶
The general form of an assumption view tactic is:
-
Variant
[move | case] / term
The term , called the view lemma can be:
- a (term coercible to a) function;
- a (possibly quantified) implication;
- a (possibly quantified) double implication;
- a (possibly quantified) instance of the reflect predicate (see section Views and reflection).
Let top be the top assumption in the goal.
There are three steps in the behavior of an assumption view tactic:
- It first introduces
top. - If the type of
termis neither a double implication nor an instance of the reflect predicate, then the tactic automatically generalises a term of the form:term term1 … termnwhere the termsterm1 … termninstantiate the possible quantified variables ofterm, in order for(term term1 … termn top)to be well typed. - If the type of
termis an equivalence, or an instance of the reflect predicate, it generalises a term of the form:(termvh (term term1 … termn ))where the termtermvhinserted is called an assumption interpretation view hint. - It finally clears top.
For a case/term tactic, the generalisation step is replaced by a
case analysis step.
View hints are declared by the user (see section:ref:views_and_reflection_ssr.8) and are
stored in the Hint View database. The proof engine automatically
detects from the shape of the top assumption top and of the view lemma
term provided to the tactic the appropriate view hint in the
database to be inserted.
If term is a double implication, then the view hint will be one of
the defined view hints for implication. These hints are by default the
ones present in the file ssreflect.v:
which transforms a double implication into the left-to-right one, or:
which produces the converse implication. In both cases, the two first Prop arguments are implicit.
If term is an instance of the reflect predicate, then A will be one
of the defined view hints for the reflect predicate, which are by
default the ones present in the file ssrbool.v. These hints are not
only used for choosing the appropriate direction of the translation,
but they also allow complex transformation, involving negations.
Example
- From Coq Require Import ssreflect ssrbool.
- Overwriting previous delimiting key bool in scope bool_scope Ambiguous paths: [pred_of_mem_pred; sort_of_simpl_pred] : mem_pred >-> pred_sort
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Check introN.
- introN : forall (P : Prop) (b : bool), reflect P b -> ~ P -> ~~ b
- Lemma test (a b : bool) (Ha : a) (Hb : b) : ~~ (a && b).
- 1 subgoal a, b : bool Ha : a Hb : b ============================ ~~ (a && b)
- apply/andP.
- 1 focused subgoal (shelved: 1) a, b : bool Ha : a Hb : b ============================ ~ (a /\ b)
In fact this last script does not
exactly use the hint introN, but the more general hint:
- Check introNTF.
- introNTF : forall (P : Prop) (b c : bool), reflect P b -> (if c then ~ P else P) -> ~~ b = c
The lemma introN is an instantiation of introNF using c := true.
Note that views, being part of i_pattern, can be used to interpret
assertions too. For example the following script asserts a && b but
actually uses its propositional interpretation.
Example
- From Coq Require Import ssreflect ssrbool.
- Overwriting previous delimiting key bool in scope bool_scope Ambiguous paths: [pred_of_mem_pred; sort_of_simpl_pred] : mem_pred >-> pred_sort
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Lemma test (a b : bool) (pab : b && a) : b.
- 1 subgoal a, b : bool pab : b && a ============================ b
- have /andP [pa ->] : (a && b) by rewrite andbC.
- 1 subgoal a, b : bool pab : b && a pa : a ============================ true
Interpreting goals
A goal interpretation view tactic of the form:
-
Variant
apply/term
applied to a goal top is interpreted in the following way:
- If the type of
termis not an instance of thereflectpredicate, nor an equivalence, then the termtermis applied to the current goaltop, possibly inserting implicit arguments. - If the type of
termis an instance of the reflect predicate or an equivalence, then a goal interpretation view hint can possibly be inserted, which corresponds to the application of a term(termvh (term _ … _))to the current goal, possibly inserting implicit arguments.
Like assumption interpretation view hints, goal interpretation ones
are user-defined lemmas stored (see section Views and reflection) in the Hint View
database bridging the possible gap between the type of term and the
type of the goal.
Interpreting equivalences¶
Equivalent boolean propositions are simply equal boolean terms. A special construction helps the user to prove boolean equalities by considering them as logical double implications (between their coerced versions), while performing at the same time logical operations on both sides.
The syntax of double views is:
The first term is the view lemma applied to the left hand side of the equality, while the second term is the one applied to the right hand side.
In this context, the identity view can be used when no view has to be applied:
Example
- From Coq Require Import ssreflect ssrbool.
- Overwriting previous delimiting key bool in scope bool_scope Ambiguous paths: [pred_of_mem_pred; sort_of_simpl_pred] : mem_pred >-> pred_sort
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Lemma test (b1 b2 b3 : bool) : ~~ (b1 || b2) = b3.
- 1 subgoal b1, b2, b3 : bool ============================ ~~ (b1 || b2) = b3
- apply/idP/idP.
- 2 focused subgoals (shelved: 2) b1, b2, b3 : bool ============================ ~~ (b1 || b2) -> b3 subgoal 2 is: b3 -> ~~ (b1 || b2)
The same goal can be decomposed in several ways, and the user may choose the most convenient interpretation.
- From Coq Require Import ssreflect ssrbool.
- Overwriting previous delimiting key bool in scope bool_scope Ambiguous paths: [pred_of_mem_pred; sort_of_simpl_pred] : mem_pred >-> pred_sort
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Lemma test (b1 b2 b3 : bool) : ~~ (b1 || b2) = b3.
- 1 subgoal b1, b2, b3 : bool ============================ ~~ (b1 || b2) = b3
- apply/norP/idP.
- 2 focused subgoals (shelved: 2) b1, b2, b3 : bool ============================ ~~ b1 /\ ~~ b2 -> b3 subgoal 2 is: b3 -> ~~ b1 /\ ~~ b2
Declaring new Hint Views¶
The database of hints for the view mechanism is extensible via a
dedicated vernacular command. As library ssrbool.v already declares a
corpus of hints, this feature is probably useful only for users who
define their own logical connectives. Users can declare their own
hints following the syntax used in ssrbool.v:
The ident is the name of the lemma to be
declared as a hint. If move is used as
tactic, the hint is declared for assumption interpretation tactics,
apply declares hints for goal interpretations. Goal interpretation
view hints are declared for both simple views and left hand side
views. The optional natural number is the number of implicit
arguments to be considered for the declared hint view lemma.
The command:
with a double slash //, declares hint views for right hand sides of
double views.
See the files ssreflect.v and ssrbool.v for examples.
Multiple views¶
The hypotheses and the goal can be interpreted by applying multiple views
in sequence. Both move and apply can be followed by an arbitrary
number of /term. The main difference between the following two
tactics
is that the former applies all the views to the principal goal.
Applying a view with hypotheses generates new goals, and the second
line would apply the view v2 to all the goals generated by apply/v1.
Note that the NO-OP intro pattern - can be used to separate two views,
making the two following examples equivalent:
The tactic move can be used together with the in tactical to
pass a given hypothesis to a lemma.
Example
- From Coq Require Import ssreflect ssrbool.
- Overwriting previous delimiting key bool in scope bool_scope Ambiguous paths: [pred_of_mem_pred; sort_of_simpl_pred] : mem_pred >-> pred_sort
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Section Test.
- Variables P Q R : Prop.
- P is declared Q is declared R is declared
- Variable P2Q : P -> Q.
- P2Q is declared
- Variable Q2R : Q -> R.
- Q2R is declared
- Lemma test (p : P) : True.
- 1 subgoal P, Q, R : Prop P2Q : P -> Q Q2R : Q -> R p : P ============================ True
- move/P2Q/Q2R in p.
- 1 subgoal P, Q, R : Prop P2Q : P -> Q Q2R : Q -> R p : R ============================ True
If the list of views is of length two, Hint Views for interpreting
equivalences are indeed taken into account, otherwise only single
Hint Views are used.
SSReflect searching tool¶
SSReflect proposes an extension of the Search command. Its syntax is:
where qualid is the name of an open module. This command returns
the list of lemmas:
- whose conclusion contains a subterm matching the optional first pattern. A - reverses the test, producing the list of lemmas whose conclusion does not contain any subterm matching the pattern;
- whose name contains the given string. A
-prefix reverses the test, producing the list of lemmas whose name does not contain the string. A string that contains symbols or is followed by a scope key, is interpreted as the constant whose notation involves that string (e.g.,+foraddn), if this is unambiguous; otherwise the diagnostic includes the output of theLocatevernacular command. - whose statement, including assumptions and types, contains a subterm
matching the next patterns. If a pattern is prefixed by
-, the test is reversed; - contained in the given list of modules, except the ones in the
modules prefixed by a
-.
Note that:
As for regular terms, patterns can feature scope indications. For instance, the command:
Search _ (_ + _)%N.lists all the lemmas whose statement (conclusion or hypotheses) involves an application of the binary operation denoted by the infix+symbol in theNscope (which is SSReflect scope for natural numbers).Patterns with holes should be surrounded by parentheses.
Search always volunteers the expansion of the notation, avoiding the need to execute Locate independently. Moreover, a string fragment looks for any notation that contains fragment as a substring. If the
ssrbool.vlibrary is imported, the command:Search "~~".answers :Example
- From Coq Require Import ssreflect ssrbool.
- Overwriting previous delimiting key bool in scope bool_scope Ambiguous paths: [pred_of_mem_pred; sort_of_simpl_pred] : mem_pred >-> pred_sort
- Set Implicit Arguments.
- Unset Strict Implicit.
- Unset Printing Implicit Defensive.
- Search "~~".
- "~~" is part of notation ("~~ _") In bool_scope, ("~~ b") denotes negb b Toplevel input, characters 0-12: > Search "~~". > ^^^^^^^^^^^^ Warning: Listing only lemmas with conclusion matching (~~ ?b) negbT forall b : bool, b = false -> ~~ b contra forall c b : bool, (c -> b) -> ~~ b -> ~~ c contraNN forall c b : bool, (c -> b) -> ~~ b -> ~~ c contraL forall c b : bool, (c -> ~~ b) -> b -> ~~ c contraTN forall c b : bool, (c -> ~~ b) -> b -> ~~ c contraFN forall c b : bool, (c -> b) -> b = false -> ~~ c introN forall (P : Prop) (b : bool), reflect P b -> ~ P -> ~~ b
A diagnostic is issued if there are different matching notations; it is an error if all matches are partial.
Similarly, a diagnostic warns about multiple interpretations, and signals an error if there is no default one.
The command
Search in M.is a way of obtaining the complete signature of the moduleM.Strings and pattern indications can be interleaved, but the first indication has a special status if it is a pattern, and only filters the conclusion of lemmas:
- The command :
Search (_ =1 _) "bij".lists all the lemmas whose conclusion features a=1and whose name contains the stringbij. - The command :
Search "bij" (_ =1 _).lists all the lemmas whose statement, including hypotheses, features a=1and whose name contains the stringbij.
- The command :
Synopsis and Index¶
Parameters¶
SSReflect tactics
-
d_tactic ::=elim case congr apply exact move
Notation scope
Module name
Natural number
where ident is an Ltac variable denoting a standard Coq numeral
(should not be the name of a tactic which can be followed by a
bracket [, like do, have,…)
Items and switches¶
binder see Abbreviations.
-
clear_switch ::={ ident+ }
clear switch see Discharge
context pattern see Contextual patterns
-
d_item ::=occ_switch clear_switch? term ( c_pattern )?
discharge item see Discharge
generalization item see Structure
-
i_pattern ::=ident _ ? * occ_switch? -> occ_switch? <- [ i_item**| - [: ident+ ]
intro pattern Introduction in the context
-
i_item ::=clear_switch s_item i_pattern {}? / term
intro item see Introduction in the context
multiplier see Iteration
-
occ_switch ::={ + -? num* }
occur. switch see Occurrence selection
multiplier see Iteration
-
mult_mark ::=? !
multiplier mark see Iteration
rewrite item see Rewriting
-
r_prefix ::=-? int_mult? occ_switch clear_switch? [ r_pattern ]?
rewrite prefix see Rewriting
rewrite pattern see Rewriting
rewrite step see Rewriting
-
s_item ::=/= // //=
simplify switch see Introduction in the context
Tactics¶
Note: without loss and suffices are synonyms for wlog and suff
respectively.
-
move
idtac or hnf see Bookkeeping
-
apply
-
exact
application see The defective tactics
-
abstract see The abstract tactic and Generating let in context entries with have
-
elim
induction see The defective tactics
-
case
case analysis see The defective tactics
-
rewrite r_step+
rewrite see Rewriting
-
Variant
have i_item* i_pattern? s_item ssr_binder+? : term by tactic?
-
have suff clear_switch? i_pattern? : term? := term
-
Variant
have suff clear_switch? i_pattern? : term by tactic?
forward chaining see Structure
-
wlog suff? i_item? : gen_item clear_switch* / term
specializing see Structure
-
suff i_item* i_pattern? ssr_binder+ : term by tactic?¶
-
Variant
suffices i_item* i_pattern? ssr_binder+ : term by tactic?¶
-
Variant
suff have? clear_switch? i_pattern? : term by tactic?
-
Variant
suffices have? clear_switch? i_pattern? : term by tactic?
backchaining see Structure
local definition Definitions
-
Variant
pose ident ssr_binder+ := term
local function definition
-
Variant
pose fix fix_body
local fix definition
-
Variant
pose cofix fix_body
local cofix definition
-
set ident : term? := occ_switch? term ( c_pattern)
abbreviation see Abbreviations
unlock see Locking, unlocking
congruence Congruence
Tacticals¶
-
tactic +=d_tactic ident? : d_item+ clear_switch?
discharge Discharge
-
tactic +=tactic => i_item+
introduction see Introduction in the context
-
tactic +=tactic in gen_item clear_switch+ *?
localization see Localization
-
tactic +=do mult? tactic [ tactic+| ]
iteration see Iteration
-
tactic +=tactic ; first last num? tactic [ tactic+| ]
selector see Selectors
-
tactic +=tactic ; first last num?
rotation see Selectors
-
tactic +=by tactic [ tactic*| ]
closing see Terminators
Commands¶
view hint declaration see Declaring new Hint Views
right hand side double , view hint declaration see Declaring new Hint Views
-
Command
Prenex Implicits ident+
prenex implicits declaration see Parametric polymorphism
Footnotes
| [10] | Unfortunately, even after a call to the Set Printing All command, some occurrences are still not displayed to the user, essentially the ones possibly hidden in the predicate of a dependent match structure. |
| [11] | Thus scripts that depend on bound variable names, e.g., via intros or with, are inherently fragile. |
| [12] | The name subnK reads as “right cancellation rule for nat
subtraction”. |
| [13] | Also, a slightly different variant may be used for the first d_item
of case and elim; see section Type families. |
| [14] | Except /= does not expand the local definitions created by the SSReflect in tactical. |
| [15] | SSReflect reserves all identifiers of the form “_x_”, which is used for such generated names. |
| [16] | More precisely, it should have a quantified inductive type with a assumptions and m − a constructors. |
| [17] | This is an implementation feature: there is no such obstruction in the metatheory |
| [18] | The current state of the proof shall be displayed by the Show Proof command of Coq proof mode. |