Ltac2¶
The L
_{tac} tactic language is probably one of the ingredients of the success of
Coq, yet it is at the same time its Achilles' heel. Indeed, L
_{tac}:
has often unclear semantics
is very nonuniform due to organic growth
lacks expressivity (data structures, combinators, types, ...)
is slow
is errorprone and fragile
has an intricate implementation
Following the need of users who are developing huge projects relying critically on Ltac, we believe that we should offer a proper modern language that features at least the following:
at least informal, predictable semantics
a type system
standard programming facilities (e.g., datatypes)
This new language, called Ltac2, is described in this chapter. It is still experimental but we nonetheless encourage users to start testing it, especially wherever an advanced tactic language is needed. The previous implementation of Ltac, described in the previous chapter, will be referred to as Ltac1.
Current limitations include:
There are a number of tactics that are not yet supported in Ltac2 because the interface OCaml and/or Ltac2 notations haven't been written. See Defining tactics.
Missing usability features such as:
Printing functions are limited and awkward to use. Only a few data types are printable.
Deep pattern matching and matching on tuples don't work.
A convenient way to build terms with casts through the lowlevel API. Because the cast type is opaque, building terms with casts currently requires an awkward construction like the following, which also incurs extra overhead to repeat typechecking for each call to
get_vm_cast
:Constr.Unsafe.make (Constr.Unsafe.Cast 'I (get_vm_cast ()) 'True)with:
 From Ltac2 Require Import Ltac2.
 [Loading ML file ltac2_plugin.cmxs ... done]
 Ltac2 get_vm_cast () := match Constr.Unsafe.kind '(I <: True) with  Constr.Unsafe.Cast _ cst _ => cst  _ => Control.throw Not_found end.
Missing lowlevel primitives that are convenient for writing automation, such as:
An easy way to get the number of constructors of an inductive type. Currently only way to do this is to destruct a variable of the inductive type and count the number of goals that result.
The
deprecated
attribute is not supported for Ltac2 definitions.Error messages may be cryptic.
General design¶
There are various alternatives to Ltac1, such as Mtac or Rtac for instance. While those alternatives can be quite different from Ltac1, we designed Ltac2 to be as close as reasonably possible to Ltac1, while fixing the aforementioned defects.
In particular, Ltac2 is:
a member of the ML family of languages, i.e.
a callbyvalue functional language
with effects
together with the HindleyMilner type system
a language featuring metaprogramming facilities for the manipulation of Coqside terms
a language featuring notation facilities to help write palatable scripts
We describe these in more detail in the remainder of this document.
ML component¶
Overview¶
Ltac2 is a member of the ML family of languages, in the sense that it is an effectful callbyvalue functional language, with static typing à la HindleyMilner (see [DM82]). It is commonly accepted that ML constitutes a sweet spot in PL design, as it is relatively expressive while not being either too lax (unlike dynamic typing) nor too strict (unlike, say, dependent types).
The main goal of Ltac2 is to serve as a metalanguage for Coq. As such, it naturally fits in the ML lineage, just as the historical ML was designed as the tactic language for the LCF prover. It can also be seen as a generalpurpose language, by simply forgetting about the Coqspecific features.
Sticking to a standard ML type system can be considered somewhat weak for a metalanguage designed to manipulate Coq terms. In particular, there is no way to statically guarantee that a Coq term resulting from an Ltac2 computation will be welltyped. This is actually a design choice, motivated by backward compatibility with Ltac1. Instead, welltypedness is deferred to dynamic checks, allowing many primitive functions to fail whenever they are provided with an illtyped term.
The language is naturally effectful as it manipulates the global state of the proof engine. This allows to think of proofmodifying primitives as effects in a straightforward way. Semantically, proof manipulation lives in a monad, which allows to ensure that Ltac2 satisfies the same equations as a generic ML with unspecified effects would do, e.g. function reduction is substitution by a value.
Use the following command to import Ltac2:
 From Ltac2 Require Import Ltac2.
Type Syntax¶
At the level of terms, we simply elaborate on Ltac1 syntax, which is quite close to OCaml. Types follow the simplytyped syntax of OCaml.
The set of base types can be extended thanks to the usual ML type declarations such as algebraic datatypes and records.
Builtin types include:
int
, machine integers (size not specified, in practice inherited from OCaml)string
, mutable strings'a array
, mutable arraysexn
, exceptionsconstr
, kernelside termspattern
, term patternsident
, wellformed identifiers
Type declarations¶
One can define new types with the following commands.

Command
Ltac2 Type rec? tac2typ_def with tac2typ_def*
¶  [ .. ]
:=
Defines a type with with an explicit set of constructors
::=
Extends an existing open variant type, a special kind of variant type whose constructors are not statically defined, but can instead be extended dynamically. A typical example is the standard
exn
type for exceptions. Pattern matching on open variants must always include a catchall clause. They can be extended with this form, in which casetac2typ_knd
should be in the form[ ? tac2alg_constructor+? ]
. Without
:=::=
Defines an abstract type for use representing data from OCaml. Not for end users.
with tac2typ_def
Permits definition of mutually recursive type definitions.
Each production of
tac2typ_knd
defines one of four possible kinds of definitions, respectively: alias, variant, open variant and record types.Aliases are names for a given type expression and are transparently unfoldable to that expression. They cannot be recursive.
Variants are sum types defined by constructors and eliminated by patternmatching. They can be recursive, but the
rec
flag must be explicitly set. Pattern matching must be exhaustive.Open variants can be extended with additional constructors using the
::=
form.Records are product types with named fields and eliminated by projection. Likewise they can be recursive if the
rec
flag is set.

Command
Ltac2 @ external ident : ltac2_type := string string
¶ Declares abstract terms. Frequently, these declare OCaml functions defined in Coq and give their type information. They can also declare data structures from OCaml. This command has no use for the end user.
APIs¶
Ltac2 provides over 150 API functions that provide various capabilities. These
are declared with Ltac2 external
in lib/coq/usercontrib/Ltac2/*.v
.
For example, Message.print
defined in Message.v
is used to print messages:
 Goal True.
 1 subgoal ============================ True
 Message.print (Message.of_string "fully qualified calls").
 fully qualified calls
 From Ltac2 Require Import Message.
 print (of_string "unqualified calls").
 unqualified calls
Term Syntax¶
The syntax of the functional fragment is very close to that of Ltac1, except that it adds a true patternmatching feature, as well as a few standard constructs from ML.
In practice, there is some additional syntactic sugar that allows the user to bind a variable and match on it at the same time, in the usual ML style.
There is dedicated syntax for list and array literals.
The nonterminal lident
designates identifiers starting with a
lowercase letter.
Ltac2 Definitions¶

Command
Ltac2 mutable? rec? tac2def_body with tac2def_body*
¶ This command defines a new global Ltac2 value. If one or more
tac2pat0
are specified, the new value is a function. This is a shortcut for one of theltac2_expr5
productions. For example:Ltac2 foo a b := …
is equivalent toLtac2 foo := fun a b => …
.The body of an Ltac2 definition is required to be a syntactical value that is, a function, a constant, a pure constructor recursively applied to values or a (nonrecursive) let binding of a value in a value.
If
rec
is set, the tactic is expanded into a recursive binding.If
mutable
is set, the definition can be redefined at a later stage (see below).

Command
Ltac2 Set qualid as ident? := ltac2_expr
¶ This command redefines a previous
mutable
definition. Mutable definitions act like dynamic binding, i.e. at runtime, the last defined value for this entry is chosen. This is useful for global flags and the like. The previous value of the binding can be optionally accessed using theas
binding syntax.Example: Dynamic nature of mutable cells
 Ltac2 mutable x := true.
 Ltac2 y := x.
 Ltac2 Eval y.
  : bool = true
 Ltac2 Set x := false.
 Ltac2 Eval y.
  : bool = false
Example: Interaction with recursive calls
 Ltac2 mutable rec f b := if b then 0 else f true.
 Ltac2 Set f := fun b => if b then 1 else f true.
 Ltac2 Eval (f false).
  : int = 1
 Ltac2 Set f as oldf := fun b => if b then 2 else oldf false.
 Ltac2 Eval (f false).
  : int = 2
In the definition, the
f
in the body is resolved statically because the definition is marked recursive. In the first redefinition, thef
in the body is resolved dynamically. This is witnessed by the second redefinition.
Reduction¶
We use the usual ML callbyvalue reduction, with an otherwise unspecified evaluation order. This is a design choice making it compatible with OCaml, if ever we implement native compilation. The expected equations are as follows:
(fun x => t) V ≡ t{x := V} (βv)
let x := V in t ≡ t{x := V} (let)
match C V₀ ... Vₙ with ...  C x₀ ... xₙ => t  ... end ≡ t {xᵢ := Vᵢ} (ι)
(t any term, V values, C constructor)
Note that callbyvalue reduction is already a departure from Ltac1 which uses heuristics to decide when to evaluate an expression. For instance, the following expressions do not evaluate the same way in Ltac1.
foo (idtac; let x := 0 in bar)
foo (let x := 0 in bar)
Instead of relying on the idtac
idiom, we would now require an explicit thunk
to not compute the argument, and foo
would have e.g. type
(unit > unit) > unit
.
foo (fun () => let x := 0 in bar)
Typing¶
Typing is strict and follows the HindleyMilner system. Unlike Ltac1, there are no type casts at runtime, and one has to resort to conversion functions. See notations though to make things more palatable.
In this setting, all the usual argumentfree tactics have type unit > unit
, but
one can return a value of type t
thanks to terms of type unit > t
,
or take additional arguments.
Effects¶
Effects in Ltac2 are straightforward, except that instead of using the standard IO monad as the ambient effectful world, Ltac2 is has a tactic monad.
Note that the order of evaluation of application is not specified and is implementationdependent, as in OCaml.
We recall that the Proofview.tactic
monad is essentially a IO monad together
with backtracking state representing the proof state.
Intuitively a thunk of type unit > 'a
can do the following:
It can perform nonbacktracking IO like printing and setting mutable variables
It can fail in a nonrecoverable way
It can use firstclass backtracking. One way to think about this is that thunks are isomorphic to this type:
(unit > 'a) ~ (unit > exn + ('a * (exn > 'a)))
i.e. thunks can produce a lazy list of results where each tail is waiting for a continuation exception.It can access a backtracking proof state, consisting among other things of the current evar assignment and the list of goals under focus.
We now describe more thoroughly the various effects in Ltac2.
Standard IO¶
The Ltac2 language features nonbacktracking IO, notably mutable data and printing operations.
Mutable fields of records can be modified using the set syntax. Likewise,
builtin types like string
and array
feature imperative assignment. See
modules String
and Array
respectively.
A few printing primitives are provided in the Message
module for
displaying information to the user.
Fatal errors¶
The Ltac2 language provides nonbacktracking exceptions, also known as panics,
through the following primitive in module Control
:
val throw : exn > 'a
Unlike backtracking exceptions from the next section, this kind of error
is never caught by backtracking primitives, that is, throwing an exception
destroys the stack. This is codified by the following equation, where E
is an evaluation context:
E[throw e] ≡ throw e
(e value)
There is currently no way to catch such an exception, which is a deliberate design choice. Eventually there might be a way to catch it and destroy all backtrack and return values.
Backtracking¶
In Ltac2, we have the following backtracking primitives, defined in the
Control
module:
Ltac2 Type 'a result := [ Val ('a)  Err (exn) ].
val zero : exn > 'a
val plus : (unit > 'a) > (exn > 'a) > 'a
val case : (unit > 'a) > ('a * (exn > 'a)) result
If one views thunks as lazy lists, then zero
is the empty list and plus
is
list concatenation, while case
is patternmatching.
The backtracking is firstclass, i.e. one can write
plus (fun () => "x") (fun _ => "y") : string
producing a backtracking string.
These operations are expected to satisfy a few equations, most notably that they form a monoid compatible with sequentialization.:
plus t zero ≡ t ()
plus (fun () => zero e) f ≡ f e
plus (plus t f) g ≡ plus t (fun e => plus (f e) g)
case (fun () => zero e) ≡ Err e
case (fun () => plus (fun () => t) f) ≡ Val (t,f)
let x := zero e in u ≡ zero e
let x := plus t f in u ≡ plus (fun () => let x := t in u) (fun e => let x := f e in u)
(t, u, f, g, e values)
Goals¶
A goal is given by the data of its conclusion and hypotheses, i.e. it can be
represented as [Γ ⊢ A]
.
The tactic monad naturally operates over the whole proofview, which may represent several goals, including none. Thus, there is no such thing as the current goal. Goals are naturally ordered, though.
It is natural to do the same in Ltac2, but we must provide a way to get access
to a given goal. This is the role of the enter
primitive, which applies a
tactic to each currently focused goal in turn:
val enter : (unit > unit) > unit
It is guaranteed that when evaluating enter f
, f
is called with exactly one
goal under focus. Note that f
may be called several times, or never, depending
on the number of goals under focus before the call to enter
.
Accessing the goal data is then implicit in the Ltac2 primitives, and may panic if the invariants are not respected. The two essential functions for observing goals are given below.:
val hyp : ident > constr
val goal : unit > constr
The two above functions panic if there is not exactly one goal under focus.
In addition, hyp
may also fail if there is no hypothesis with the
corresponding name.
Metaprogramming¶
Overview¶
One of the major implementation issues of Ltac1 is the fact that it is never clear whether an object refers to the object world or the metaworld. This is an incredible source of slowness, as the interpretation must be aware of bound variables and must use heuristics to decide whether a variable is a proper one or referring to something in the Ltac context.
Likewise, in Ltac1, constr parsing is implicit, so that foo 0
is
not foo
applied to the Ltac integer expression 0
(L
_{tac} does have a
notion of integers, though it is not firstclass), but rather the Coq term
Datatypes.O
.
The implicit parsing is confusing to users and often gives unexpected results. Ltac2 makes these explicit using quoting and unquoting notation, although there are notations to do it in a short and elegant way so as not to be too cumbersome to the user.
Quotations¶
Builtin quotations¶
The current implementation recognizes the following builtin quotations:
ident
, which parses identifiers (typeInit.ident
).constr
, which parses Coq terms and produces anevar free term at runtime (typeInit.constr
).open_constr
, which parses Coq terms and produces a term potentially with holes at runtime (typeInit.constr
as well).pattern
, which parses Coq patterns and produces a pattern used for term matching (typeInit.pattern
).reference
Qualified names are globalized at internalization into the corresponding global reference, while&id
is turned intoStd.VarRef id
. This produces at runtime aStd.reference
.ltac1
, for calling Ltac1 code, described in Simple API.ltac1val
, for manipulating Ltac1 values, described in Lowlevel API.
The following syntactic sugar is provided for two common cases:
Strict vs. nonstrict mode¶
Depending on the context, quotationproducing terms (i.e. constr
or
open_constr
) are not internalized in the same way. There are two possible
modes, the strict and the nonstrict mode.
In strict mode, all simple identifiers appearing in a term quotation are required to be resolvable statically. That is, they must be the short name of a declaration which is defined globally, excluding section variables and hypotheses. If this doesn't hold, internalization will fail. To work around this error, one has to specifically use the
&
notation.In nonstrict mode, any simple identifier appearing in a term quotation which is not bound in the global environment is turned into a dynamic reference to a hypothesis. That is to say, internalization will succeed, but the evaluation of the term at runtime will fail if there is no such variable in the dynamic context.
Strict mode is enforced by default, such as for all Ltac2 definitions. Nonstrict
mode is only set when evaluating Ltac2 snippets in interactive proof mode. The
rationale is that it is cumbersome to explicitly add &
interactively, while it
is expected that global tactics enforce more invariants on their code.
Term Antiquotations¶
Syntax¶
One can also insert Ltac2 code into Coq terms, similar to what is possible in Ltac1.
Antiquoted terms are expected to have type unit
, as they are only evaluated
for their sideeffects.
Semantics¶
A quoted Coq term is interpreted in two phases, internalization and evaluation.
Internalization is part of the static semantics, that is, it is done at Ltac2 typing time.
Evaluation is part of the dynamic semantics, that is, it is done when a term gets effectively computed by Ltac2.
Note that typing of Coq terms is a dynamic process occurring at Ltac2 evaluation time, and not at Ltac2 typing time.
Static semantics¶
During internalization, Coq variables are resolved and antiquotations are
type checked as Ltac2 terms, effectively producing a glob_constr
in Coq
implementation terminology. Note that although it went through the
type checking of Ltac2, the resulting term has not been fully computed and
is potentially illtyped as a runtime Coq term.
Example
The following term is valid (with type unit > constr
), but will fail at runtime:
 Ltac2 myconstr () := constr:(nat > 0).
Term antiquotations are type checked in the enclosing Ltac2 typing context of the corresponding term expression.
Example
The following will type check, with type constr
.
Beware that the typing environment of antiquotations is not expanded by the Coq binders from the term.
Example
The following Ltac2 expression will not type check:
`constr:(fun x : nat => ltac2:(exact x))` `(* Error: Unbound variable 'x' *)`
There is a simple reason for that, which is that the following expression would not make sense in general.
constr:(fun x : nat => ltac2:(clear @x; exact x))
Indeed, a hypothesis can suddenly disappear from the runtime context if some other tactic pulls the rug from under you.
Rather, the tactic writer has to resort to the dynamic goal environment, and must write instead explicitly that she is accessing a hypothesis, typically as follows.
constr:(fun x : nat => ltac2:(exact (hyp @x)))
This pattern is so common that we provide dedicated Ltac2 and Coq term notations for it.
&x
as an Ltac2 expression expands tohyp @x
.&x
as a Coq constr expression expands toltac2:(Control.refine (fun () => hyp @x))
.
In the special case where Ltac2 antiquotations appear inside a Coq term
notation, the notation variables are systematically bound in the body
of the tactic expression with type Ltac2.Init.preterm
. Such a type represents
untyped syntactic Coq expressions, which can by typed in the
current context using the Ltac2.Constr.pretype
function.
Example
The following notation is essentially the identity.
 Notation "[ x ]" := ltac2:(let x := Ltac2.Constr.pretype x in exact $x) (only parsing).
 Setting notation at level 0.
Dynamic semantics¶
During evaluation, a quoted term is fully evaluated to a kernel term, and is in particular type checked in the current environment.
Evaluation of a quoted term goes as follows.
The quoted term is first evaluated by the pretyper.
Antiquotations are then evaluated in a context where there is exactly one goal under focus, with the hypotheses coming from the current environment extended with the bound variables of the term, and the resulting term is fed into the quoted term.
Relative orders of evaluation of antiquotations and quoted term are not specified.
For instance, in the following example, tac
will be evaluated in a context
with exactly one goal under focus, whose last hypothesis is H : nat
. The
whole expression will thus evaluate to the term fun H : nat => H
.
let tac () := hyp @H in constr:(fun H : nat => ltac2:(tac ()))
Many standard tactics perform type checking of their argument before going further. It is your duty to ensure that terms are welltyped when calling such tactics. Failure to do so will result in nonrecoverable exceptions.
Trivial Term Antiquotations
It is possible to refer to a variable of type constr
in the Ltac2 environment
through a specific syntax consistent with the antiquotations presented in
the notation section.
In a Coq term, writing $x
is semantically equivalent to
ltac2:(Control.refine (fun () => x))
, up to retypechecking. It allows to
insert in a concise way an Ltac2 variable of type constr
into a Coq term.
Match over terms¶
Ltac2 features a construction similar to Ltac1 match
over terms, although
in a less hardwired way.

Tactic
ltac2_match_key ltac2_expr_{term} with ltac2_match_list end
¶  match!multi_match!
Evaluates
ltac2_expr_{term}
, which must yield a term, and matches it sequentially with theltac2_match_pattern
s, which may contain metavariables. When a match is found, metavariable values are substituted intoltac2_expr
, which is then applied.Matching may continue depending on whether
lazy_match!
,match!
ormulti_match!
is specified.In the
ltac2_match_pattern
s, metavariables have the form?ident
, whereas in theltac2_expr
s, the question mark is omitted.Matching is nonlinear: if a metavariable occurs more than once, each occurrence must match the same expression. Expressions match if they are syntactically equal or are αconvertible. Matching is firstorder except on variables of the form
@?ident
that occur in the head position of an application. For these variables, matching is secondorder and returns a functional term.lazy_match!
Causes the match to commit to the first matching branch rather than trying a new match if
ltac2_expr
fails. Example.match!
If
ltac2_expr
fails, continue matching with the next branch. Failures in subsequent tactics (after thematch!
) will not cause selection of a new branch. Examples here and here.multi_match!
If
ltac2_expr
fails, continue matching with the next branch. When altac2_expr
succeeds for a branch, subsequent failures (after themulti_match!
) causing consumption of all the successes ofltac2_expr
trigger selection of a new matching branch. Example.cpattern
The syntax of
cpattern
is the same as that ofterm
s, but it can contain pattern matching metavariables in the form?ident
and@?ident
._
can be used to match irrelevant terms.Unlike Ltac1, Ltac2
?id
metavariables only match closed terms.There is also a special notation for secondorder pattern matching: in an applicative pattern of the form
@?ident ident_{1} … ident_{n}
, the variableident
matches any complex expression with (possible) dependencies in the variablesident_{i}
and returns a functional term of the formfun ident_{1} … ident_{n} => term
.
context ident? [ cpattern ]
Matches any term with a subterm matching
cpattern
. If there is a match andident
is present, it is assigned the "matched context", i.e. the initial term where the matched subterm is replaced by a hole. This hole in the matched context can be filled with the expressionPattern.instantiate ident cpattern
.For
match!
andmulti_match!
, if the evaluation of theltac2_expr
fails, the next matching subterm is tried. If no further subterm matches, the next branch is tried. Matching subterms are considered from top to bottom and from left to right (with respect to the raw printing obtained by setting thePrinting All
flag). Example.
ltac2_expr
The tactic to apply if the construct matches. Metavariable values from the pattern match are statically bound as Ltac2 variables in
ltac2_expr
before it is applied.If
ltac2_expr
is a tactic with backtracking points, then subsequent failures after alazy_match!
ormulti_match!
(but notmatch!
) can cause backtracking intoltac2_expr
to select its next success.
Variables from the
tac2pat1
are statically bound in the body of the branch. Variables from theterm
pattern have values of typeconstr
. Variables from theident
in thecontext
construct have values of typePattern.context
(defined inPattern.v
).
Note that unlike Ltac1, only lowercase identifiers are valid as Ltac2 bindings. Ltac2 will report an error if one of the bound variables starts with an uppercase character.
The semantics of this construction are otherwise the same as the corresponding one from Ltac1, except that it requires the goal to be focused.
Example: Ltac2 Comparison of lazy_match! and match!
(Equivalent to this Ltac1 example.)
These lines define a msg
tactic that's used in several examples as a moresuccinct
alternative to print (to_string "...")
:
 From Ltac2 Require Import Message.
 Ltac2 msg x := print (of_string x).
 Goal True.
 1 subgoal ============================ True
In lazy_match!
, if ltac2_expr
fails, the lazy_match!
fails;
it doesn't look for further matches. In match!
, if ltac2_expr
fails
in a matching branch, it will try to match on subsequent branches. Note that
'term
below is equivalent to open_constr:(term)
.
 Fail lazy_match! 'True with  True => msg "branch 1"; fail  _ => msg "branch 2" end.
 branch 1 The command has indeed failed with message: Uncaught Ltac2 exception: Tactic_failure (None)
 match! 'True with  True => msg "branch 1"; fail  _ => msg "branch 2" end.
 branch 1 branch 2
Example: Ltac2 Comparison of match! and multi_match!
(Equivalent to this Ltac1 example.)
match!
tactics are only evaluated once, whereas multi_match!
tactics may be evaluated more than once if the following constructs trigger backtracking:
 Fail match! 'True with  True => msg "branch 1"  _ => msg "branch 2" end ; msg "branch A"; fail.
 branch 1 branch A The command has indeed failed with message: Uncaught Ltac2 exception: Tactic_failure (None)
 Fail multi_match! 'True with  True => msg "branch 1"  _ => msg "branch 2" end ; msg "branch A"; fail.
 branch 1 branch A branch 2 branch A The command has indeed failed with message: Uncaught Ltac2 exception: Match_failure
Example: Ltac2 Multiple matches for a "context" pattern.
(Equivalent to this Ltac1 example.)
Internally "x <> y" is represented as "(~ (x = y))", which produces the first match.
 Ltac2 f2 t := match! t with  context [ (~ ?t) ] => print (of_constr t); fail  _ => () end.
 f2 constr:((~ True) <> (~ False)).
 ((~ True) = (~ False)) True False
Match over goals¶

Tactic
ltac2_match_key reverse? goal with goal_match_list end
¶ Matches over goals, similar to Ltac1
match goal
. Use this form to match hypotheses and/or goals in the local context. These patterns have zero or more subpatterns to match hypotheses followed by a subpattern to match the conclusion. Except for the differences noted below, this works the same as the correspondingltac2_match_key ltac2_expr
construct (seematch!
). Each current goal is processed independently.Matching is nonlinear: if a metavariable occurs more than once, each occurrence must match the same expression. Within a single term, expressions match if they are syntactically equal or αconvertible. When a metavariable is used across multiple hypotheses or across a hypothesis and the current goal, the expressions match if they are convertible.
gmatch_pattern*,
Patterns to match with hypotheses. Each pattern must match a distinct hypothesis in order for the branch to match.
Hypotheses have the form
name := term_{binder}? : type
. Currently Ltac2 doesn't allow matching on or capturing the value ofterm_{binder}
. It only supports matching on thename
and thetype
, for examplen : ?t
.If there are multiple
gmatch_hyp_pattern
s in a branch, there may be multiple ways to match them to hypotheses. Formatch! goal
andmulti_match! goal
, if the evaluation of theltac2_expr
fails, matching will continue with the next hypothesis combination. When those are exhausted, the next alternative from anycontext
construct in theltac2_match_pattern
s is tried and then, when the context alternatives are exhausted, the next branch is tried. Example.reverse
Hypothesis matching for
gmatch_hyp_pattern
s normally begins by matching them from left to right, to hypotheses, last to first. Specifyingreverse
begins matching in the reverse order, from first to last. Normal and reverse examples. ltac2_match_pattern
A pattern to match with the current goal
Note that unlike Ltac1, only lowercase identifiers are valid as Ltac2 bindings. Ltac2 will report an error if you try to use a bound variable that starts with an uppercase character.
Variables from
gmatch_hyp_pattern
andltac2_match_pattern
are bound in the body of the branch. Their types are:constr
for pattern variables appearing in aterm
Pattern.context
for variables binding a contextident
for variables binding a hypothesis name.
The same identifier caveat as in the case of matching over constr applies, and this feature has the same semantics as in Ltac1.
Example: Ltac2 Matching hypotheses
(Equivalent to this Ltac1 example.)
Hypotheses are matched from the last hypothesis (which is by default the newest
hypothesis) to the first until the apply
succeeds.
 Goal forall A B : Prop, A > B > (A>B).
 1 subgoal ============================ forall A B : Prop, A > B > A > B
 intros.
 1 subgoal A, B : Prop H : A H0 : B H1 : A ============================ B
 match! goal with  [ h : _  _ ] => let h := Control.hyp h in print (of_constr h); apply $h end.
 H1 H0 No more subgoals.
Example: Matching hypotheses with reverse
(Equivalent to this Ltac1 example.)
Hypotheses are matched from the first hypothesis to the last until the apply
succeeds.
 Goal forall A B : Prop, A > B > (A>B).
 1 subgoal ============================ forall A B : Prop, A > B > A > B
 intros.
 1 subgoal A, B : Prop H : A H0 : B H1 : A ============================ B
 match! reverse goal with  [ h : _  _ ] => let h := Control.hyp h in print (of_constr h); apply $h end.
 A B H H0 No more subgoals.
Example: Multiple ways to match a hypotheses
(Equivalent to this Ltac1 example.)
Every possible match for the hypotheses is evaluated until the righthand
side succeeds. Note that h1
and h2
are never matched to the same hypothesis.
Observe that the number of permutations can grow as the factorial
of the number of hypotheses and hypothesis patterns.
 Goal forall A B : Prop, A > B > (A>B).
 1 subgoal ============================ forall A B : Prop, A > B > A > B
 intros A B H.
 1 subgoal A, B : Prop H : A ============================ B > A > B
 match! goal with  [ h1 : _, h2 : _  _ ] => print (concat (of_string "match ") (concat (of_constr (Control.hyp h1)) (concat (of_string " ") (of_constr (Control.hyp h2))))); fail  [  _ ] => () end.
 match B H match A H match H B match A B match H A match B A
Match on values¶

Tactic
match ltac2_expr5 with ltac2_branches? end
¶ Matches a value, akin to the OCaml
match
construct. By itself, it doesn't cause backtracking as do the*match*!
and*match*! goal
constructs.[ ]()

Tactic
if ltac2_expr5_{test} then ltac2_expr5_{then} else ltac2_expr5_{else}
¶ Equivalent to a
match
on a boolean value. If theltac2_expr5_{test}
evaluates to true,ltac2_expr5_{then}
is evaluated. Otherwiseltac2_expr5_{else}
is evaluated.
Note
For now, deep pattern matching is not implemented.
Notations¶

Command
Ltac2 Notation ltac2_scope+ : natural? := ltac2_expr
¶ Ltac2 Notation
provides a way to extend the syntax of Ltac2 tactics. The lefthand side (before the:=
) defines the syntax to recognize and gives formal parameter names for the syntactic values.integer
is the level of the notation. When the notation is used, the values are substituted into the righthand side. The righthand side is typechecked when the notation is used, not when it is defined. In the following example,x
is the formal parameter name andconstr
is its syntactic class.print
andof_constr
are functions provided by Coq throughMessage.v
.Example: Printing a
term
 Goal True.
 1 subgoal ============================ True
 From Ltac2 Require Import Message.
 Ltac2 Notation "ex1" x(constr) := print (of_constr x).
 ex1 (1 + 2).
 (1 + 2)
You can also print terms with a regular Ltac2 definition, but then the
term
must be in the quotationconstr:( … )
: Ltac2 ex2 x := print (of_constr x).
 ex2 constr:(1+2).
 (1 + 2)
There are also metasyntactic classes described here that combine other items. For example,
list1(constr, ",")
recognizes a commaseparated list of one or moreterm
s.Example: Parsing a list of
term
s Ltac2 rec print_list x := match x with  a :: t => print (of_constr a); print_list t  [] => () end.
 Ltac2 Notation "ex2" x(list1(constr, ",")) := print_list x.
 ex2 1, 2, 3.
 1 2 3
An Ltac2 notation adds a parsing rule to the Ltac2 grammar, which is expanded to the provided body where every token from the notation is letbound to the corresponding generated expression.
Example
Assume we perform:
Ltac2 Notation "foo" c(thunk(constr)) ids(list0(ident)) := Bar.f c ids.Then the following expression
let y := @X in foo (nat > nat) x $y
will expand at parsing time to
let y := @X in
let c := fun () => constr:(nat > nat) with ids := [@x; y] in Bar.f c ids
Beware that the order of evaluation of multiple letbindings is not specified, so that you may have to resort to thunking to ensure that sideeffects are performed at the right time.
Abbreviations¶

Command
Ltac2 Notation stringlident := ltac2_expr
¶ Introduces a special kind of notation, called an abbreviation, that does not add any parsing rules. It is similar in spirit to Coq abbreviations (see
Notation (abbreviation)
, insofar as its main purpose is to give an absolute name to a piece of pure syntax, which can be transparently referred to by this name as if it were a proper definition.The abbreviation can then be manipulated just like a normal Ltac2 definition, except that it is expanded at internalization time into the given expression. Furthermore, in order to make this kind of construction useful in practice in an effectful language such as Ltac2, any syntactic argument to an abbreviation is thunked onthefly during its expansion.
For instance, suppose that we define the following.
Ltac2 Notation foo := fun x => x ().
Then we have the following expansion at internalization time.
foo 0 ↦ (fun x => x ()) (fun _ => 0)
Note that abbreviations are not type checked at all, and may result in typing errors after expansion.
Defining tactics¶
Builtin tactics (those defined in OCaml code in the Coq executable) and Ltac1 tactics,
which are defined in .v
files, must be defined through notations. (Ltac2 tactics can be
defined with Ltac2
.
Notations for many but not all builtin tactics are defined in Notations.v
, which is automatically
loaded with Ltac2. The Ltac2 syntax for these tactics is often identical or very similar to the
tactic syntax described in other chapters of this documentation. These notations rely on tactic functions
declared in Std.v
. Functions corresponding to some builtin tactics may not yet be defined in the
Coq executable or declared in Std.v
. Adding them may require code changes to Coq or defining
workarounds through Ltac1 (described below).
Two examples of syntax differences:
There is no notation defined that's equivalent to
intros until identnatural
. There is, however, already anintros_until
tactic function definedStd.v
, so it may be possible for a user to add the necessary notation.The builtin
simpl
tactic in Ltac1 supports the use of scope keys in delta flags, e.g.simpl ["+"%nat]
which is not accepted by Ltac2. This is because Ltac2 uses a different definition fordelta_flag
; compare it toltac2_delta_flag
. This also affectscompute
.
Ltac1 tactics are not automatically available in Ltac2. (Note that some of the tactics described in the documentation are defined with Ltac1.) You can make them accessible in Ltac2 with commands similar to the following:
 From Coq Require Import Lia.
 [Loading ML file ring_plugin.cmxs ... done] [Loading ML file zify_plugin.cmxs ... done] [Loading ML file micromega_plugin.cmxs ... done]
 Local Ltac2 lia_ltac1 () := ltac1:(lia).
 Ltac2 Notation "lia" := lia_ltac1 ().
A similar approach can be used to access missing builtin tactics. See Simple API for an example that passes two parameters to a missing buildin tactic.
Syntactic classes¶
The simplest syntactic classes in Ltac2 notations represent individual nonterminals from the Coq grammar. Only a few selected nonterminals are available as syntactic classes. In addition, there are metasyntactic operations for describing more complex syntax, such as making an item optional or representing a list of items. When parsing, each syntactic class expression returns a value that's bound to a name in the notation definition.
Syntactic classes are described with a form of Sexpression:
Metasyntactic operations that can be applied to other syntactic classes are:
opt(ltac2_scope)
Parses an optional
ltac2_scope
. The associated value is eitherNone
or enclosed inSome
list1(ltac2_scope , string?)
Parses a list of one or more
ltac2_scope
s. Ifstring
is specified, items must be separated bystring
.list0(ltac2_scope , string?)
Parses a list of zero or more
ltac2_scope
s. Ifstring
is specified, items must be separated bystring
. For zero items, the associated value is an empty list.seq(ltac2_scope+,)
Parses the
ltac2_scope
s in order. The associated value is a tuple, omittingltac2_scope
s that arestring
s.self
andnext
are not permitted withinseq
.
The following classes represent nonterminals with some special handling. The table further down lists the classes that that are handled plainly.
constr ( scope_key+, )?
Parses a
term
. If specified, thescope_key
s are used to interpret the term (as described in Local interpretation rules for notations). The lastscope_key
is the top of the scope stack that's applied to theterm
.open_constr
Parses an open
term
.ident
Parses
ident
or$ident
. The first form returnsident:(ident)
, while the latter form returns the variableident
.string
Accepts the specified string that is not a keyword, returning a value of
()
.keyword(string)
Accepts the specified string that is a keyword, returning a value of
()
.terminal(string)
Accepts the specified string whether it's a keyword or not, returning a value of
()
.tactic (integer)?
Parses an
ltac2_expr
. Ifinteger
is specified, the construct parses altac2_exprinteger
, for exampletactic(5)
parsesltac2_expr5
.tactic(6)
parsesltac2_expr
.integer
must be in the range0 .. 6
.You can also use
tactic
to accept aninteger
or astring
, but there's no syntactic class that accepts only aninteger
or astring
.
self
parses an Ltac2 expression at the current level and returns it as is.
next
parses an Ltac2 expression at the next level and returns it as is.
thunk(ltac2_scope)
Used for semantic effect only, parses the same as
ltac2_scope
. Ife
is the parsed expression forltac2_scope
,thunk
returnsfun () => e
.pattern
parses a
cpattern
A few syntactic classes contain antiquotation features. For the sake of uniformity, all
antiquotations are introduced by the syntax $lident
.
A few other specific syntactic classes exist to handle Ltac1like syntax, but their use is discouraged and they are thus not documented.
For now there is no way to declare new syntactic classes from the Ltac2 side, but this is planned.
Other nonterminals that have syntactic classes are listed here.
Syntactic class name
Nonterminal
Similar nonLtac2 syntax
intropatterns
intropattern
ident
destruction_arg
destruction_arg
with_bindings
with bindings?
bindings
strategy
reference
clause
occurrences
at occs_nums?
induction_clause
induction_clause
conversion
rewriting
dispatch
hintdb
move_location
where
pose
bindings_with_parameters
assert
constr_matching
See
match
goal_matching
See
match goal
Here is the syntax for the q_*
nonterminals:
Evaluation¶
Ltac2 features a toplevel loop that can be used to evaluate expressions.

Command
Ltac2 Eval ltac2_expr
¶ This command evaluates the term in the current proof if there is one, or in the global environment otherwise, and displays the resulting value to the user together with its type. This command is pure in the sense that it does not modify the state of the proof, and in particular all sideeffects are discarded.
Debug¶

Flag
Ltac2 Backtrace
¶ When this flag is set, toplevel failures will be printed with a backtrace.
Compatibility layer with Ltac1¶
Ltac1 from Ltac2¶
Simple API¶
One can call Ltac1 code from Ltac2 by using the ltac1:(ltac1_expr_in_env)
quotation.
See Builtin quotations. It parses
a Ltac1 expression, and semantics of this quotation is the evaluation of the
corresponding code for its side effects. In particular, it cannot return values,
and the quotation has type unit
.
Ltac1 cannot implicitly access variables from the Ltac2 scope, but this can
be done with an explicit annotation on the ltac1:(ident*  ltac_expr)
quotation. See Builtin quotations. For example:
 Local Ltac2 replace_with (lhs: constr) (rhs: constr) := ltac1:(lhs rhs  replace lhs with rhs) (Ltac1.of_constr lhs) (Ltac1.of_constr rhs).
 Ltac2 Notation "replace" lhs(constr) "with" rhs(constr) := replace_with lhs rhs.
The return type of this expression is a function of the same arity as the number
of identifiers, with arguments of type Ltac2.Ltac1.t
(see below). This syntax
will bind the variables in the quoted Ltac1 code as if they had been bound from
Ltac1 itself. Similarly, the arguments applied to the quotation will be passed
at runtime to the Ltac1 code.
Lowlevel API¶
There exists a lowerlevel FFI into Ltac1 that is not recommended for daily use,
which is available in the Ltac2.Ltac1
module. This API allows to directly
manipulate dynamicallytyped Ltac1 values, either through the function calls,
or using the ltac1val
quotation. The latter parses the same as ltac1
, but
has type Ltac2.Ltac1.t
instead of unit
, and dynamically behaves as an Ltac1
thunk, i.e. ltac1val:(foo)
corresponds to the tactic closure that Ltac1
would generate from idtac; foo
.
Due to intricate dynamic semantics, understanding when Ltac1 value quotations focus is very hard. This is why some functions return a continuationpassing style value, as it can dispatch dynamically between focused and unfocused behaviour.
The same mechanism for explicit binding of variables as described in the previous section applies.
Ltac2 from Ltac1¶
Same as above by switching Ltac1 by Ltac2 and using the ltac2
quotation
instead.
The typing rules are dual, that is, the optional identifiers are bound
with type Ltac2.Ltac1.t
in the Ltac2 expression, which is expected to have
type unit. The value returned by this quotation is an Ltac1 function with the
same arity as the number of bound variables.
Note that when no variables are bound, the inner tactic expression is evaluated
eagerly, if one wants to use it as an argument to a Ltac1 function, one has to
resort to the good old idtac; ltac2:(foo)
trick. For instance, the code
below will fail immediately and won't print anything.
 From Ltac2 Require Import Ltac2.
 Set Default Proof Mode "Classic".
 Ltac mytac tac := idtac "I am being evaluated"; tac.
 mytac is defined
 Goal True.
 1 subgoal ============================ True
 Proof.
 Fail mytac ltac2:(fail).
 The command has indeed failed with message: Uncaught Ltac2 exception: Tactic_failure (None)
 Fail mytac ltac:(idtac; ltac2:(fail)).
 I am being evaluated The command has indeed failed with message: Uncaught Ltac2 exception: Tactic_failure (None)
In any case, the value returned by the fully applied quotation is an unspecified dummy Ltac1 closure and should not be further used.
Switching between Ltac languages¶
We recommend using the Default Proof Mode
option to switch between tactic
languages with a proofbased granularity. This allows to incrementally port
the proof scripts.
Transition from Ltac1¶
Owing to the use of a lot of notations, the transition should not be too difficult. In particular, it should be possible to do it incrementally. That said, we do not guarantee it will be a blissful walk either. Hopefully, owing to the fact Ltac2 is typed, the interactive dialogue with Coq will help you.
We list the major changes and the transition strategies hereafter.
Syntax changes¶
Due to conflicts, a few syntactic rules have changed.
The dispatch tactical
tac; [foobar]
is now writtentac > [foobar]
.Levels of a few operators have been revised. Some tacticals now parse as if they were normal functions. Parentheses are now required around complex arguments, such as abstractions. The tacticals affected are:
try
,repeat
,do
,once
,progress
,time
,abstract
.idtac
is no more. Either use()
if you expect nothing to happen,(fun () => ())
if you want a thunk (see next section), or use printing primitives from theMessage
module if you want to display something.
Tactic delay¶
Tactics are not magically delayed anymore, neither as functions nor as arguments. It is your responsibility to thunk them beforehand and apply them at the call site.
A typical example of a delayed function:
Ltac foo := blah.
becomes
Ltac2 foo () := blah.
All subsequent calls to foo
must be applied to perform the same effect as
before.
Likewise, for arguments:
Ltac bar tac := tac; tac; tac.
becomes
Ltac2 bar tac := tac (); tac (); tac ().
We recommend the use of syntactic notations to ease the transition. For instance, the first example can alternatively be written as:
Ltac2 foo0 () := blah.
Ltac2 Notation foo := foo0 ().
This allows to keep the subsequent calls to the tactic asis, as the
expression foo
will be implicitly expanded everywhere into foo0 ()
. Such
a trick also works for arguments, as arguments of syntactic notations are
implicitly thunked. The second example could thus be written as follows.
Ltac2 bar0 tac := tac (); tac (); tac ().
Ltac2 Notation bar := bar0.
Variable binding¶
Ltac1 relies on complex dynamic trickery to be able to tell apart bound variables from terms, hypotheses, etc. There is no such thing in Ltac2, as variables are recognized statically and other constructions do not live in the same syntactic world. Due to the abuse of quotations, it can sometimes be complicated to know what a mere identifier represents in a tactic expression. We recommend tracking the context and letting the compiler print typing errors to understand what is going on.
We list below the typical changes one has to perform depending on the static errors produced by the typechecker.
Exception catching¶
Ltac2 features a proper exceptioncatching mechanism. For this reason, the
Ltac1 mechanism relying on fail
taking integers, and tacticals decreasing it,
has been removed. Now exceptions are preserved by all tacticals, and it is
your duty to catch them and reraise them as needed.