Chapter 10  Detailed examples of tactics

• 10.1  dependent induction
• 10.2  autorewrite
• 10.3  quote
• 10.4  Using the tactical language

This chapter presents detailed examples of certain tactics, to illustrate their behavior.

10.1  dependent induction

The tactics dependent induction and dependent destruction are another solution for inverting inductive predicate instances and potentially doing induction at the same time. It is based on the BasicElim tactic of Conor McBride which works by abstracting each argument of an inductive instance by a variable and constraining it by equalities afterwards. This way, the usual induction and destruct tactics can be applied to the abstracted instance and after simplification of the equalities we get the expected goals.

The abstracting tactic is called generalize_eqs and it takes as argument an hypothesis to generalize. It uses the JMeq datatype defined in Coq.Logic.JMeq, hence we need to require it before. For example, revisiting the first example of the inversion documentation above:

The index S n gets abstracted by a variable here, but a corresponding equality is added under the abstract instance so that no information is actually lost. The goal is now almost amenable to do induction or case analysis. One should indeed first move n into the goal to strengthen it before doing induction, or n will be fixed in the inductive hypotheses (this does not matter for case analysis). As a rule of thumb, all the variables that appear inside constructors in the indices of the hypothesis should be generalized. This is exactly what the generalize_eqs_vars variant does:

As the hypothesis itself did not appear in the goal, we did not need to use an heterogeneous equality to relate the new hypothesis to the old one (which just disappeared here). However, the tactic works just as well in this case, e.g.:

One drawback of this approach is that in the branches one will have to substitute the equalities back into the instance to get the right assumptions. Sometimes injection of constructors will also be needed to recover the needed equalities. Also, some subgoals should be directly solved because of inconsistent contexts arising from the constraints on indexes. The nice thing is that we can make a tactic based on discriminate, injection and variants of substitution to automatically do such simplifications (which may involve the K axiom). This is what the simplify_dep_elim tactic from Coq.Program.Equality does. For example, we might simplify the previous goals considerably:

The higher-order tactic do_depind defined in Coq.Program.Equality takes a tactic and combines the building blocks we have seen with it: generalizing by equalities calling the given tactic with the generalized induction hypothesis as argument and cleaning the subgoals with respect to equalities. Its most important instantiations are dependent induction and dependent destruction that do induction or simply case analysis on the generalized hypothesis. For example we can redo what we’ve done manually with dependent destruction :

This gives essentially the same result as inversion. Now if the destructed hypothesis actually appeared in the goal, the tactic would still be able to invert it, contrary to dependent inversion. Consider the following example on vectors:

In this case, the v variable can be replaced in the goal by the generalized hypothesis only when it has a type of the form vector (S n), that is only in the second case of the destruct. The first one is dismissed because S n <> 0.

10.1.1  A larger example

Let’s see how the technique works with induction on inductive predicates on a real example. We will develop an example application to the theory of simply-typed lambda-calculus formalized in a dependently-typed style:

We have defined types and contexts which are snoc-lists of types. We also have a conc operation that concatenates two contexts. The term datatype represents in fact the possible typing derivations of the calculus, which are isomorphic to the well-typed terms, hence the name. A term is either an application of:

• the axiom rule to type a reference to the first variable in a context,
• the weakening rule to type an object in a larger context
• the abstraction or lambda rule to type a function
• the application to type an application of a function to an argument

Once we have this datatype we want to do proofs on it, like weakening:

The problem here is that we can’t just use induction on the typing derivation because it will forget about the G ; D constraint appearing in the instance. A solution would be to rewrite the goal as:

With this proper separation of the index from the instance and the right induction loading (putting G and D after the inducted-on hypothesis), the proof will go through, but it is a very tedious process. One is also forced to make a wrapper lemma to get back the more natural statement. The dependent induction tactic alleviates this trouble by doing all of this plumbing of generalizing and substituting back automatically. Indeed we can simply write:

This call to dependent induction has an additional arguments which is a list of variables appearing in the instance that should be generalized in the goal, so that they can vary in the induction hypotheses. By default, all variables appearing inside constructors (except in a parameter position) of the instantiated hypothesis will be generalized automatically but one can always give the list explicitly.

The simpl_depind tactic includes an automatic tactic that tries to simplify equalities appearing at the beginning of induction hypotheses, generally using trivial applications of reflexivity. In cases where the equality is not between constructor forms though, one must help the automation by giving some arguments, using the specialize tactic for example.

Once the induction hypothesis has been narrowed to the right equality, it can be used directly.

If there is an easy first-order solution to these equations as in this subgoal, the specialize_eqs tactic can be used instead of giving explicit proof terms:

This concludes our example.
See also: The induction 9, case 9 and inversion 8.14 tactics.

10.2  autorewrite

Here are two examples of autorewrite use. The first one (Ackermann function) shows actually a quite basic use where there is no conditional rewriting. The second one (Mac Carthy function) involves conditional rewritings and shows how to deal with them using the optional tactic of the Hint Rewrite command.

Example 1: Ackermann function

Example 2: Mac Carthy function

10.3  quote

The tactic quote allows using Barendregt’s so-called 2-level approach without writing any ML code. Suppose you have a language L of ’abstract terms’ and a type A of ’concrete terms’ and a function f : L -> A. If L is a simple inductive datatype and f a simple fixpoint, quote f will replace the head of current goal by a convertible term of the form (f t). L must have a constructor of type: A -> L.

Here is an example:

The algorithm to perform this inversion is: try to match the term with right-hand sides expression of f. If there is a match, apply the corresponding left-hand side and call yourself recursively on sub-terms. If there is no match, we are at a leaf: return the corresponding constructor (here f_const) applied to the term.

Error messages:

1. quote: not a simple fixpoint
   Happens when quote is not able to perform inversion properly.

10.3.1  Introducing variables map

The normal use of quote is to make proofs by reflection: one defines a function simplify : formula -> formula and proves a theorem simplify_ok: (f:formula)(interp_f (simplify f)) -> (interp_f f). Then, one can simplify formulas by doing:

   quote interp_f.
   apply simplify_ok.
   compute.

But there is a problem with leafs: in the example above one cannot write a function that implements, for example, the logical simplifications A ∧ A → A or A ∧ ¬ A → False. This is because the Prop is impredicative.

It is better to use that type of formulas:

index is defined in module quote. Equality on that type is decidable so we are able to simplify A ∧ A into A at the abstract level.

When there are variables, there are bindings, and quote provides also a type (varmap A) of bindings from index to any set A, and a function varmap_find to search in such maps. The interpretation function has now another argument, a variables map:

quote handles this second case properly:

It builds vm and t such that (f vm t) is convertible with the conclusion of current goal.

10.3.2  Combining variables and constants

One can have both variables and constants in abstracts terms; that is the case, for example, for the ring tactic (chapter 25). Then one must provide to quote a list of constructors of constants. For example, if the list is [O S] then closed natural numbers will be considered as constants and other terms as variables.

Example:

Warning: Since function inversion is undecidable in general case, don’t expect miracles from it!

Variants:

1. quote ident in term using tactic

   tactic must be a functional tactic (starting with fun x =>) and will be called with the quoted version of term according to ident.

2. quote ident [ ident₁ … ident_n ] in term using tactic

   Same as above, but will use ident₁, …, ident_n to chose which subterms are constants (see above).

See also: comments of source file plugins/quote/quote.ml

See also: the ring tactic (Chapter 25)

10.4  Using the tactical language

10.4.1  About the cardinality of the set of natural numbers

A first example which shows how to use the pattern matching over the proof contexts is the proof that natural numbers have more than two elements. The proof of such a lemma can be done as follows:

We can notice that all the (very similar) cases coming from the three eliminations (with three distinct natural numbers) are successfully solved by a match goal structure and, in particular, with only one pattern (use of non-linear matching).

10.4.2  Permutation on closed lists

Another more complex example is the problem of permutation on closed lists. The aim is to show that a closed list is a permutation of another one.

First, we define the permutation predicate as shown in table 10.1.

---

Coq < Section Sort.

Coq < Variable A : Set.

Coq < Inductive permut : list A -> list A -> Prop :=
        | permut_refl   : forall l, permut l l
        | permut_cons   :
            forall a l0 l1, permut l0 l1 -> permut (a :: l0) (a :: l1)
        | permut_append : forall a l, permut (a :: l) (l ++ a :: nil)
        | permut_trans  :
            forall l0 l1 l2, permut l0 l1 -> permut l1 l2 -> permut l0 l2.

Coq < End Sort.

Figure 10.1: Definition of the permutation predicate

---

A more complex example is the problem of permutation on closed lists. The aim is to show that a closed list is a permutation of another one. First, we define the permutation predicate as shown on Figure 10.1.

---

Coq < Ltac Permut n :=
        match goal with
        | |- (permut _ ?l ?l) => apply permut_refl
        | |- (permut _ (?a :: ?l1) (?a :: ?l2)) =>
            let newn := eval compute in (length l1) in
            (apply permut_cons; Permut newn)
        | |- (permut ?A (?a :: ?l1) ?l2) =>
            match eval compute in n with
            | 1 => fail
            | _ =>
                let l1' := constr:(l1 ++ a :: nil) in
                (apply (permut_trans A (a :: l1) l1' l2);
                  [ apply permut_append | compute; Permut (pred n) ])
            end
        end.
Permut is defined

Coq < Ltac PermutProve :=
        match goal with
        | |- (permut _ ?l1 ?l2) =>
            match eval compute in (length l1 = length l2) with
            | (?n = ?n) => Permut n
            end
        end.
PermutProve is defined

Figure 10.2: Permutation tactic

---

Next, we can write naturally the tactic and the result can be seen on Figure 10.2. We can notice that we use two toplevel definitions PermutProve and Permut. The function to be called is PermutProve which computes the lengths of the two lists and calls Permut with the length if the two lists have the same length. Permut works as expected. If the two lists are equal, it concludes. Otherwise, if the lists have identical first elements, it applies Permut on the tail of the lists. Finally, if the lists have different first elements, it puts the first element of one of the lists (here the second one which appears in the permut predicate) at the end if that is possible, i.e., if the new first element has been at this place previously. To verify that all rotations have been done for a list, we use the length of the list as an argument for Permut and this length is decremented for each rotation down to, but not including, 1 because for a list of length n, we can make exactly n−1 rotations to generate at most n distinct lists. Here, it must be noticed that we use the natural numbers of Coq for the rotation counter. On Figure 9.1, we can see that it is possible to use usual natural numbers but they are only used as arguments for primitive tactics and they cannot be handled, in particular, we cannot make computations with them. So, a natural choice is to use Coq data structures so that Coq makes the computations (reductions) by eval compute in and we can get the terms back by match.

With PermutProve, we can now prove lemmas as follows:

10.4.3  Deciding intuitionistic propositional logic

---

Coq < Ltac Axioms :=
        match goal with
        | |- True => trivial
        | _:False |- _  => elimtype False; assumption
        | _:?A |- ?A  => auto
        end.
Axioms is defined

Figure 10.3: Deciding intuitionistic propositions (1)

---

---

Coq < Ltac DSimplif :=
        repeat
         (intros;
          match goal with
           | id:(~ _) |- _ => red in id
           | id:(_ /\ _) |- _ =>
               elim id; do 2 intro; clear id
           | id:(_ \/ _) |- _ =>
               elim id; intro; clear id
           | id:(?A /\ ?B -> ?C) |- _ =>
               cut (A -> B -> C);
                [ intro | intros; apply id; split; assumption ]
           | id:(?A \/ ?B -> ?C) |- _ =>
               cut (B -> C);
                [ cut (A -> C);
                   [ intros; clear id
                   | intro; apply id; left; assumption ]
                | intro; apply id; right; assumption ]
           | id0:(?A -> ?B),id1:?A |- _ =>
               cut B; [ intro; clear id0 | apply id0; assumption ]
           | |- (_ /\ _) => split
           | |- (~ _) => red
           end).
DSimplif is defined

Coq < Ltac TautoProp :=
        DSimplif;
         Axioms ||
           match goal with
           | id:((?A -> ?B) -> ?C) |- _ =>
                cut (B -> C);
                [ intro; cut (A -> B);
                   [ intro; cut C;
                      [ intro; clear id | apply id; assumption ]
                   | clear id ]
                | intro; apply id; intro; assumption ]; TautoProp
           | id:(~ ?A -> ?B) |- _ =>
               cut (False -> B);
                [ intro; cut (A -> False);
                   [ intro; cut B;
                      [ intro; clear id | apply id; assumption ]
                   | clear id ]
                | intro; apply id; red; intro; assumption ]; TautoProp
           | |- (_ \/ _) => (left; TautoProp) || (right; TautoProp)
           end.
TautoProp is defined

Figure 10.4: Deciding intuitionistic propositions (2)

---

The pattern matching on goals allows a complete and so a powerful backtracking when returning tactic values. An interesting application is the problem of deciding intuitionistic propositional logic. Considering the contraction-free sequent calculi LJT* of Roy Dyckhoff ([56]), it is quite natural to code such a tactic using the tactic language as shown on Figures 10.3 and 10.4. The tactic Axioms tries to conclude using usual axioms. The tactic DSimplif applies all the reversible rules of Dyckhoff’s system. Finally, the tactic TautoProp (the main tactic to be called) simplifies with DSimplif, tries to conclude with Axioms and tries several paths using the backtracking rules (one of the four Dyckhoff’s rules for the left implication to get rid of the contraction and the right or).

For example, with TautoProp, we can prove tautologies like those:

10.4.4  Deciding type isomorphisms

A more tricky problem is to decide equalities between types and modulo isomorphisms. Here, we choose to use the isomorphisms of the simply typed λ-calculus with Cartesian product and unit type (see, for example, [45]). The axioms of this λ-calculus are given by table 10.5.

---

Coq < Open Scope type_scope.

Coq < Section Iso_axioms.

Coq < Variables A B C : Set.

Coq < Axiom Com : A * B = B * A.

Coq < Axiom Ass : A * (B * C) = A * B * C.

Coq < Axiom Cur : (A * B -> C) = (A -> B -> C).

Coq < Axiom Dis : (A -> B * C) = (A -> B) * (A -> C).

Coq < Axiom P_unit : A * unit = A.

Coq < Axiom AR_unit : (A -> unit) = unit.

Coq < Axiom AL_unit : (unit -> A) = A.

Coq < Lemma Cons : B = C -> A * B = A * C.

Coq < Proof.

Coq < intro Heq; rewrite Heq; reflexivity.

Coq < Qed.

Coq < End Iso_axioms.

Figure 10.5: Type isomorphism axioms

---

A more tricky problem is to decide equalities between types and modulo isomorphisms. Here, we choose to use the isomorphisms of the simply typed λ-calculus with Cartesian product and unit type (see, for example, [45]). The axioms of this λ-calculus are given on Figure 10.5.

---

Coq < Ltac DSimplif trm :=
        match trm with
        | (?A * ?B * ?C) =>
            rewrite <- (Ass A B C); try MainSimplif
        | (?A * ?B -> ?C) =>
            rewrite (Cur A B C); try MainSimplif
        | (?A -> ?B * ?C) =>
            rewrite (Dis A B C); try MainSimplif
        | (?A * unit) =>
            rewrite (P_unit A); try MainSimplif
        | (unit * ?B) =>
            rewrite (Com unit B); try MainSimplif
        | (?A -> unit) =>
            rewrite (AR_unit A); try MainSimplif
        | (unit -> ?B) =>
            rewrite (AL_unit B); try MainSimplif
        | (?A * ?B) =>
            (DSimplif A; try MainSimplif) || (DSimplif B; try MainSimplif)
        | (?A -> ?B) =>
            (DSimplif A; try MainSimplif) || (DSimplif B; try MainSimplif)
        end
       with MainSimplif :=
        match goal with
        | |- (?A = ?B) => try DSimplif A; try DSimplif B
        end.
DSimplif is defined
MainSimplif is defined

Coq < Ltac Length trm :=
        match trm with
        | (_ * ?B) => let succ := Length B in constr:(S succ)
        | _ => constr:1
        end.
Toplevel input, characters 107-108:
>   | _ => constr:1
>                ^
Syntax error: 'end' expected after [match_list] (in [tactic:tactic_expr]).

Coq < Ltac assoc := repeat rewrite <- Ass.
assoc is defined

Figure 10.6: Type isomorphism tactic (1)

---

---

Coq < Ltac DoCompare n :=
        match goal with
        | [ |- (?A = ?A) ] => reflexivity
        | [ |- (?A * ?B = ?A * ?C) ] =>
            apply Cons; let newn := Length B in
                        DoCompare newn
        | [ |- (?A * ?B = ?C) ] =>
            match eval compute in n with
            | 1 => fail
            | _ =>
                pattern (A * B) at 1; rewrite Com; assoc; DoCompare (pred n)
            end
        end.
Toplevel input, characters 138-144:
>       apply Cons; let newn := Length B in
>                               ^^^^^^
Error: The reference Length was not found in the current environment.

Coq < Ltac CompareStruct :=
        match goal with
        | [ |- (?A = ?B) ] =>
            let l1 := Length A
            with l2 := Length B in
            match eval compute in (l1 = l2) with
            | (?n = ?n) => DoCompare n
            end
        end.
Toplevel input, characters 80-86:
>       let l1 := Length A
>                 ^^^^^^
Error: The reference Length was not found in the current environment.

Coq < Ltac IsoProve := MainSimplif; CompareStruct.
Toplevel input, characters 30-43:
> Ltac IsoProve := MainSimplif; CompareStruct.
>                               ^^^^^^^^^^^^^
Error: The reference CompareStruct was not found in the current environment.

Figure 10.7: Type isomorphism tactic (2)

---

The tactic to judge equalities modulo this axiomatization can be written as shown on Figures 10.6 and 10.7. The algorithm is quite simple. Types are reduced using axioms that can be oriented (this done by MainSimplif). The normal forms are sequences of Cartesian products without Cartesian product in the left component. These normal forms are then compared modulo permutation of the components (this is done by CompareStruct). The main tactic to be called and realizing this algorithm is IsoProve.

Here are examples of what can be solved by IsoProve.