Pattern Matching on Types and Contexts

In the previous article of our series on Ltac, we have shown how tactics allow for constructing Coq terms incrementally. Ltac programs (“proof scripts”) generate terms, and the shape of said terms can be very different regarding the initial context. For instance, induction x will refine the current goal using an inductive principle dedicated to the type of x.
This is possible because Ltac allows for pattern matching on types and contexts. In this article, we give a short introduction on this feature of key importance.
  1. To lazymatch or to match
  2. Pattern Matching on Types with type of
  3. Pattern Matching on the Context with goal
  4. Conclusion
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2020-08-28 Heavy reworking of the Ltac series a7e4531

To lazymatch or to match

Gallina provides one pattern matching construction, whose semantics is always the same: for a given term, the first pattern to match will always be selected. On the contrary, Ltac provides several pattern matching constructions with different semantics. This key difference has probably been motivated because Ltac is not a total language: a tactic may not always succeed.
Ltac programmers can use match or lazymatch. One the one hand, with match, if one pattern matches, but the branch body fails, Coq will backtrack and try the next branch. On the other hand, lazymatch will stop on error.
So, for instance, the two following tactics will print two different messages:

Ltac match_failure :=
  match goal with
  | _
    => fail "fail in first branch"
  | _
    => fail "fail in second branch"
  end.

Ltac lazymatch_failure :=
  lazymatch goal with
  | _
    => fail "fail in first branch"
  | _
    => fail "fail in second branch"
  end.

We can try that quite easily by starting a dumb goal (eg. Goal (True).) and call our tactic. For match_failure, we get:
Ltac call to "match_failure" failed.
Error: Tactic failure: fail in second branch.
On the other hand, for lazymatch_failure, we get:
Ltac call to "match_failure'" failed.
Error: Tactic failure: fail in first branch.
Moreover, pattern matching allows for matching over patterns, not just constants. Here, Ltac syntax differs from Gallina’s. In Gallina, if a variable name is used in a pattern, Gallina creates a new variable in the scope of the related branch. If a variable with the same name already existed, it is shadowed. On the contrary, in Ltac, using a variable name as-is in a pattern implies an equality check.
To illustrate this difference, we can take the example of the following tactic.

Ltac successive x y :=
  lazymatch y with
  | S x => idtac
  | _ => fail
  end.

successive x y will fails if y is not the successor of x. On the contrary, the “syntactically equivalent” function in Gallina will exhibit a totally different behavior.

Definition successor (x y : nat) : bool :=
  match y with
  | S x => true
  | _ => false
  end.

Here, the first branch of the pattern match is systematically selected when y is not O, and in this branch, the argument x is shadowed by the predecessor of y.
For Ltac to adopt a behavior similar to Gallina, the ? prefix shall be used. For instance, the following tactic will check whether its argument has a known successor, prints it if it does, or fail otherwise.

Ltac print_pred_or_zero x :=
  match x with
  | S ?x => idtac x
  | _ => fail
  end.

On the one hand, print_pred_or_zero 3 will print 2. On the other hand, if there exists a variable x : nat in the context, calling print_pred_or_zero x will fail, since the exact value of x is not known.

Pattern Matching on Types with type of

The type of operator can be used in conjunction to pattern matching to generate different terms according to the type of a variable. We could leverage this to reimplement induction for instance.
As an example, we propose the following not_nat tactic which, given an argument x, fails if x is of type nat.

Ltac not_nat x :=
  lazymatch type of x with
  | nat => fail "argument is of type nat"
  | _ => idtac
  end.

With this definition, not_nat true succeeds since true is of type bool, and not_nat O since O encodes 0 0 in nat.
We can also use the ? prefix to write true pattern. For instance, the following tactic will fail if the type of its supplied argument has at least one parameter.

Ltac not_param_type x :=
  lazymatch type of x with
  | ?f ?a => fail "argument is of type with parameter"
  | _ => idtac
  end.

Both not_param_type (@nil nat) of type list nat and (@eq_refl nat 0) of type 0 = 0 fail, but not_param_type 0 of type nat succeeds.

Pattern Matching on the Context with goal

Lastly, we discuss how Ltac allows for inspecting the context (i.e., the hypotheses and the goal) using the goal keyword.
For instance, we propose a naive implementation of the subst tactic as follows.

Ltac subst' :=
  repeat
    match goal with
    | H : ?x = _ |- context[?x]
      => repeat rewrite H; clear H
    end.

With goal, patterns are of the form H : (pattern), ... |- (pattern) .
In both cases, Ltac makes available an interesting operator, context[(pattern)], which is satisfies if (pattern) appears somewhere in the object we are pattern matching against. So, the branch of the match reads as follows: we are looking for an equation H which specifies the value of an object x which appears in the goal. If such an equation exists, subst' tries to rewrite x as many time as possible.
This implementation of subst' is very fragile, and will not work if the equation is of the form _ = ?x, and it may behaves poorly if we have “transitive equations”, such as there exists hypotheses ?x = ?y and ?y = _. Motivated readers may be interested in proposing more robust implementation of subst'.

Conclusion

This concludes our tour on Ltac pattern matching capabilities. In the next article of this series, we explain how Ltac and Gallina can actually be used simultaneously.