How to debug tactic failure in a match goal branch?

Let's say I have some complex tactics in the body of a match goal branch that can easily go wrong in a way I might need to debug. Is there a way to get the "real" error message from the branch if some tactic fails, rather than simply getting "Error: No matching clauses for match goal"?

Take as an example this fake tactic where apply A, B, C has several chances for something to go wrong. I've been fighting with a real tactic somewhat similar to this today.

Ltac three_applications :=
    match goal with
        | [
            A : (* something reasonable *),
            B : (* something reasonable *),
            C : (* something reasonable *)
        |- _ ]  =>
            idtac A B C;
            assert (F: (* something reasonable *))
                by apply A, B, C;
            solve [discriminate F]
    end.

Thank you in advance!

Solution

The easiest is to use lazymatch goal instead but it has different semantics, but as I see you match only on one shape, it might be ok for you.

The idea is that match goal might backtrack: if you have

match goal with
| n : nat |- _ => destruct n ; reflexivity
| |- nat => exact 0
end.

it will first try to find some n : nat in your hypotheses, starting from the most recent, and then try the tactic destruct n ; reflexivity. If it fails, it will try to find another natural number. If all of them fail, then it will look if the goal matches the second clause instead, and if so execute exact 0. If that fails again, it will backtrack once again and conclude that No matching clauses for match goal.

On the other hand,

lazymatch goal with
| n : nat |- _ => destruct n ; reflexivity
| |- nat => exact 0
end.

will find the first branch that matches, and then apply the tactic, if it fails, no backtracking in the lazymatch, and it will give you the error of the tactic in the corresponding branch.

Note that I would always use lazymatch, not just for debugging purposes, when I don't find a use case for backtracking.

In case, lazymatch is not semantically equivalent to your match and backtracking needs to be involved then it will be harder, but indeed, using idtac A B C can help you see which branch it selected before launching the error. Sometimes, writing

Fail three_applications.

instead of just

three_applications.

will help because Fail will usually keep the printing that you made in the command, even if it failed in the end.

Once you know the branch, just apply your tactic manually.