How to do "negative" match in Ltac?

Question

I want to apply a rule in a case when some hypothesis present, and another is not. How can I check for this condition?

For example:

Variable X Y : Prop.
Axiom A: X -> Y.
Axiom B: X -> Z.

Ltac more_detail :=
    match goal with
     |[H1:X,<not H2:Y>|-_]  =>
      let H' := fresh "DET" in assert Y as H'
                                   by (apply A;assumption)
     |[H1:X,<not H2:Z>|-_]  =>
      let H' := fresh "DET" in assert Z as H'
                                   by (apply B;assumption)
    end.

Such that, for this goal:

> Goal X->True. intros.

H:X
=====
True

more_detail. would introduce a second hypothesis DET:

H:X
DET:Y
DET0:Z
=====
True

And a successive invocation more_detail. would fail.

However more_detail. should always ensure, that both Y and Z are there, i.e. if only one of them present, it should run a rule for another:

Goal X->Y->True. intros.

H:X
H1:Y
=====
True

> more_detail.

H:X
H1:Y
DET:Z
=====
True

And:

> Goal X->Z->True. intros.

H:X
H0:Z
=====
True

> more_detail.

H:X
H0:Z
DET:Y
=====
True

Answer 1

This is a common Ltac pattern. You can use the fail tactic to avoid executing a branch when some condition matches:

Variable X Y Z : Prop.
Hypothesis A : X -> Y.
Hypothesis B : X -> Z.

Ltac does_not_have Z :=
  match goal with
  | _ : Z |- _ => fail 1
  | |- _ => idtac
  end.

Ltac more_detail :=
  match goal with
  | H : X |- _ =>
    first [ does_not_have Y;
            let DET := fresh "DET" in
            assert (DET := A H)
          | does_not_have Z;
            let DET := fresh "DET" in
            assert (DET := B H) ]
  end.

Goal X -> True.
intros X. more_detail. more_detail.
(* This fails *)
more_detail.
Abort.

The does_not_have tactic acts as a negative match: it only succeeds if its argument is not present in the context. Here's how it works: if H : Z is present in the context, the first branch will match. Calling simply fail or fail 0 would cause that branch to fail, but would allow Ltac to try other branches of the same match. Using fail 1 causes the current branch and the entire match to fail. If H : Z is not present in the context, the first branch will never match, and Coq will skip it and try the second branch. Since this branch doesn't do anything, execution will proceed with whichever tactics come after the match.

In more_detail, the first tactical can be used to combine several invocations of does_not_have; since each branch of first will fail if the context contains the corresponding hypothesis, the construction as a whole will have the effect of your match with negative patterns.