[mercury-users] type-equivalence for DUs

[doug.auclair at logicaltypes.com]

I wish to define my own equality for a simple DU:

:- type espresso ---> single(float); duo(float, float)
        where equality is coffee_goodness.

The compiler complains at my "ignore args" equality:

:- pred coffee_goodness(espresso::in, espresso:in) is semidet.
coffee_goodness(single(_), single(_)).
coffee_goodness(duo(_, _), duo(_, _)).

Saying that unification may fail.  I thought that's what
'semidet' meant.  Any rewrite of the above that I've
tried leads to the same error, e.g.:

coffee_goodness(A, B) :-
  A = single(_) -> B = single(_); B = duo(_, _).

The refman documentation isn't helping me any more ... what
am I doing incorrectly?

Sincerely,
Doug Auclair

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