[mercury-users] existential constraints

[Michael Day]

Which of the following declarations is more correct:

:- all [S] some [T] ((pred foo(S, T) => c2(T)) <= c1(S)).
:- mode foo(in, out) is det.
:- mode foo(out, in) is det.

:- all [S] some [T] ((pred foo(T, S) => c2(T)) <= c1(S)).
:- mode foo(in, out) is det.
:- mode foo(out, in) is det.

I really have trouble making sense of the interaction between modes, type
class constraints and existential qualification.

Incidentally, will the 'new functor' syntax for existentially quanitifed
data go away any time soon?

Michael

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