[mercury-users] Existential type constraints

[Peter Hawkins]

Hi...

Why is this legal mercury?

:- typeclass c(T).
:- pred foo(T::in) => (c(T)).

Shouldn't it be illegal to add an existential type class constraint on 
an non-existentially typed variable?

(I just spent an enjoyable 15 minutes tracking down a missing 'some [T]' 
on a predicate declaration, and I wasn't helped by a misleading error 
message).

Thanks,
Peter
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