[mercury-users] Existential type constraints
Peter Hawkins
peter at hawkins.emu.id.au
Fri Jul 29 09:33:28 AEST 2005
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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