[mercury-users] variable parameters in typeclass instances

[Michael Day]

> That is unfortunate, it seems like such declarations would be quite
> useful, even in the absence of constructor classes. I can't speculate on a
> revised implementation that could handle such declarations so I will
> assume they are not feasible.

Just a quick follow up to my previous message. As I think Ralph suggested,
equivalent functionality to this:

:- typeclass container(C, T) <= arbitrary_constraint(T) where
	[
		pred insert(T, C(T), C(T)),
		mode insert(in, in, out) is det,
		...
	].

can simply be achieved by this:

:- typeclass container(C) where
	[
		pred insert(T, C(T), C(T)) <= arbitrary_constraint(T),
		mode insert(in, in, out) is det,
		...
	].

(Assuming support for constructor classes of course). Adding the arbitrary
constraint to each predicate in the type class is a little dissatisfying
but hardly a bar to such usage. I'm still wondering what it *means* for
some of the predicates to have the constraint and some not, but it all
seems to work. I still think it would be nice to be able to specify
universal type class instances, but it could be that I'm just wrong...

Michael

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