[mercury-users] constructor class syntax

[Michael Day]

Hi,

making the assumption that Mercury will have support for constructor
classes in the not impossibly distant future, would it be reasonable to
expect syntax roughly like this:

:- class mappable(F(T)) where
	[
		pred map(pred(X, Y), F(A), F(B)),
		mode map(in, in, out) is det
	].

:- instance mappable(list(T)) where
	[
		pred(map/3) is list__map
	].

I'm assuming here that type classes are merely a special case of
constructor classes and that both can be declared in a single unified
syntax. With that in mind it seems easier to combine :- typeclass and a
hypothetical :- constructorclass into :- class, and save typing all round.

It seems constraints on constructors as with normal type classes would be
useful, but what about constraints on constructor arguments?

:- class mappable(F(T)) <= container(F) where
	[
		pred map(pred(X, Y), F(A), F(B)),
		mode map(in, in, out) is det
	].

I can't see any use of expressing constraints on T, at this point. Would
it in fact be desirable to rephrase the declaration as:

:- class mappable(F/1) ...

or can useful constraints be placed on the constructor arguments? Can
current type class declarations be seen as having an implicit zero arity:

:- typeclass ordered(T/0) ...

given that they're listed in instance declarations with all arguments
specified (fully curried?).

Any words of wisedom regarding the future direction of Mercury classes
syntax would be appreciated.

Michael


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