[mercury-users] Re: default methods (was: boolean expressions from semidet predicates)

[David Overton]

On 20/05/06, Jonathan Morgan <jonmmorgan at gmail.com> wrote:> The point I am making is that a typeclass system with defaults allows> implementors to define two methods in terms of each other, leaving the> user to choose which of the two operations they define.  Sometimes> they will want to define both, if they can implement them more> efficiently.  This cannot be done with Mercury's typeclass-system, and> I don't see that it would hurt the type-system to include them.
You can get the equivalent of Haskell's default methods by doingsomething like this:
	:- typeclass eq_class(T) where [		pred eq(T::in, T::in) is semidet,		pred neq(T::in, T::in) is nondet	].
	:- pred default_neq(T, T) <= eq_class(T).	:- mode default_neq(in, in) is semidet.
	default_neq(A, B) :- not eq(A, B).

This provides a default implementation of neq/2 method which can beused by any instance, e.g.:
	:- instance eq_class(int) where [		pred(eq/2) is eq_int,		pred(neq/2) is default_neq	].
	:- pred eq_int(int::in, int::in) is semidet.	eq_int(A, B) :- A = B.
So the only real difference from Haskell is that you have to explicitly mentionthe use of the "default" method in the instance declaration.  But this is not abad thing, IMHO.
David
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