[m-dev.] Foreign type compare and unification

[Mark Brown]

On 21-May-2002, Ralph Becket <rafe at cs.mu.OZ.AU> wrote:
> Mark Brown, Tuesday, 21 May 2002:
> > It would be nice to provide a generic method implementation, along the
> > lines of the function pprint__to_doc in the standard library.  Perhaps
> > something like the following (note that, unlike ordinary instance
> > declarations, the parameter is a type variable):
> > 
> > 	:- generic_instance printable(T) where [
> > 		(to_doc(X) = pprint__to_doc(X))
> > 	].
> > 
> > This would _not_ imply that all types are members of the typeclass.  In
> > order to declare a type to be in the typeclass, we could use an ordinary
> > instance declaration, or we could use something like:
> > 
> > 	:- derived_instance printable(foo).
> > 
> > This would use the generic method, binding the hidden type parameter to
> > the type_info for foo.
> 
> Surely this is the same as providing default method implementations?

No.  The difference is that a default method implementation would be
used implicitly, whereas a derived instance of the above form must be
explicit.

Cheers,
Mark.

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