[mercury-users] variable parameters in typeclass instances

Ralph Becket rbeck at microsoft.com
Thu Nov 4 01:19:36 AEDT 1999




> it does not appear to be possible to state the following:
> 
> % everything is an instance of the "pointless" typeclass:
> :- instance pointless(_).
> 
> % everything and int satisfy the "strange_relation" typeclass:
> :- instance strange_relation(int, T).
> 
> do these declarations have semantic problems that make them undesirable,
> or will they be supported in a future version of the compiler?

Er, why not just define polymorphic predicates for this sort of thing.
There's no need to use typeclasses here.

Ralph
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