wiki:ClassMethodTypes

Version 8 (modified by john@…, 8 years ago) (diff)

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Class Method Types

Brief Explanation

In Haskell 98, s4.3.1, the signatures of methods in a class may contain constraints, but these constraints must not mention the argument of the type class. The following is illegal:

class Foo a where
    op :: Num a => a -> a -> a

The motivation was perhaps that without such constraints, class dictionaries could be represented as records with PolymorphicComponents.

However this restriction is not implemented by Hugs, following a suggestion of Mark Jones in Typing Haskell in Haskell, and can be turned off in GHC with -fglasgow-exts.

interaction with existentials

If this were allowed than existential types of the form

data SomeFoo = exists a . Foo a => SomeFoo a

would have to carry around a dictionary for Num as well as Foo in dictionary passing implementation of type classes. In general some sort of fixpoint iteration would be needed to determine the set of dictionaries needed on an existential type.

This would not affect typecase based implementations of classes such as jhcs.

Cons

Tickets

#17
relax restriction on signatures of class methods