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.

Cons

• In the absence of this restriction, a type-preserving translation of a type system including either AssociatedTypes or FunctionalDependencies becomes much more difficult (see Section 4.3.4 of  Associated Types with Class, POPL'05, and ClassMethodTypes/TypedTrans). Even if we adopt neither of these two type system features for Haskell', we will impact future directions of Haskell.

Tickets

#17

relax restriction on signatures of class methods