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.

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