|Version 5 (modified by chak@…, 10 years ago) (diff)|
Class Method Types
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.
- 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.
- relax restriction on signatures of class methods