On Jun 8, 2007, at 16:25 , Emilio Jesús Gallego Arias wrote:
Yeah, in general Haskell types don't carry constraints, however, I don't see the reason that this doesn't work when using type level macros, as
type F a = C a => a
should just be a macro and substitute.
It is. That's the problem. Macros can't know anything about the contexts in which they're used. In the type system, this translates to implicit "forall" constraints:
type F' a = forall a. C a => a
The result of this is that a simple use like
foo :: F a -> F a
expands the macros literally, with the implicit "forall" because the macro expansion has to assume it is independent of everything else:
foo' :: (forall a. C a => a) -> (forall a. C a => a)
This means the two "a"s are independent and can't be unified by the typechecker. In theory I suppose the simple macro expansion could be replaced by some type expression which would allow the typechecker to recognize that any (F a)s used in a given type can be unified, but I have no idea what it would look like. (Then again, I'm no type hacker.) And I find myself wondering if that would lead to problems when expanding complex types. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH