You might want to wait a bit until the new type checker has made it
into mainline. The ideas behind the new type checker are explained in
the paper linked from here:
http://www.haskell.org/haskellwiki/Simonpj/Talk:OutsideIn
Here's an earlier thread about the new type checker:
http://comments.gmane.org/gmane.comp.lang.haskell.cafe/77413
On 23 July 2010 21:47, Matt Brown
Hi all, I've been hacking on GHC for a couple months now, experimenting with some different ideas I find interesting. One thing I'm trying to do is allow instance unifs (when there's an unambiguous choice, a question which is simplified in this case by there being only one), and force the required unification. Here's a simple example:
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances #-} class Apply a b c | a b -> c where applyInst :: a -> b -> c
instance (Monad m) => Apply (a -> m b) (m a) (m b) where applyInst = (=<<)
apply :: (Monad m) => (a -> m b) -> (m a) -> (m b) apply = (=<<)
ioStr :: IO String ioStr = return "foo"
printStr :: String -> IO () printStr = print
main = do print `apply` (return "foo") printStr `applyInst` ioStr print `applyInst` (return "bar") -- this fails
With my code to use the unif instance enabled, I get Ambiguous type variable errors for "Show a" (from print) and "Monad m" (from return).
My question is: in the case of apply (which isn't implemented by a class), how does the typechecker propagate "a ~ String" and "m ~ IO" to the predicates for print and return? If someone (such as myself) had sufficient time and energy to spend trying to achieve similar behavior for applyInst, where might he/I start?
Thanks and Regards, -matt _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
-- If it looks like a duck, and quacks like a duck, we have at least to consider the possibility that we have a small aquatic bird of the family Anatidae on our hands.