Matt I afraid I didn't understand your email well enough to offer a coherent response. For example I have no clue what "instance unifs" might mean. Nor do I understand what your program seeks to achieve. Thomas is right to say that the type checker is in upheaval at the moment. I'm actively working on it with Dimitrios (http://darcs.haskell.org/ghc-new-tc/ghc for the over-interested), but it'll be a month or two before it gets into HEAD. However the plan is to do so for the 6.14 release. Simon | -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users- | bounces@haskell.org] On Behalf Of Matt Brown | Sent: 23 July 2010 21:47 | To: glasgow-haskell-users | Subject: Allowing Instances to Unify Types | | 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