Allowing Instances to Unify Types

[Thomas Schilling]

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 <matt at softmechanics.net> wrote:
> 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
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