Allowing Instances to Unify Types

[Simon Peyton-Jones]

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 at haskell.org [mailto:glasgow-haskell-users-
|  bounces at 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
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