ghc hung by FunctionalDependencies/UndecidableInstances

[Simon Peyton-Jones]

No, it's behaving exactly as expected.  If you omit UndecidableInstances the program is rejected.  If you add that flag you are saying "you are allowed to diverge if I screw up".  And indeed you wrote a looping type problem.

I added some comments below that may help explain.

Simon


{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, UndecidableInstances #-}

module X() where

class C a b | a -> b where f :: a -> b
newtype T a = T a

instance (C a b, Eq b) => Eq (T a) where (==) = undefined

g x = (undefined :: d -> d -> d -> ()) (T x) (f x) (undefined :: Eq e => e)

  -- f :: C a b => a -> b
  -- x :: a
  -- b ~ T a
  -- C a b
  -- b ~ e
  -- Eq e

{- 
Hence need (C a (T a), Eq (T a))
Apply instance for Eq
     = (C a (T a), C a g, Eq g)
Apply functional dependency: g ~ T a
     = (C a (T a), C a (T a), Eq (T a))
And now we are back where we started
-}

| -----Original Message-----
| From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-
| users-bounces at haskell.org] On Behalf Of Roland Zumkeller
| Sent: 29 October 2009 04:55
| To: glasgow-haskell-users at haskell.org
| Subject: ghc hung by FunctionalDependencies/UndecidableInstances
| 
| Hi,
| 
| ghc seems to hang and eat memory when fed the following code:
| 
| {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,
| UndecidableInstances #-}
| class C a b | a -> b where f :: a -> b
| newtype T a = T a
| instance (C a b, Eq b) => Eq (T a) where (==) = undefined
| g x = (undefined :: a -> a -> a -> ()) (T x) (f x) (undefined :: Eq a => a)
| 
| Is this a bug?
| 
| Best,
| 
| Roland
| 
| --
| http://alacave.net/~roland/
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