[GHC] #9210: "overlapping instances" through FunctionalDependencies

[GHC]

#9210: "overlapping instances" through FunctionalDependencies
-------------------------------------------+-------------------------------
       Reporter:  rwbarton                 |             Owner:
           Type:  bug                      |            Status:  new
       Priority:  normal                   |         Milestone:
      Component:  Compiler (Type checker)  |           Version:  7.8.1
       Keywords:                           |  Operating System:
   Architecture:  Unknown/Multiple         |  Unknown/Multiple
     Difficulty:  Unknown                  |   Type of failure:
     Blocked By:                           |  None/Unknown
Related Tickets:                           |         Test Case:
                                           |          Blocking:
-------------------------------------------+-------------------------------
 This program prints `("1",2)`, but if you reverse the order of the two
 instances, it prints `(1,"2")`.

 {{{
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE FunctionalDependencies #-}

 -- extracted from http://lpaste.net/105656

 import Control.Applicative
 import Data.Functor.Identity

 modify :: ((a -> Identity b) -> s -> Identity t) -> (a -> b) -> (s -> t)
 modify l f s = runIdentity (l (Identity . f) s)

 class Foo s t a b | a b s -> t where
   foo :: Applicative f => (a -> f b) -> s -> f t

 instance Foo (x, a) (y, a) x y where
   foo f (a,b) = (\fa -> (fa,b)) <$> f a

 instance Foo (a, x) (a, y) x y where
   foo f (a,b) = (\fb -> (a,fb)) <$> f b

 main = print $ modify foo (show :: Int -> String) (1 :: Int, 2 :: Int)
 }}}

 Note that the two instances involved `Foo (Int, Int) (String, Int) Int
 String` and `Foo (Int, Int) (Int, String) Int String` are not actually
 overlapping. But, they have the same `a`, `b`, and `s` fields and it seems
 that this makes GHC think that either one is equally valid, thanks to the
 fundep.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9210>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler