[GHC] #7641: Incorrect reporting of overlapping instances

31 Jan 2013

      #7641: Incorrect reporting of overlapping instances
-----------------------------+----------------------------------------------
Reporter:  snoyberg          |          Owner:                  
    Type:  bug               |         Status:  new             
Priority:  normal            |      Component:  Compiler        
 Version:  7.6.1             |       Keywords:                  
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
 Failure:  None/Unknown      |      Blockedby:                  
Blocking:                    |        Related:                  
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 I've reproduced this issue on both 7.6.1 and 7.6.2. It does not exist on
 7.4.2.

 Given the following code (simplified from an actual case in Yesod):

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

 data Foo b = Foo deriving Show

 class ToFoo a b where
     toFoo :: a -> Foo b

 instance ToFoo (c -> ()) b where
     toFoo _ = Foo

 bar :: Foo ()
 bar =
     baz ()
   where
     baz () = toFoo $ \_ -> ()

 main :: IO ()
 main = print bar
 }}}

 This compiles and runs correctly with 7.4.2, producing the output "Foo".
 However, with 7.6.1 and 7.6.2 I get the following error message (identical
 between the two versions):

 {{{
 test.hs:16:5:
     Overlapping instances for ToFoo (t0 -> ()) b
       arising from the ambiguity check for `baz'
     Matching givens (or their superclasses):
       (ToFoo (t -> ()) b)
         bound by the inferred type for `baz':
                    ToFoo (t -> ()) b => () -> Foo b
         at test.hs:16:5-29
     Matching instances:
       instance ToFoo (c -> ()) b -- Defined at test.hs:9:10
     (The choice depends on the instantiation of `b, t0')
     When checking that `baz'
       has the inferred type `forall b t.
                              ToFoo (t -> ()) b =>
                              () -> Foo b'
     Probable cause: the inferred type is ambiguous
     In an equation for `bar':
         bar
           = baz ()
           where
               baz () = toFoo $ \ _ -> ()
 }}}

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7641
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