[GHC] #8030: FlexibleContexts PolyKinds Type Families bug

[GHC]

#8030: FlexibleContexts PolyKinds Type Families bug
--------------------------------------+-------------------------------------
Reporter:  wvv                        |          Owner:                  
    Type:  bug                        |         Status:  new             
Priority:  normal                     |      Component:  Compiler        
 Version:  7.6.3                      |       Keywords:                  
      Os:  Unknown/Multiple           |   Architecture:  Unknown/Multiple
 Failure:  GHC rejects valid program  |      Blockedby:                  
Blocking:                             |        Related:                  
--------------------------------------+-------------------------------------
 A bug with TypeFamilies + FlexibleContexts + PolyKinds

 {{{
 {-# LANGUAGE PolyKinds, FlexibleContexts, TypeFamilies #-}
 class Monoid (a :: k) where
         type Pr a :: *
         mempty :: Pr a
         mappend :: Pr a -> Pr a -> Pr a

 instance Monoid [b] where
         type Pr [b] = [b]
         mempty = []
         mappend = (++)
 }}}
 This is compilable.
 But this is not:
 {{{
 t :: (Monoid [b]) => b -> [b]
 t b = [b] `mappend` mempty
 }}}


 {{{
     Could not deduce ([b] ~ Pr k0 a0)
     from the context (Monoid * [b])
       bound by the type signature for t :: Monoid * [b] => b -> [b]
       at t.hs:26:6-29
     The type variables `k0', `a0' are ambiguous
     Possible fix: add a type signature that fixes these type variable(s)
     In the expression: [b] `mappend` mempty
     In an equation for `t': t b = [b] `mappend` mempty

     Could not deduce (Pr k1 a1 ~ [b])
     from the context (Monoid * [b])
       bound by the type signature for t :: Monoid * [b] => b -> [b]
       at test4.hs:26:6-29
     The type variables `k1', `a1' are ambiguous
     Possible fix: add a type signature that fixes these type variable(s)
     Expected type: Pr k0 a0
       Actual type: Pr k1 a1
     In the second argument of `mappend', namely `mempty'
     In the expression: [b] `mappend` mempty
     In an equation for `t': t b = [b] `mappend` mempty
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/8030>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler