[GHC] #5600: Instantiation of a rank 2 type fails for type synonyms

[GHC]

#5600: Instantiation of a rank 2 type fails for type synonyms
---------------------------------+------------------------------------------
    Reporter:  paba              |       Owner:                         
        Type:  bug               |      Status:  new                    
    Priority:  normal            |   Component:  Compiler (Type checker)
     Version:  7.2.1             |    Keywords:                         
    Testcase:                    |   Blockedby:                         
          Os:  Unknown/Multiple  |    Blocking:                         
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown           
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 The instantiation of a rank 2 type with a type containing type synonyms
 that rearrange or omit type variables fails.

 For example, consider the following function:
 {{{
 mapR :: Functor f => (forall a. g a -> b) -> f (g a) -> f b
 mapR f t = fmap f t
 }}}

 Now we want to instantiate the type variable g in the above type:
 {{{
 data F a = F a
 type G a = F a

 mapG :: Functor f =>  (forall a. G a -> b) -> f (G a) -> f b
 mapG f t = mapR f t
 }}}

 This works fine. But if we change the definition of the type synonym G to
 {{{
 type G a = F ()
 }}}
 we get the following error message:
 {{{
 /home/paba/code/bug.hs:11:17:
     Could not deduce (a1 ~ ())
     from the context (Functor f)
       bound by the type signature for
                  mapG :: Functor f => (forall a1. G a1 -> b) -> f (G a) ->
 f b
       at /home/paba/code/bug.hs:11:1-19
       `a1' is a rigid type variable bound by
            a type expected by the context: F a1 -> b
            at /home/paba/code/bug.hs:11:12
     Expected type: F a -> b
       Actual type: G a0 -> b
     In the first argument of `mapR', namely `f'
     In the expression: mapR f t
 }}}

 This only happens for rank 2 types. If we change the type of mapR by
 removing the quantifier, the instantiation of the type of mapR succeeds.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5600>
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