[GHC] #6074: Instance inference failure with GADTs

[GHC]

#6074: Instance inference failure with GADTs
------------------------------+---------------------------------------------
 Reporter:  goldfire          |          Owner:                  
     Type:  bug               |         Status:  new             
 Priority:  normal            |      Component:  Compiler        
  Version:  7.5               |       Keywords:  GADTs           
       Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
  Failure:  None/Unknown      |       Testcase:                  
Blockedby:                    |       Blocking:                  
  Related:                    |  
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Comment(by dreixel):

 I think this is a feature request. Indeed michalt points out that this
 cannot work in the way presented. But what if we use `XDataKinds`?
 {{{
 data Index = TInt | TBool

 data GADT (a :: Index) where
   GInt  :: GADT TInt
   GBool :: GADT TBool


 class Stringable a where
   toString :: a -> String

 instance Stringable (GADT TInt)  where
   toString _ = "constructor GInt"
 instance Stringable (GADT TBool) where
   toString _ = "constructor GBool"


 mkString :: GADT a -> String
 mkString g = toString g
 }}}

 Now we know that the instances of `Stringable` for `GADT a` cover all
 possible types of `a`!

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6074#comment:2>
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