[GHC] #7494: Allow compatible type synonyms to be the return type of a GADT data constructor.

[GHC]

#7494: Allow compatible type synonyms to be the return type of a GADT data
constructor.
-----------------------------+----------------------------------------------
Reporter:  topi              |          Owner:                         
    Type:  feature request   |         Status:  new                    
Priority:  normal            |      Component:  Compiler (Type checker)
 Version:  7.4.1             |       Keywords:  GADT                   
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple       
 Failure:  None/Unknown      |      Blockedby:                         
Blocking:                    |        Related:                         
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 Please consider adding support for something like:

 {{{
 {-# LANGUAGE GADTs #-}

 data Steps s y where
   Yield :: y -> FK s y -> FK s y
   Done  :: Steps s y

 newtype M y s a = M { unM :: (a -> FK s y) -> FK s y }
 type FK s y = s -> Steps s y
 }}}

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7494>
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