[GHC] #7939: RHS of associated type not kind-checked

[GHC]

#7939: RHS of associated type not kind-checked
------------------------------------------+---------------------------------
  Reporter:  goldfire                     |          Owner:                  
      Type:  bug                          |         Status:  new             
  Priority:  normal                       |      Milestone:                  
 Component:  Compiler                     |        Version:  7.7             
Resolution:                               |       Keywords:                  
        Os:  Unknown/Multiple             |   Architecture:  Unknown/Multiple
   Failure:  GHC accepts invalid program  |     Difficulty:  Unknown         
  Testcase:  ghci/scripts/T7939           |      Blockedby:                  
  Blocking:                               |        Related:                  
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Changes (by monoidal):

  * owner:  goldfire =>
  * status:  closed => new
  * resolution:  fixed =>


Comment:

 Something is suspicious here.

 GHC accepts original program

 {{{
 {-# LANGUAGE TypeFamilies, PolyKinds #-}

 class Foo a where
   type Bar a

 instance Foo Int where
   type Bar Int = Maybe

 instance Foo Bool where
   type Bar Bool = Double
 }}}

 but rejects

 {{{
 {-# LANGUAGE TypeFamilies, PolyKinds #-}

 type family Baz a
 type instance Baz Int = Maybe
 type instance Baz Bool = Double
 }}}

 Is this really expected? Even more, you can write

 {{{
 instance Foo Bool where
   type Bar Bool = Double
   type Bar Bool = Maybe
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7939#comment:4>
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