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

[GHC]

#7939: RHS of associated type not kind-checked
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  Reporter:  goldfire                     |          Owner:  goldfire        
      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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Comment(by monoidal):

 Replying to [comment:11 goldfire]:

 > Now, {{{D}}} doesn't look GADT-like in its kind parameter, which is
 always just {{{k}}}. The difference between the version that works and the
 version that doesn't is that pattern-matching on the version that works
 does ''not'' refine information about the kind {{{k}}}. In fact, you can
 see this easily because you can write more instances for either {{{A}}} or
 {{{B}}} showing that the kind argument is not really fixed by this
 construction.

 I see, thanks. One last question - does anything change in your comment if
 I use a closed type family? Like this:

 {{{
 type family F :: k where
   F = Maybe

 x :: Maybe F
 x = Nothing
 }}}

 Should this be legal? While the type `F` has kind `k`, it works
 effectively as a synonym for `Maybe`, which is kind `* -> *`.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7939#comment:14>
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