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

[GHC]

#7939: RHS of associated type not kind-checked
------------------------------------------+---------------------------------
  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:                  
------------------------------------------+---------------------------------

Comment(by goldfire):

 This, too, is fine. `F` does have kind `k`, but it will only be a synonym
 to `Maybe` in contexts that expect a type of kind `* -> *`:

 {{{
 Prelude> type family F :: k where F = Maybe
 Prelude> :kind! F
 F :: k
 = forall (k :: BOX). F k
 Prelude> :kind! F Int
 F Int :: k
 = forall (k :: BOX). F (* -> k) Int
 Prelude> :kind! (F Int :: *)
 (F Int :: *) :: *
 = Maybe Int
 }}}

 So, I do think that beasts like `F` are strange, but I think (hope?) that
 a programmer is unlikely to stumble upon an `F` without knowing what they
 are doing.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7939#comment:15>
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