[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:                  
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Comment(by monoidal):

 I think it's fine to default to *. But, again, consider

 {{{
 type family A :: k
 type instance A = Double
 type instance A = Maybe
 }}}

 Should this really compile? I don't know, it looks suspicious but perhaps
 it is OK.

 What makes me suspicious is this. Consider

 {{{
 data D :: k -> * where
   D1 :: D Bool
   D2 :: D Maybe
 }}}

 GHC gives an error:

 {{{
     Data constructor `D1' cannot be GADT-like in its *kind* arguments
       D1 :: D * Bool
 }}}

 But this seemingly equivalent code compiles:

 {{{
 {-# LANGUAGE TypeFamilies, GADTs, PolyKinds #-}
 type family A :: k
 type instance A = Bool

 type family B :: k
 type instance B = Maybe

 data D :: k -> * where
   D1 :: D A
   D2 :: D B
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7939#comment:9>
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