[GHC] #7347: Existential data constructors should not be promoted

[GHC]

#7347: Existential data constructors should not be promoted
---------------------------------+------------------------------------------
    Reporter:  simonpj           |       Owner:                  
        Type:  bug               |      Status:  merge           
    Priority:  normal            |   Milestone:                  
   Component:  Compiler          |     Version:  7.6.1           
    Keywords:                    |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown    
  Difficulty:  Unknown           |    Testcase:  polykinds/T7347 
   Blockedby:                    |    Blocking:                  
     Related:                    |  
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Comment(by simonpj):

 What if I declare `Ex` like this?
 {{{
 data KEx :: * where
   MkKEx :: forall (a::k). Proxy a -> KEx
 }}}
 Now the kind variable `k` as well as the type variable `a` is
 existentially quantified, and NOW we will have to worry about existential
 escape, notwithstanding your comment about "no phase separation".  Indeed
 I can't give a kind to `UnKEx`:
 {{{
 type family UnKEx :: KEx -> ???
 }}}
 So maybe what saves us here is that type families have user-specified kind
 signatures, and that in turn means we don't need to check for existential
 escape.

 So I think I agree with your point, but I don't know how urgent/important
 it is, nor how hard it would be to implement.  At its easiest it might
 mean just removing a restriction, but I'm not certain.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7347#comment:12>
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