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

[GHC]

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

 Well, this works in 7.6.1:

 {{{
 data Ex = forall a. MkEx a

 type family F (t :: Ex) :: *
 type instance F (MkEx Int)  = Int
 type instance F (MkEx Bool) = String
 }}}

 And I don't see how it's dangerous or any different from this:

 {{{
 data Wrap = MkWrap Int

 f :: Wrap -> Int
 f (MkWrap 0) = 0
 f (MkWrap 1) = 42
 }}}

 The point is still that if the kind of the promoted constructor is
 {{{
 MkEx :: * -> Ex
 }}}
 then there's no actual existential on the type level. We've just created a
 wrapper for values of kind *. I'm ''not'' arguing that we should promote
 constructors that would get polymorphic kinds of the form
 {{{
 MkStrange :: forall k. k -> Ex
 }}}

 AFAIK, Pedro makes use of promoted existentials in at least one of his
 generic universe encodings. So it'd be nice if they keep working, unless
 there's actually a problem with them.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7347#comment:5>
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