#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>
GHC <http://www.haskell.org/ghc/>
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