#7347: Existential data constructors should not be promoted
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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 goldfire):
If you declare {{{KEx}}} as above, something weird happens when you
promote (in 7.6.1): the type {{{KEx}}} gets promoted to a kind, but the
kind-polymorphic data constructor {{{MkKEx}}} does not get promoted to a
type. So, {{{KEx}}} becomes an uninhabited kind. This behavior is weird,
but it seems not to violate any description of promotion: kind-polymorphic
things are not promoted, and other (suitable) things are.
So, one cannot write an {{{UnKEx}}} type instance, and thus there is no
problem.
I agree that this is far from urgent. But, if the checks you added to fix
this bug added complexity, they could perhaps be removed. I believe the
original implementation of naive promotion of existentials is the right
one.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7347#comment:13>
GHC <http://www.haskell.org/ghc/>
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