[GHC] #8566: Given kind equalities are discarded (was: Panic with kindFunResult)

[GHC]

#8566: Given kind equalities are discarded
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        Reporter:  dreixel           |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.7
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:  polykinds/T8566   |       Blocked By:
        Blocking:                    |  Related Tickets:
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Changes (by simonpj):

 * status:  closed => new
 * resolution:  fixed =>


Comment:

 Hmm.  It occurs to me that with these GADTs it isn't just record selectors
 that are going to bad on us.  Consider
 {{{
 -- Proxy :: forall k. k -> *
 data T a where
   MkT :: Proxy b -> T (Proxy b)
 }}}
 So the real type of `MkT` is (putting in the kinds)
 {{{
   MkT :: forall u. forall k, b:k. (u ~ Proxy k b) => Proxy k b -> T u
 }}}
 Now that `k` is existential.  So pattern-matching is going to be
 problematic.  For example, if we write
 {{{
 f :: forall kc, c:kc. T (Proxy kc c) -> Proxy kc c
 f (MkT x) = x
 }}}
 we'll get a Given equality `Proxy kc c ~ Proxy k b`, where the 'k' and 'b'
 are existentially bound; since we can't make use of such Given equalities,
 we'll get obscure failures.

 I rather think that we should '''reject any type signature (including GADT
 ones) with an equality constraint that mentions a kind variable'''.  That
 would be more restrictive, but it is at least clear.  How bad would that
 be?

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8566#comment:8>
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