Re: [GHC] #15810: Kind inference error in classes

23 Nov 2018

      #15810: Kind inference error in classes
-------------------------------------+-------------------------------------
        Reporter:  goldfire          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D5305
       Wiki Page:                    |
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Comment (by simonpj):

 Hmph.  Given
 {{{
 class C a where
   meth :: Proxy (a :: k)
 }}}
 where is the implicit forall for that `k`?   Currently it's quantifed at
 the `::`, thus:
 {{{
 class C a where
   meth :: forall k. Proxy (a :: k)
 }}}
 And indeed that is ill-typed, and elicits
 {{{
 T15810.hs:9:18: error:
     • Expected kind ‘k’, but ‘a’ has kind ‘k0’
     • In the first argument of ‘Proxy’, namely ‘(a :: k)’
       In the type signature: meth :: Proxy (a :: k)
 }}}
 If you brought `k` into scope further out it'd be fine
 {{{
 class C a where
   meth :: Proxy (a :: k)
 }}}
 In short I say this is not a bug. Do you disagree?

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15810#comment:4
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