Re: [GHC] #14190: Typeable imposes seemingly redundant constraints on polykinded instances

7 Sep 2017

      #14190: Typeable imposes seemingly redundant constraints on polykinded instances
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler (Type    |              Version:  8.2.1
  checker)                           |             Keywords:
      Resolution:                    |  TypeableReflection
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 The reason you need a `Typeable k` constraint is quite simple: `k` is just
 as much of a type as `a` or `b`, so in order to obtain a proper `TypeRep`
 for `Const`, you need to also obtain the `TypeRep`s for `a`, `b`, and `k`,
 requiring them all to be `Typeable` instances:

 {{{
 λ> typeRep :: TypeRep (Const String Type)
 Const * [Char] *
 λ> typeRep :: TypeRep (Const String (Nothing :: Maybe Bool))
 Const (Maybe Bool) [Char] ('Nothing Bool)
 }}}

 I don't see a bug here.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14190#comment:2
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