Re: [GHC] #13337: GHC doesn't think a type is of kind *, despite having evidence for it

26 Feb 2017

      #13337: GHC doesn't think a type is of kind *, despite having evidence for it
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.2
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Another way to phrase my question is: I don't see what the distinction
 between the first and second examples is. After all, you can rewrite the
 second example like this:

 {{{#!hs
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE TypeOperators #-}
 module Foo where

 import Data.Kind (Type)
 import Data.Typeable

 foo :: forall k (a :: k) proxy. (Typeable k, Typeable a)
     => proxy a -> String
 foo _ =
   case eqT :: Maybe (k :~: Type) of
     Nothing   -> "Non-Type kind"
     Just Refl -> go_on eqT
  where
   go_on :: Maybe (a :~: Int) -> String
   go_on Nothing     = "Non-Int type"
   go_on (Just Refl) = "It's an Int!"
 }}}

 Now the equality assumption isn't coming into scope "only in an expression
 of that type", it's coming into scope in a type signature! So again,
 unless I'm misunderstanding something, I strongly suspect that fixing the
 second program would naturally lend itself towards fixing the first
 program.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13337#comment:3
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