[GHC] #9882: Kind mismatch with singleton [Nat]

13 Dec 2014

      #9882: Kind mismatch with singleton [Nat]
-------------------------------------+-------------------------------------
       Reporter:  Roel van Dijk      |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.8.3
       Keywords:                     |        Operating System:  Linux
   Architecture:  Unknown/Multiple   |         Type of failure:  GHC
     Difficulty:  Unknown            |  rejects valid program
     Blocked By:                     |               Test Case:
Related Tickets:                     |                Blocking:
                                     |  Differential Revisions:
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 {{{#!hs
 {-# LANGUAGE DataKinds      #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE TypeOperators  #-}

 import GHC.TypeLits ( Nat )

 data Foo (n :: [Nat]) = Foo

 x :: Foo ('(:) 42 '[])
 x = Foo

 y :: Foo (42 ': '[])
 y = Foo

 z :: Foo [42]
 z = Foo
 }}}

 {{{
 Expected kind ‘*’, but ‘42’ has kind ‘Nat’
 In the type signature for ‘z’: z :: Foo [42]
 }}}

 I would expected z to be identical to both x and y. The error occurs in
 both GHCi and GHC.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9882
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