Re: [GHC] #9357: Kind-polymorphic type family accepts unlifted type arguments

21 Jan 2016

      #9357: Kind-polymorphic type family accepts unlifted type arguments
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        Reporter:  kosmikus          |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.8.2
  checker)                           |
      Resolution:  fixed             |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by bgamari):

 If Phab:D1807 is merged then this limitation will once again be lifted
 allowing. As it stands, this works,
 {{{#!hs
 {-# LANGUAGE PolyKinds TypeFamilies MagicHash DataKinds TypeInType #-}

 import GHC.Exts
 import GHC.Types

 class BoxIt (a :: TYPE 'Unlifted) where
     type Boxed a :: *
     boxed :: a -> Boxed a

 instance BoxIt Char# where
     type Boxed Char# = Char
     boxed x = C# x

 main :: IO ()
 main = print $ boxed 'c'#
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9357#comment:4
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