Re: [GHC] #14131: Difference between newtype and newtype instance

17 Aug 2017

      #14131: Difference between newtype and newtype instance
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  TypeFamilies
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):

 Note that this error does not occur if the kind variables are bound in an
 earlier type pattern. That is, this equivalent instance typechecks:

 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE TypeFamilies #-}

 data family Nat :: k -> k -> *

 newtype instance Nat (f :: k -> *) :: (k -> *) -> * where
   Nat :: (forall xx. f xx -> g xx) -> Nat f g
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14131#comment:3
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