Re: [GHC] #10428: GHC cannot match representations using Coercible constraint

18 May 2015

      #10428: GHC cannot match representations using Coercible constraint
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        Reporter:  crockeea          |                   Owner:  goldfire
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by simonpj):

 Curious. With HEAD I get
 {{{
 T10428.hs:5:18: error:
     Couldn't match representation of type ‘o r’ with that of ‘n m' r’
     Inaccessible code in
       the type signature for:
       coerceNewtype :: Coercible (o r) (n m' r) => [o r] -> [n m' r]
     In the ambiguity check for the type signature for ‘coerceNewtype’:
       coerceNewtype :: forall (o :: * -> *) r (n :: * -> * -> *) m'.
                        Coercible (o r) (n m' r) =>
                        [o r] -> [n m' r]
     To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
     In the type signature for ‘coerceNewtype’:
       coerceNewtype :: (Coercible (o r) (n m' r)) => [o r] -> [n m' r]
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10428#comment:2
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