[GHC] #10494: Representational equalities over AppTys are not hard failures

5 Jun 2015

      #10494: Representational equalities over AppTys are not hard failures
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              Reporter:  goldfire    |             Owner:  goldfire
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.11
              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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 If I say

 {{{
 module App where

 import Data.Coerce

 foo :: Coercible (a b) (c d) => a b -> c d
 foo = undefined
 }}}

 I get

 {{{
 App.hs:5:8: error:
     Couldn't match representation of type ‘a b’ with that of ‘c d’
     Inaccessible code in
       the type signature for: foo :: Coercible (a b) (c d) => a b -> c d
     In the ambiguity check for the type signature for ‘foo’:
       foo :: forall (a :: * -> *) b (c :: * -> *) d.
              Coercible (a b) (c d) =>
              a b -> c d
     To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
     In the type signature for ‘foo’:
       foo :: Coercible (a b) (c d) => a b -> c d
 }}}

 This is very silly, indeed.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10494
GHC http://www.haskell.org/ghc/
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