[GHC] #9730: Polymorphism and type classes

[GHC]

#9730: Polymorphism and type classes
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              Reporter:  mjaskel     |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
  (Type checker)                     |         Keywords:  Typeclass,
            Resolution:              |  Polymorphism, ImpredicativeTypes
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:  GHC         |       Blocked By:
  rejects valid program              |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by simonpj):

 I'm sorry but you really need impredicative polymorphism here, and GHC
 simply does not support it properly at the moment.  (The
 `ImpredicativeTypes` stuff is badly broken.)

 However in this case there's an easy workaround
 {{{
 a2c x = b2c (a2b x)
 }}}
 Now you don't need impredicativity

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9730#comment:1>
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