[GHC] #1749: forall type gives "not polymorphic enough" error incorrectly

[GHC]

#1749: forall type gives "not polymorphic enough" error incorrectly
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    Reporter:  guest                    |        Owner:         
        Type:  bug                      |       Status:  closed 
    Priority:  normal                   |    Milestone:         
   Component:  Compiler (Type checker)  |      Version:  6.6.1  
    Severity:  normal                   |   Resolution:  invalid
    Keywords:                           |   Difficulty:  Unknown
          Os:  Windows                  |     Testcase:         
Architecture:  x86                      |  
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Comment (by simonpj):

 [http://www.haskell.org/ghc/docs/latest/html/users_guide/type-
 extensions.html#scoped-type-variables] explains the way GHC does scoped
 type variables.  The two type sigs you thought are equivalent, aren't
 equivalent in this design.  Yes, that's odd, but I've been though several
 designs for scoped type variables and this seems the least bad.

 Scoped type variables, in some form, are ''essential'' else you simply
 can't write some type signatures, and that's unacceptable.  (Esp with
 higer rank types.)

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1749#comment:6>
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