[GHC] #2157: Equality Constraints with Type Families

[GHC]

#2157: Equality Constraints with Type Families
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    Reporter:  hpacheco                 |        Owner:  chak    
        Type:  feature request          |       Status:  new     
    Priority:  normal                   |    Milestone:          
   Component:  Compiler (Type checker)  |      Version:  6.9     
    Severity:  normal                   |   Resolution:          
    Keywords:                           |     Testcase:          
Architecture:  Multiple                 |           Os:  Multiple
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Comment (by hpacheco):

 Well, it seems to work if the add a "magic" dummy argument to fmapF.


 {{{
 class FunctorF x where
         fmapF :: d -> (a -> b) -> F x a -> F x b

 hylo :: (FunctorF d) => d -> (F d c -> c) -> (a -> F d a) -> a -> c
 hylo d g h = g . fmapF d (hylo d g h) . h
 }}}

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2157#comment:5>
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