[GHC] #7026: Impredicative implicit parameters

[GHC]

#7026: Impredicative implicit parameters
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    Reporter:  Ashley Yakeley           |       Owner:                           
        Type:  bug                      |      Status:  new                      
    Priority:  normal                   |   Milestone:  _|_                      
   Component:  Compiler (Type checker)  |     Version:  7.4.2                    
    Keywords:                           |          Os:  Unknown/Multiple         
Architecture:  Unknown/Multiple         |     Failure:  GHC rejects valid program
  Difficulty:  Unknown                  |    Testcase:                           
   Blockedby:                           |    Blocking:                           
     Related:                           |  
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Comment(by Ashley Yakeley):

 A nicer alternative to using a reader monad transformer is to use an
 implicit parameter. In my Java VM project, I define this:

 type VM a = (?jvmenv :: VMEnv) => IO a

 So I can use this without any lifting. Ideally VM would act like a "real"
 type wherever, but I run into this problem with a function that returns
 "Maybe (VM t)".

 There are a couple of possible workarounds of course, Pulling the implicit
 parameter outside the "Maybe" might be acceptable, or else I can use some
 kind of wrapper.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7026#comment:2>
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