[GHC] #2855: Surprising type (family) type derived

[GHC]

#2855: Surprising type (family) type derived
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    Reporter:  guest             |        Owner:                  
        Type:  feature request   |       Status:  new             
    Priority:  normal            |    Milestone:                  
   Component:  Compiler          |      Version:  6.10.1          
    Severity:  normal            |   Resolution:                  
    Keywords:                    |   Difficulty:  Unknown         
    Testcase:                    |           Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |  
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Comment (by guest):

 So what about my attached last example?  It's another one of these the
 type that ghc reports cannot be inserted in the code.  Very frustrating.

 You say "At what type should we call the poymorphic function x?"  Well, I
 want it to be the x from the dictionary (C a) that I'm passing.  So there
 is no doubt at what type it should be called, it's at type (U a).  I guess
 there's no way to express that.

 This is very annoying, because it means that the associated types simply
 don't work for some of the tasks that I thought they were supposed to
 solve.
 I can provide a real life example if that helps.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2855#comment:3>
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