[GHC] #8030: FlexibleContexts PolyKinds Type Families bug

[GHC]

#8030: FlexibleContexts PolyKinds Type Families bug
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    Reporter:  wvv               |        Owner:                           
        Type:  bug               |       Status:  closed                   
    Priority:  normal            |    Component:  Compiler                 
     Version:  7.6.3             |   Resolution:  invalid                  
    Keywords:                    |           Os:  Unknown/Multiple         
Architecture:  Unknown/Multiple  |      Failure:  GHC rejects valid program
   Blockedby:                    |     Blocking:                           
     Related:                    |  
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Changes (by goldfire):

  * status:  new => closed
  * resolution:  => invalid


Comment:

 This looks like correct behavior to me.

 The problem is that the type signatures for `mempty` and `mappend` are
 inherently ambiguous. Because GHC cannot know whether `Pr` is injective,
 there is no way to get the identity of the type `a` from the identity of
 `Pr a`. Thus, all uses of `mempty` and `mappend` will be ambiguous. This
 ambiguity is what is causing the error you see.

 It's possible that the definitions of `mempty` and `mappend` as given
 should be rejected, but the error you're seeing looks correct to me.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/8030#comment:1>
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