[GHC] #6150: Nested instances

[GHC]

#6150: Nested instances
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  Reporter:  exFalso           |          Owner:                              
      Type:  feature request   |         Status:  closed                      
  Priority:  normal            |      Milestone:                              
 Component:  Compiler          |        Version:  7.4.1                       
Resolution:  invalid           |       Keywords:  DataKinds, typeclass, nested
        Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple            
   Failure:  None/Unknown      |     Difficulty:  Unknown                     
  Testcase:                    |      Blockedby:                              
  Blocking:                    |        Related:                              
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Changes (by simonpj):

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


Comment:

 Lots of reasons not to do this:
  * You propose to give `printBool` this type
 {{{
 printBool :: Proxy (b :: Bool) -> IO ()
 }}}
  but it really is not as polymorphic as that, and the "free theorems"
 won't hold. The class restriction tells you that the polymorphism is not
 parametric.

  * GHC uses "evidence passing" to implement type classes. That means that
 even in your example, `printBool` must take a hidden value parameter,
 which is the dictionary of methods for `SomeClass`.  So if you suppressed
 it from the visible type, it would still have to be present in the real
 type.

  * In your example the instances occur right with the class declaration.
 But what if they don't. Then in some places you'd see it as exhaustive and
 in some not.

 Sorry, but I don't think this is a runner.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6150#comment:1>
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