[GHC] #6102: Subclass Specialization in Rewrite Rules

[GHC]

#6102: Subclass Specialization in Rewrite Rules
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  Reporter:  SamAnklesaria     |          Owner:  SamAnklesaria   
      Type:  feature request   |         Status:  closed          
  Priority:  normal            |      Milestone:                  
 Component:  Compiler          |        Version:  7.4.1           
Resolution:  invalid           |       Keywords:                  
        Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
   Failure:  None/Unknown      |     Difficulty:  Unknown         
  Testcase:                    |      Blockedby:                  
  Blocking:                    |        Related:                  
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Comment(by SamAnklesaria):

 I don't think I expressed the feature request very well. The idea was that
 rewriting would only occur when the types matched. "willcompile" will only
 change classOneOp to specialFunc when the result is a ClassInstance.
 Likewise, I was thinking it would be useful to have "wontcompile" only
 change classOne to classTwoOp when the result is shown to be an instance
 of ClassTwo.
 In your example, one couldn't assume that the result of classOneOp
 (classOneOp a) was an instance of ClassTwo, and the rewrite wouldn't
 happen.

 I don't know if this kind of behavior is possible, but it isn't something
 that would produce ill typed expressions.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6102#comment:4>
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