[GHC] #5590: "guarded instances": instance selection can add extra parameters to the class

[GHC]

#5590: "guarded instances": instance selection can add extra parameters to the
class
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    Reporter:  nfrisby                  |       Owner:  simonpj         
        Type:  feature request          |      Status:  new             
    Priority:  normal                   |   Milestone:  7.6.1           
   Component:  Compiler (Type checker)  |     Version:  7.2.1           
    Keywords:                           |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple         |     Failure:  None/Unknown    
  Difficulty:                           |    Testcase:                  
   Blockedby:                           |    Blocking:                  
     Related:                           |  
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Comment(by nfrisby):

 Yet more motivation: guarded instances would simply the approach to
 "overcoming overlapping instances", as described by Oleg here. (Indeed,
 that's the same context in which I thought of the feature.)

   http://okmij.org/ftp/Haskell/typeEQ.html#without-over

 In particular, "guarded instances" would mitigate the need for "replica
 classes", such as his `ShowList'`.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5590#comment:4>
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