[GHC] #9107: An alternative to witness values

[GHC]

#9107: An alternative to witness values
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        Reporter:           |            Owner:
  spacekitteh               |           Status:  new
            Type:  feature  |        Milestone:
  request                   |          Version:  7.9
        Priority:  normal   |         Keywords:
       Component:           |     Architecture:  Unknown/Multiple
  Compiler                  |       Difficulty:  Moderate (less than a day)
      Resolution:           |       Blocked By:
Operating System:           |  Related Tickets:
  Unknown/Multiple          |
 Type of failure:           |
  None/Unknown              |
       Test Case:           |
        Blocking:           |
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Comment (by spacekitteh):

 Such an extension would appear to be relatively straightforward to
 implement, by simply adding extra constraints to the typechecker at the
 call site, and removing the "The class method 'measure' mentions none of
 the type variables of the class Metric k a" error during compilation of
 the typeclass.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9107#comment:1>
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