[GHC] #12240: Common Sense for Type Classes

[GHC]

#12240: Common Sense for Type Classes
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        Reporter:  Mathnerd314       |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Mathnerd314):

 I discussed the proposal with some people on IRC, and the consensus seems
 to be that, while nobody besides me thinks that implementing this is
 particularly useful (or possible), they would not immediately reject an
 implementation.

 I have started work on this in https://github.com/Mathnerd314/ghc. I am
 also planning to solve #11948 "correctly" in the same branch, as outlined
 in the OutsideIn(X) paper:

 > ... the behavior of the solving algorithm [with respect to dictionary
 instantiation] remains non-deterministic. This [can be fixed] by
 refraining from applying DINSTW if a local given could match with the
 wanted constraint (perhaps after instantiating unification variables)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12240#comment:5>
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