[GHC] #8171: Extending ExtendedDefaultRules

[GHC]

#8171: Extending ExtendedDefaultRules
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        Reporter:  ekmett            |            Owner:
            Type:  feature request   |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.6.3
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by simonpj):

 The extended-default rules for a bunch of constraints (C1 a1, C2 a2, ...)
 are these:
  1. The type variable a appears in no other constraints
  2. All of the classes Ci are single-parameter type classes.
  3. At least one of the classes Ci is numeric, or is Show, Eq, or Ord.

 We could, I suppose, relax it by simply abolishing Rule 3.  That would
 certainly solve your problem, and would require little engineering effort.
 I don't know what the unintended consequences might be.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8171#comment:5>
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