[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:
                                     |  ExtendedDefaultRules
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #2641             |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by thomie):

 * keywords:   => ExtendedDefaultRules
 * related:   => #2641


Comment:

 Replying to [comment:5 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.

 See also #2641 (still open), where you proposed to make these rules
 //less// liberal, albeit 7 years ago.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8171#comment:12>
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