[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 ekmett):

 Personally, I think that sounds like a pretty good fix.

 I'd want to float it past the libraries mailing list to see what issues
 folks can foresee, but literally none of the packages I maintain on
 hackage would have an issue with this change, but you already get a
 warning with -Wall for defaulting, and all programs that typechecked
 before would necessarily continue to typecheck, more defaulting would work
 by default in future code though.

 This interacts with the current libraries discussion on how to improve the
 rules about the shape of constraints (C1 a1, ...) to make it possible to
 support defaulting IsString when something like `length "hello"` on an
 OverloadedString happens.

 With appropriate rephrasing of the conditions on the constraints above, we
 may be able to consolidate support both of these scenarios.

 That said, I'm not sure off the top of my head how to rephrase that
 condition on the initial batch of constraints.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8171#comment:6>
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