[GHC] #14824: automatically select instance

[GHC]

#14824: automatically select instance
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        Reporter:  zaoqi             |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by zaoqi):

 Replying to [comment:3 zaoqi]:
 > Replying to [comment:2 AntC]:
 > > I'm afraid we'll need a bit of elaboration to know what this means.
 > >
 > > What do you expect to see? Your instance declarations do not give
 implementations for method `m`.
 > >
 > > There are no `FunctionalDependencies` so all of those instances are
 valid.
 > If I write `b -> a` in that, `Type inference` can't work correctly.
 >
 > I can't write `a -> b`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14824#comment:4>
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