[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 AntC):

 Thank you zaoqi. The ticket is now very different to your first post.

 GHC is behaving as I would expect. It doesn't know which instance of `C`
 to select for the invocation of `m`. That's because it doesn't know the
 type of the result for `m [X1]`. You could go:

 {{{
 *Main> m [X1] :: X2
 }}}

 And get result `X2`, because GHC can select `instance C X1 X2`.

 That's similar to `read "5" :: Int` vs `read "5" :: Float` to disambiguate
 number formats.

 GHC does not automatically select an instance just because it seems to be
 the only one that fits: maybe there is another instance declared in
 another module that fits just as well.

 I could tell you about `FunctionalDependencies` combined with
 `OverlappingInstances` and `UndecidableInstances`. But first consider
 whether Closed `TypeFamilies` will provide a better approach.

 I can't see any fault. I can't see any "new feature" needed. Please do not
 use Trac tickets for help requests.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14824#comment:9>
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