[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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Description changed by zaoqi:

Old description:

> There is
> {{{#!hs
> class C a b where m :: [a] -> b
> instance C X0 X0
> instance C X1 X2
> instance C X0 X3
> }}}
> FunctionalDependencies can't work.

New description:

 There is
 {{{#!hs
 {-# LANGUAGE OverlappingInstances, MultiParamTypeClasses #-}
 data X0 = X0 deriving Show
 data X1 = X1 deriving Show
 data X2 = X2 deriving Show
 data X3 = X3 deriving Show
 class C a b where m :: [a] -> b
 instance C X0 X0 where m _ = X0
 instance C X1 X2 where m _ = X2
 instance C X0 X3 where m _ = X3
 }}}
 X0 X1 X2 X3 is 4 types in real program.
 FunctionalDependencies can't work.
 {{{
 *Main> m [X1]

 <interactive>:6:1: error:
     • Ambiguous type variable ‘a0’ arising from a use of ‘print’
       prevents the constraint ‘(Show a0)’ from being solved.
       Probable fix: use a type annotation to specify what ‘a0’ should be.
       These potential instances exist:
         instance Show Ordering -- Defined in ‘GHC.Show’
         instance Show Integer -- Defined in ‘GHC.Show’
         instance [overlap ok] [safe] Show X0 -- Defined at a.hs:2:23
         ...plus 26 others
         ...plus 11 instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In a stmt of an interactive GHCi command: print it
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14824#comment:7>
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