[GHC] #13946: 自動選擇實例

[GHC]

#13946: 自動選擇實例
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           Reporter:  zaoqi          |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{#!hs
 {-# LANGUAGE MultiParamTypeClasses #-}

 class B a b where
     b :: a -> b

 instance B Int Char where
     b = error "B Int Char"

 instance B Int () where
     b = error "B Int ()"
 }}}

 {{{
 *Main> :t (b undefined)::Char

 <interactive>:1:2: error:
     • Ambiguous type variable ‘a0’ arising from a use of ‘b’
       prevents the constraint ‘(B a0 Char)’ from being solved.
       Probable fix: use a type annotation to specify what ‘a0’ should be.
       These potential instance exist:
         instance [safe] B Int Char -- Defined at t.hs:6:10
     • In the expression: (b undefined) :: Char
 }}}
 怎麼讓GHC在這種情況下自動選擇任意一個實例

 [WARING]Google Translate
 How to make GHC automatically select any instance in this case

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13946>
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