Re: [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 request   |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2
      Resolution:  invalid           |             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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Changes (by RyanGlScott):

 * status:  new => closed
 * resolution:   => invalid


Comment:

 This is behaving as expected. The issue is that when you type `(b
 undefined) :: Char`, GHC attempts to look for a `B a Char` instance. But
 `a` is underdetermined, so GHC cannot successfully find an instance.

 What you //can// do is use `FunctionalDependencies` to inform GHC that `b`
 uniquely determines `a`. That is, you can do this:

 {{{#!hs
 {-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE MultiParamTypeClasses #-}

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

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

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

 Note the use of `| b -> a` in the class declaration for `B`. This is the
 notation used to express that `b` uniquely determines `a`. Now, if you try
 this:

 {{{#!hs
 > :t (b undefined) :: Char
 (b undefined) :: Char :: Char
 }}}

 It works, since when GHC tries to find a `B a Char` instance, it knows
 that `Char` uniquely determines what `a` should be (in this case, `Int`),
 so it finds the `B Int Char` and proceeds.

 I'm going to close this as invalid - please reopen if this didn't answer
 your question in a satisfactory way.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13946#comment:2>
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