[GHC] #12240: Common Sense for Type Classes

[GHC]

#12240: Common Sense for Type Classes
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        Reporter:  Mathnerd314       |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ezyang):

 If you want refinement of this type to occur, you need to use a functional
 dependency, ala:

 {{{
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE FunctionalDependencies #-}
 module T4921 where

 class C a b | a -> b where
     f :: (a,b)

 instance C Int Char where
     f = undefined

 -- x = fst f
 y = fst f :: Int
 }}}

 Otherwise, a user can legally, WITHOUT `IncoherentInstances`, define
 another `instance C Int Int` which would cause the instance resolution to
 be ambiguous.

 Maybe we can make the error message better though.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12240#comment:3>
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