[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 Mathnerd314):

 Functional dependencies imply too much structure. Consider

 {{{#!hs
 {-# LANGUAGE MultiParamTypeClasses #-}
 class C a b
 instance C Int Bool
 instance C Int Char
 instance C Word8 String
 instance C Word16 String
 }}}

 If we have `instance C a Bool`, we know that `a ~ Int`. But if we have
 `instance C Word8 b`, we know `b ~ String`. Functional dependencies cannot
 express this logic, but it is still "common sense logic".

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12240#comment:4>
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