[GHC] #9627: Type error with functional dependencies

[GHC]

#9627: Type error with functional dependencies
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              Reporter:  augustss    |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by simonpj):

 Indeed.  Here is a more direct example:
 {{{
 class C a b | a -> b

 instance C Int Bool

 f :: C Int b => b -> Bool
 f x = x
 }}}
 From which we get
 {{{
 Foo.hs:9:7:
     Could not deduce (b ~ Bool)
     from the context (C Int b)
 }}}
 Remember that GHC elaborates the program into System FC, generating
 evidence.
  * For type families, we know how to elaborate the program with proper,
 abstractable evidence in System FC
  * For functional dependencies I have no clue what evidence might look
 like.  All GHC does (and all Hugs does) is to generate extra unification
 constraints; think of them as hints to guide the choice of how to
 instantiate  unification variables.

 In the exmaple above there are no unification variables, so these hints do
 nothing, and the program is rejected.  It's the same with your larger
 example.

 If someone can explain how to elaborate functional dependencies into well-
 typed evidence in System FC, that would be good.  My inability to do so
 was one of the reasons we developed type families and System FC in the
 first place.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9627#comment:1>
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