[GHC] #2256: Incompleteness of type inference: must quantify over implication constraints

[GHC]

#2256: Incompleteness of type inference: must quantify over implication
constraints
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              Reporter:  simonpj     |            Owner:  simonpj
                  Type:  bug         |           Status:  new
              Priority:  lowest      |        Milestone:  7.10.1
             Component:  Compiler    |          Version:  6.8.2
  (Type checker)                     |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:  GHC         |  Related Tickets:
  rejects valid program              |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by simonpj):

 I agree that much of this is not hard. And it's certainly useful. Indeed
 it is explicitly discussed in our Haskell Workshop 2000 paper
 [http://research.microsoft.com/en-us/um/people/simonpj/papers/derive.htm
 Derivable Type Classes].

 The issues I see are:

  * Inference.  If users can write such instances, they will probably
 expect them to be inferred.  But that may wrongly defer a type error from
 a function to its call sites, by inferring an implication which is
 actually unsatisfiable.

  * Completeness.  "Given" constraints interact with each other.  How do
 "given" implications interact with simple "givens"?

  * Impredicativity.  Can a constraint variable `c` be instantiated with
 one of these implications? Perhaps it's just a question of ruling that
 out.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/2256#comment:15>
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