[GHC] #9587: Type checking with type functions introduces many type variables, which remain ambiguous. The code no longer type checks.

[GHC]

#9587: Type checking with type functions introduces many type variables, which
remain ambiguous. The code no longer type checks.
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              Reporter:  oleg        |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
  (Type checker)                     |         Keywords:  type family,
            Resolution:              |  ambiguity check
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:  GHC         |       Blocked By:
  rejects valid program              |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by oleg):

 Correct: uncommenting the  line {-# LANGUAGE AllowAmbiguousTypes #-} does
 not change the behavior of the type checker for the code in question. And
 looking at the error message, I didn't think it would. The program is
 plainly rejected and I don't see what I can possibly do to get it accepted
 again. I have tried many annotations and small re-writings, to no avail.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9587#comment:2>
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