[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 (Type    |                 Version:  7.8.3
  checker)                           |                Keywords:  type
      Resolution:                    |  family, ambiguity check
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:  #9607             |  Differential Revisions:
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Comment (by jstolarek):

 > one may well define Arr R' a b = String

 But the question is whether `Arr R' a b = String` is an acceptable
 definition? If yes, then of course injectivity is not a solution.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9587#comment:11>
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