[GHC] #12823: Inconsistency in acceptance of equality constraints in different forms

[GHC]

#12823: Inconsistency in acceptance of equality constraints in different forms
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        Reporter:  dfeuer            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:  GADTs
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 Replying to [comment:1 rwbarton]:
 > I assume `EqualF` should be `EqualS`.

 Indeed.

 > As long as we're allowing the full application of a multiparameter type
 class in a module with no extensions in the first place (strictly
 speaking, it should be a syntax error in Haskell 2010), I see no purpose
 in rejecting either of your functions in module `B`.

 My concern: `GADTs` implies `MonoLocalBinds` because (for reasons I don't
 personally understand) GADTs don't play well with let generalization.
 Since the `Equal` class and `ExistentialQuantification` are sufficient to
 simulate GADTs, I would expect them to run into the same inference
 fragility issues without `MonoLocalBinds`. Am I missing something?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12823#comment:2>
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