[GHC] #11982: Typechecking fails for parallel monad comprehensions with polymorphic let (GHC 7.10.3 through 8.6.3)

[GHC]

#11982: Typechecking fails for parallel monad comprehensions with polymorphic let
(GHC 7.10.3 through 8.6.3)
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        Reporter:  simonpj           |                Owner:  josefs
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:  ApplicativeDo
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by josef):

 simonpj, I don't agree with how you characterized my comments. You wrote:

 > Second, for the program in comment:5, as josef points out in comment:11,
 the renamer rewrites the program to a form that really would > require
 full-on impredicative polymorphism to propagate f's polymorphism.

 I must have explained myself very poorly. The point I was trying to get
 across is that we **don't** need full-on impredicative polymorphism.
 Instead, I believe we can solve the problem for ApplicativeDo by simply
 changing the algorithm that ApplicativeDo uses to handle `let` statements,
 just as simonmar suggested.

 In fact, I think that this ticket should be split up so that we have a
 separate ticket for ApplicativeDo. The remedies for parallell monad
 comprehension and ApplicativeDo seem very different. I'll create a new
 ticket for ApplicativeDo unless someone yells.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11982#comment:16>
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