[GHC] #11982: Typechecking fails for parallel monad comprehensions with polymorphic let

[GHC]

#11982: Typechecking fails for parallel monad comprehensions with polymorphic let
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           Reporter:  simonpj        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{
 {-# LANGUAGE MonadComprehensions, ParallelListComp #-}

 module Foo where

 foo xs ys = [ (f y True, f x 'c')
             | let f _ z = z, x <- xs
             | y <- ys ]
 }}}
 This fails with
 {{{
 Foo.hs:5:52: error:
     * Cannot instantiate unification variable `t0'
       with a type involving foralls: forall t2 t3. t3 -> t2 -> t2
         GHC doesn't yet support impredicative polymorphism
     * In a stmt of a monad comprehension:
         let f _ z = z, x <- xs |  y <- ys
       In the expression:
         [(f y True, f x 'c') | let f _ z = z, x <- xs |  y <- ys]
       In an equation for `foo':
           foo xs ys
             = [(f y True, f x 'c') | let f _ z = z, x <- xs |  y <- ys]
 }}}
 NB: `ApplicativeDo` has a related problem: the implementation is quite
 different and has the effect of monomorphising the let-bound variable.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11982>
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