[GHC] #10651: Type checking issue with existential quantification, rank-n types and constraint kinds

[GHC]

#10651: Type checking issue with existential quantification, rank-n types and
constraint kinds
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        Reporter:  Roboguy           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.10.1
  checker)                           |
      Resolution:                    |             Keywords:
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 Roboguy):

 It looks like the following works (on GHC 8.2.1 at least). Is this because
 the `Proxy b` makes it so that `b` is no longer untouchable? I also notice
 the explicit type annotation in the lambda argument is still necessary.

 {{{
 constrMapM :: Monad m => (forall a. c a => a -> m b) -> ConstrList c -> m
 [b]
 constrMapM = ...

 constrMapM_ :: forall m c b. Monad m =>
     (forall a. (c a) => a -> m b) -> Proxy b -> ConstrList c -> m ()
 constrMapM_ f Proxy x =
   constrMapM f x >>= (\(x1 :: [b]) -> return ())
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10651#comment:3>
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