[GHC] #12652: Type checker no longer accepting code using function composition and rank-n types

[GHC]

#12652: Type checker no longer accepting code using function composition and rank-n
types
-------------------------------------+-------------------------------------
        Reporter:  ezyang            |                Owner:  simonpj
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Richard is right on all counts.  This works
 {{{
 h = (.) @(M ()) @(M ()) @Int f g
 }}}
 recalling that `(.)` has type
 {{{
 (.) :: forall b c a. (b -> c) -> (a -> b) -> a -> c
         -- Defined in ‘GHC.Base’
 }}}
 It seems to me that if a special case for `($)` is justified, then having
 one for `(.)` as well would make sense.  Perhaps using it should require
 `-XImpredicativeTypes` though?

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12652#comment:5>
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