Re: [GHC] #10709: Using ($) allows sneaky impredicativity on its left

29 Jul 2015

      #10709: Using ($) allows sneaky impredicativity on its left
-------------------------------------+-------------------------------------
        Reporter:  goldfire          |                   Owner:  goldfire
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler (Type    |                 Version:  7.10.1
  checker)                           |
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
-------------------------------------+-------------------------------------

Comment (by rwbarton):

 From IRC discussion, something weird is happening that is not specific to
 `$`. All these type check:
 {{{
  (replicateM 2 . mask) (\_ -> return ())
  (replicateM 2 . mask) (\x -> undefined x)
  (replicateM 2 . mask) (id (\_ -> undefined))
 }}}
 but these do not:
 {{{
  (replicateM 2 . mask) (const undefined)
  (replicateM 2 . mask) ((\x -> undefined x) :: a -> b)
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10709#comment:1
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler