Re: [GHC] #9569: Tuple constraints don't work right

8 Dec 2014

      #9569: Tuple constraints don't work right
-------------------------------------+-------------------------------------
              Reporter:  simonpj     |            Owner:
                  Type:  bug         |           Status:  closed
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.2
            Resolution:  fixed       |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
  polykinds/T9569,                   |
  typecheck/should_compile/T9569a    |
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------

Comment (by luite):

 The commit for this changes behaviour of impredicative types, the
 following does not typecheck anymore:

 {{{#!hs
 {-# LANGUAGE ImpredicativeTypes #-}

 f :: ((forall a. a -> a) -> b) -> b
 f x = x id

 g :: ((a -> b) -> c) -> ((a -> b) -> c)
 g = id

 h :: ((forall a. a -> a) -> b) -> b
 h = g f
 }}}

 I get the following error:

 {{{
 test.hs:10:7:
     Couldn't match expected type ‘forall a. a -> a’
                 with actual type ‘a0 -> a0’
     In the first argument of ‘g’, namely ‘f’
     In the expression: g f
 Failed, modules loaded: none.
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9569#comment:5
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler