[GHC] #8739: ($) returning kind # no longer type checks

5 Feb 2014

      #8739: ($) returning kind # no longer type checks
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       Reporter:  NeilMitchell      |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.8.1-rc1
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 Given:

 {{{
 {-# LANGUAGE MagicHash #-}
 module InnerLoop(go) where
 import GHC.Exts
 go :: () -> Int#
 go () = go $ ()
 }}}

 This type checks fine on GHC 7.2.3, and I have a similar version that
 works fine with 7.4 and 7.6. However, with GHC 7.8.20140130, I get:

 {{{
 InnerLoop.hs:5:9:
     Kind incompatibility when matching types:
       s0 :: *
       Int# :: #
     In the expression: go $ ()
     In an equation for `go': go () = go $ ()
 }}}

 This code was reduced from [http://neilmitchell.blogspot.co.uk/2014/01
 /optimising-haskell-for-tight-inner-loop.html this blog post], and code
 which triggers a similar bug can be
 [https://github.com/ndmitchell/shake/issues/94 found in Shake].

 I know {{{$}}} has special type checking rules, were they removed or
 changed?

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8739
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