[GHC] #13178: Generalize type of runRW#

[GHC]

#13178: Generalize type of runRW#
-------------------------------------+-------------------------------------
        Reporter:  dfeuer            |                Owner:  dfeuer
            Type:  feature request   |               Status:  patch
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D3012
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by dfeuer):

 Yes. Consider `runST` for ''lazy'' `ST`. We currently have

 {{{#!hs
 runST :: (forall s. ST s a) -> a
 runST st = case st of ST the_st -> let (r,_) = the_st (S# realWorld#) in r
 {-# NOINLINE runST #-}
 }}}

 If we want to switch to `runRW#` using today's `runRW#`, we need to do
 something like this:

 {{{#!hs
 runST (ST st) =
   case runRW# (\s -> case st (S# s) of
                        (r, _s') -> (# s, r #)) of
     (# _, r #) -> r
 }}}

 That is, we need to pack up the result with a state token to satisfy the
 type of `runRW#`. Note that we'd use the token we got from `runRW#`
 because opening up the one `st` returned would force actions that may not
 be necessary. Then once we have the result from `runRW#`, we have to match
 on it again to remove the unneeded state token. With the more general
 type, we can write this as

 {{{#!hs
 runST (ST st) = runRW# (\s -> case st (S# s) of (r, _) -> r)
 }}}

 Much simpler, no?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13178#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler