#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