#9388: Narrow the scope of the notorious "state hack" -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- The "state hack" has caused any number of bug reports (just search for that string), the most recent of which is #9349. Here's an idea to make it less pervasive: (roughly) use the state hack only for top-level functions definitions. The idea is that for nested lambdas the context should give the one-shot- ness, now that we are equipped with cardinality analysis. For example, consider the call {{{ replicatM_ 1000 (\(s :: RealWorld#) -> blah) }}} The lambda is 1000-shot, not one-shot, notwithstanding the type of the binder. Moreover `replicateM_`'s strictness/cardinality signature will say just that, and GHC already knows how to propagate that information onto the `\s`. But for top level functions like {{{ pr :: String -> IO () pr x = putStrLn (reverse x) }}} we get Core {{{ pr = \x. let y = reverse x in \ (s :: State# RealWorld). putStrLn y s }}} and, since we can't see all the callers of `pr`, we don't know if work is lost by pushing the `reverse` call inside, to get {{{ pr = \x. (s :: State# RealWorld). putStrLn (reverse x) s }}} which is much more efficient. Indeed, this might not be so good if the calls looked like {{{ ... replicateM_ 1000 (pr "foo")... }}} because then "foo" will be reversed 1000 times. But arguably that's what the programmer expects anyway, looking at the code; and the efficiency hit from not eta-expanding all functions like `pr` (which are very very common) is significant. The point is the that the only ones that need hacking are the top-level guys, and maybe even the top-level ''exported'' guys. I have not fully thought this through, let alone tried it out, but I wanted to capture the thought. It would need some careful performance testing. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9388 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler