#11555: catch _|_ breaks at -O1 -------------------------------------+------------------------------------- Reporter: slyfox | Owner: Type: bug | Status: new Priority: highest | Milestone: 8.0.1 Component: Compiler | Version: 8.0.1-rc2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect result | Unknown/Multiple at runtime | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): So consider {{{ f x = catch (g x) <recovery-code> }}} where `g` is strict in `x`; perhaps {{{ g x = if x>0 then <something> else <something else> }}} Now, are we allowed to use call-by-value on `f`? If 'no' then we are forced to use lazy evaluation and thunk creation in inner loops of the I/O system (for example). This can have a very material performance impact. Being able do to this sort of code motion is what "The semantics of imprecise exceptions" was all about and, combined with `unsafePerformIO`, you can indeed get unpredictable behaviour. There certainly ought to be a way of saying "evaluate this free variable right at this point",and there is: `evaluate` is just what you need. Indeed if you replace `abort "fail"` with `evaluate (abort "fail")` then the program behaves the same no matter how you compile it. But, I grant you, this makes my head hurt, and needs a careful writeup. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11555#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler