#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 NeilMitchell): Note that the second example involves no {{{unsafePerformIO}}}, so it's surprisingly semantics just with imprecise exceptions alone. My personal expectation is that yes, you would have to create a thunk for {{{g}}} - but my expectations rarely match the lazy exception semantics. If alternatively you argue that one valid interpretation is {{{catch _|_ x = _|_}}}, then why not define: {{{#!hs catch !a b = ... }}} I suspect having a lazy first argument never improves performance, and now it's always picking the same semantics at all optimisation levels. Note that in the above examples {{{evaluate}}} is a sufficient trick, but for Shake only because I reduced the example. In the real code it takes an argument of type {{{IO a}}}. I think a correct general workaround is: {{{#!hs safeCatch a b = catch (join $ evaluate a) b }}} But it would be good to get someone who fully understands the semantics to declare this a "safe" trick. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11555#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler