#11555: catch under unsafePerformIO breaks on -O1 to 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): The current title is inaccurate - this ticket is all about {{{catch _|_}}}, and has nothing to do with {{{unsafePerformIO}}}. The question is whether {{{catch _|_}}} should evaluate to {{{_|_}}} or {{{Left}}}. I observe that in all previous releases of GHC, and at {{{-O0}}}, it's been {{{Left}}}. In two applications that have excellent test suites and have upgraded to GHC 8.0RC2 this issue has been flagged as a breaking change, so I suspect that it might be relatively widespread. As to what it should do, it seems unfortunate to argue that {{{catch undefined}}} is itself undefined. What's the semantics of {{{catch (return () >> undefined)}}}? If it's not equivalent to {{{catch undefined}}} then it's problematic for the Monad laws. I think it's reasonable in a lazy language that a function only evaluates its arguments when it has to. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11555#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler