#7411: Exceptions are optimized away in certain situations -------------------------------------+------------------------------------- Reporter: SimonHengel | Owner: tdammers Type: bug | Status: new Priority: high | Milestone: 8.6.1 Component: Compiler | Version: 7.6.1 Resolution: | Keywords: seq, deepseq, | evaluate, exceptions Operating System: Linux | Architecture: x86_64 | (amd64) Type of failure: Incorrect result | Test Case: at runtime | simplCore/should_fail/T7411 Blocked By: | Blocking: Related Tickets: #5129 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by dfeuer): I think the `-fno-state-hack` is almost certainly a clue, although I'm not exactly sure how it ends up relating in that specific example. What I ''suspect'' is happening (without actually tracing simplification) is roughly this: {{{#!hs evaluate (undefined `seq` return ()) -- ===> inline IO $ \s -> seq# (undefined `seq` IO (\s' -> (# s', () #))) s -- ===> simplify the coercions for my sanity IO $ \s -> seq# (IO $ undefined `seq` \s' -> (# s', () #)) s -- ===> This is where I suspect things go sideways. I believe we generally -- assume that it's okay to eta-expand IO things. IO $ \s -> seq# (IO $ \s'' -> (undefined `seq` (\s' -> (# s', () #))) s'') s }}} We've effectively turned a bottom with an `IO` type into a non-bottom `IO` value that only ''returns'' a bottom. Then the `seq#` ends up going away: {{{#!hs -- ===> beta reduce past seq IO $ \s -> seq# (IO $ \s'' -> undefined `seq` (# s'', () #)) s -- ===> PrelRules.seqRule says we can eliminate seq# for WHNF things IO $ \s -> (# s, IO $ \s'' -> undefined `seq` (# s'', () #)) }}} I imagine the behavior is a bit fragile because it depends on GHC not recognizing that the undefined value is in fact undefined, or at least not recognizing it too early. I suspect the solution is likely to make `-fpedantic-bottoms` go a little further than it currently does, although I don't know enough to say just what. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7411#comment:25 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler