#5129: "evaluate" optimized away -------------------------------------+------------------------------------- Reporter: dons | Owner: simonmar Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 7.0.3 Resolution: fixed | Keywords: seq, Operating System: Unknown/Multiple | evaluate Type of failure: Incorrect result | Architecture: at runtime | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by simonpj): A later note, in the light of [https://www.haskell.org/pipermail/ghc- devs/2015-January/007900.html this email thread]. Looking at the Haddock documentation for `evaluate`, Roman said: we seem to have three possible implementations for `evaluate`: {{{ 1) evaluate x = return $! x 2) evaluate x = (return $! x) >>= return 3) evaluate x = \s -> seq# x s }}} Now (1) is too strict: `(evaluate (error "urk") True)` should yield `True`. And, contrary to my claim in the email thread, (2) suffers from comment:2 above: inlining can mean that the argument to `evaluate` never gets evaluated. Consider, under (2) {{{ evaluate = \x -> (return $! x) >>= return = \x. \s. case return $! x s of (s1, r) -> return r s1 = \x s. x `seq` case (s,x) of (s1, r) -> return r s1 = \x s. x `seq` (s,x) = \x s. case x of _ -> (s,x) }}} Now consider the call in comment:2: {{{ evalutate poss_err >> assertFailure "foo"` = (inline >>) \s -> case evaluate poss_err of (s1, _) -> assertFailure "foo" s1 = (inline evaluate) \s -> case (case x of _ -> (s,x)) of (s1, _) -> assertFailure "foo" s1 = (case of case) case x of _ -> assertFailure "foo" s1 }}} and now, as explained in comment:2, since `assertFailure` diverges, GHC feel free to discard the case on `x`. Using a primop `seq# :: a -> State# -> (State#, a)` means that the I/O options that follows, which consumes the state that comes out of the `seq#` simply can't start until the `seq#` has executed. And that is what we want for `evaluate`. So only (3) will do. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/5129#comment:17 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler