#12656: ghc floats out constant despite -fno-cse -------------------------------------+------------------------------------- Reporter: nh2 | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 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 rwbarton): First note that `main` (and thus `-fno-cse`) is a red herring, since the behavior under consideration is that of `let f = foo 100000 in f >> f` which does not mention `main`. But this is just normal Haskell laziness in action, which becomes clear if you desugar `foo`. {{{ foo :: Int -> IO () foo n = (>>=) (V.unsafeThaw $ V.generate n id) (...) }}} If you bind `foo 100000` to a name `f`, then `f`'s subexpression `V.generate 100000 id` will only ever be evaluated once during the lifetime of `f`. Moral: `unsafeThaw` is unsafe. What's more mysterious is why you get the "expected" behavior from compiled code. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12656#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler