#15349: fixST is a bit wrong -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: dfeuer Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Core Libraries | Version: 8.5 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: | -------------------------------------+------------------------------------- Changes (by dfeuer): * owner: (none) => dfeuer Comment: I raised this issue on the libraries list. Levent Erkök was [https://mail.haskell.org/pipermail/libraries/2018-July/028888.html first to respond]. He recognized that this bug leads to a violation of the `MonadFix` laws, and surprising behavior, but that it was harmless enough to get away with just documenting it. Then Arseniy Alekseyev came up with [https://mail.haskell.org/pipermail/libraries/2018-July/028889.html a more frightening example], in which this feature of `fixST` turns safe code using unsafe operations into unsafe code. That was enough to [https://mail.haskell.org/pipermail/libraries/2018-July/028892.html tilt Edward Kmett] toward fixing. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15349#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler