#13457: unsafeInterleaveST is too unsafe -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.4.1 Component: Core | Version: 8.1 Libraries | 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: -------------------------------------+------------------------------------- Several years ago, Edward Kmett wrote a [https://www.schoolofhaskell.com/school/to-infinity-and-beyond/older-but- still-interesting/deamortized-st#newsflash--unsafeinterleavest-is-unsafe- blog post] that (among other things) explains that `unsafeInterleaveST` is not thread-safe; actions may be performed multiple times. The fix that would seem to match everything else best would be to rename `unsafeInterleaveST` to `unsafeDupableInterleaveST` and define `unsafeInterleaveST m = unsafeDupableInterleaveST (noDuplicate >> m)` using an `ST`ish version of `noDuplicate`. As Kmett explains, the dupable version is useful for high performance when the suspended action is known to be idempotent. Actually, the action being idempotent is insufficient, but I believe a sufficient condition is that the action does not both read and write any reference or array element. While we're at it, we need to document all the functions in `Control.Monad.ST.Unsafe`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13457 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler