#13615: Nondeterminism in ‘pure’ function w/ parallel evaluation & memo combinators -------------------------------------+------------------------------------- Reporter: pacak | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: 8.2.1 Component: Compiler | Version: 8.2.1-rc2 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 bgamari): I've put this aside for now. The current state of things is, * My current theory is that a closure is being entered multiple times, resulting in repeated observable mutation. It's still not entirely clear precisely which closure is being entered multiple times, but the evidence before us is consistent with the multiple mutation hypothesis. * compiling `unordered-containers` with `-feager-blackholing` appears to resolve the issue, however I'm pretty certain that this is merely shrinking the race window, not eliminating it altogether. * adding a `noDuplicate#` (e.g. see https://github.com/bgamari/unordered- containers/commit/bf799b2a00da8c78ac78d818de8f1963c7ff87ad) at the beginning of `unsafeInsertWith` also resolves the issue, even with lazy blackholing enabled. However, this seems like a very large hammer to place in an inner loop like this. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13615#comment:26 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler