#11760: runST with lazy blackholing breaks referential transparency -------------------------------------+------------------------------------- Reporter: Yuras | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonmar): Wow. Great bug. This should not happen. I believe the problem is specific to the lazy ST monad (`Control.Monad.ST.Lazy`), but your example relies on this so it's hard to test the theory. What happens is that the top-level runST thunk gets updated with `(x : xs)` where `xs` is itself a lazy ST computation that refers to mutable state. At this point it is possible for two threads to evaluate the inner thunk and get different results. We could use `noDuplicate#` in the lazy ST monad which should prevent this, but it will make lazy ST much more expensive. Maybe that's ok. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11760#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler