Quoth John Meacham
It is more an accident of ghc's design than anything, the same mechanism that allowed threads to call back into the runtime also allowed them to be non blocking so the previously used 'safe' and 'unsafe' terms got re-used. personally, I really don't like those terms, they are non-descriptive in terms of what they actually mean and presuppose a RTS similar to ghcs current design. 'reentrant' and 'blocking' which could be specified independently would be better and would be more future-proof against changes in the RTS or between compilers.
Is the concurrency issue documented somewhere? What does `non blocking' mean, and why would it not just always be that way? In my situation, thread creation and dispatch happens in foreign library code, and execution in the Haskell runtime happens _only_ via callbacks. I don't need those callbacks to compute in parallel, generally, but it would be disappointing to hear that a callback strictly blocks execution of any others for its entire duration, for example even during a potentially slow I/O. (Will test for that, but not sure whether it would be conclusive since the system seems to be slightly broken at this point - need to disable RTS timer signals ( -V0 ) to survive externally generated thread dispatch events.) thanks, Donn Cave, donn@avvanta.com