On Tue, Aug 3, 2010 at 3:06 PM, Gregory Crosswhite
But you've got it backwards: if the function I am calling can call back into Haskell (i.e., is marked as "safe"), then GHC *doesn't* block the world, but if the function I am calling will never call back into Haskell (i.e., is marked as "unsafe"), then GHC *does* block the world. The reasoning behind this choice of behaviors is exactly what I do not understand.
"unsafe" calls are fast but unsafe. If they call back to haskell, or take a long time, bad things will happen. "safe" calls are safer: They can call back to haskell, block on IO, or just wander off and do something CPU intensive. As is often the case, you pay a little bit of efficiency for the safety, since the RTS has to line up a few ducks to allow callbacks (and presumably concurrency). Just think of "unsafe" in relation to "unsafeIndex" or something. It's faster, but you have to be sure the index is in bounds.