Probably a stack overflow which, unfortunately, causes Hugs to segfault.

I was suspicious of that, but I wasn't certain. However, this brings up what I was really after. In all cases I was using tail recursion, although the lazy versions still create control context proportional to n. (I have a few continuation passing interpreters I wrote that mysteriously overflow stack, I've been suspicious of unneeded laziness somewhere...) Although I've mostly been programming in functional programming languages (SML and Haskell) for the last 4 years, I'm still used to thinking that the stack is used for activation records. Then, what goes on the stack? I'll venture a guess that Hugs is keeping track of the current continuation on the stack... But in a lazy language control context can't always go on the stack... How can I, as a programmer, have a good guess when it does and doesn't? best, leon