Just so Guido doesn't feel like the quesion is being ignored <wink>:
... How does Scheme do this? [continuations]
One more reference here. Previously sketched Wilson's simple heap implementation and Dybvig's simple stack one. They're easy to understand, but are (heap) slow all the time, or (stack) fast most of the time but horribly slow in some cases. For the other extreme end of things, check out: Representing Control in the Presence of First-Class Continuations Robert Hieb, R. Kent Dybvig, and Carl Bruggeman PLDI, June 1990 http://www.cs.indiana.edu/~dyb/papers/stack.ps In part: In this paper we show how stacks can be used to implement activation records in a way that is compatible with continuation operations, multiple control threads, and deep recursion. Our approach allows a small upper bound to be placed on the cost of continuation operations and stack overflow and underflow recovery. ... ordinary procedure calls and returns are not adversely affected. ... One important feature of our method is that the stack is not copied when a continuation is captured. Consequently, capturing a continuation is very efficient, and objects that are known to have dynamic extent can be stack allocated and modified since they remain in the locations in which they were originally allocated. By copying only a small portion of the stack when a continuation is reinstated, reinstatement costs are bounded by a small constant. The basic gimmick is a segmented stack, where large segments are heap-allocated and each contains multiple contiguous frames (across their code base, only 1% of frames exceeded 30 machine words). But this is a complicated approach, best suited for industrial-strength native-code compilers (speed at any cost -- the authors go thru hell to save an add here, a pointer store there, etc). At least at the time the paper was written, it was the approach implemented by Dybvig's Chez Scheme (a commercial native-code Scheme compiler noted for high speed). Given that Python allocates frames from the heap, I doubt there's a much faster approach than the one Christian has crafted out of his own sweat and blood! It's worth a paper of its own. or-at-least-two-hugs<wink>-ly y'rs - tim