[neilv]

In case first-class continuations scare anyone away from Scheme: you probably will never use it directly in practice, unless you're doing something very unusual that happens to really need it.

For example, say you have a really hairy AI search algorithm, capturing a continuation happens to make backtracking easier.

Or you're implementing another language or DSL in Scheme, and you use first-class continuations to implement some control structure semantics exactly how you want them to be.

I think the closest I've used, in writing maybe a hundred modules, is an escape procedure (like a premature `return` statement in many other languages, which you generally avoid in idiomatic Scheme).

[jfoutz]

I’m having trouble recalling, call cc just fixes the stack, right? If you mess with the heap, that sticks around?

The backtracking example is a good one. I vaguely remember needing to be careful about global state, or visible in a given context. It’s not awful, but a little tricky.

[gpderetta]

There isn't necessarily a stack with call/cc, the model is that activation frames are heap allocated[1] and can be arbitrary composed. Normally they would be GC collected when a function returns, but call/cc exposes the caller activation frame (+ a function representing the rest of the caller function body) as a first call object, that once captured prevents it from being garbage collected. The stack is recovered by realizing that activation frames have a reference to the calling function activation frame (which is passed to every function as an implicit parameter).

Also an activation frame per se is immutable, but not the objects referenced by it, so all modifications are preserved when invoking a continuation.

[1] this is just a model, in practice there are many ways to implement this that optimize for the common FIFO allocation discipline.