What is call/cc?

Question

I\'ve tried several times to grasp the concept of continuations and call/cc. Every single attempt was a failure. Can somebody please explain me these concepts, ideally with

Answer 1

The thing that helped me is the idea that in a traditional language with function calls you implicitly pass a continuation any time you make a function call.

Before jumping to a function's code you save some state on the stack (i.e. you push your return address and the stack already contains your locals). This is essentially a continuation. When the function has finished it has to determine where to send the flow of execution. It uses the continuation stored on the stack, popping the return address and jumping to it.

Other languages generalise this idea of continuations allowing you to specify explicitly where to continue the code execution, rather than implicitly continuing on from where the function call was made.

EDIT based on comment:

The continuation is the complete execution state. At any point of execution you can divide the program into two parts (in time, not space) - that which has run to this point, and everything that's going to run from here. The "current continuation" is the "everything that's going to run from here" (you can think of it kind of like a function that will do everything the rest of your program would've done). So the function you supply to call/cc gets passed the continuation that was current when call/cc was invoked. The function can use the continuation to return execution to the call/cc statement (more likely though it'll pass the continuation around to something else, because if it used it directly it could do a simple return instead).