Continuation¶
A continuation represents the rest of a program’s computation at a given point, reified as a callable value. Scheme’s call-with-current-continuation captures this rest of the program and hands it to the current function as an ordinary function value; invoking that captured continuation later, even from a completely different call frame, resumes execution exactly where it was captured, as many times as desired, effectively implementing arbitrary non-local control flow: early exits, generators, backtracking, or cooperative threads can all be built on top of it.
This is a strictly more general mechanism than exceptions or generators: a continuation can be invoked more than once, including after the function that captured it has already returned, which exceptions and generators cannot do.
PHP has no continuations and no call/cc equivalent. Non-local control flow is limited to a fixed set of built-in constructs: exceptions unwind the stack once and cannot be resumed, and Generator/Fiber can only be suspended and resumed by their own caller, in a strictly linear, single-shot fashion. There is no way to capture the current point of execution as a value and invoke it later, from anywhere else, as call/cc allows.
