callwithcurrentcontinuation

Call-with-current-continuation, commonly abbreviated call/cc, is a control operator found in Scheme and other Lisp-family languages. It captures the current continuation as a first-class procedure, allowing non-local jumps by re-entering from that point.

Semantics: The expression (call-with-current-continuation f) evaluates f to a function k representing the current continuation. When f is executed, k is bound to the current continuation; calling k with a value v aborts the current computation and returns v to the original call/cc point, causing the entire (call-with-current-continuation f) expression to evaluate to v. The continuation can be invoked multiple times, potentially yielding multiple results, depending on the evaluation order and the language's semantics.

Usage: It enables non-local exits, early returns, exception-like signaling, backtracking, and the implementation of various continuation-based abstractions such as generators and coroutines. It is often used to implement complex control flows or to construct libraries that provide features built on top of direct continuation manipulation.

Alternatives and variants: The full form name call-with-current-continuation is used in Scheme; many implementations also provide call/cc as an alias. In other languages there are related primitives for continuations, including delimited continuations such as shift and reset, which capture only a portion of the control stack.

History and notes: Call-with-current-continuation has been a foundational feature of Scheme since its early development and has influenced many languages and programming techniques that rely on continuation-passing style. It is typically treated as a low-level primitive, with higher-level abstractions built on top of it.