Delimited continuation

A delimited continuation is a computational abstraction representing "the rest of the computation" up to a specified boundary — as opposed to an undelimited continuation, which captures the entire remaining program execution. The delimiter (often called a prompt or reset point) bounds the captured context, making delimited continuations composable and tractable in ways that their undelimited cousins are not.

Delimited continuations were introduced independently by Felleisen (as "prompts and control"), Danvy and Filinski (as "shift and reset"), and others. They have since become fundamental to the implementation of algebraic effects and handlers: when an effect operation is invoked, the handler captures the delimited continuation from the operation up to the nearest handler boundary, allowing it to resume, abort, or duplicate the remaining computation.

The connection to logic is deep: the delimiter corresponds to a cut in sequent calculus, and the capture/resume mechanism corresponds to the elimination rule for the double-negation monad. Delimited continuations thus sit at the intersection of programming language semantics, proof theory, and control theory — a position that explains their power but also their notorious difficulty for programmers unfamiliar with their non-local control flow.