coroutine

A coroutine is a computer program component that generalizes subroutines for non-linear control flow. Instead of always returning to the caller, a coroutine can suspend its execution at a designated point and later be resumed, possibly from another point in the program. This enables cooperative multitasking and the construction of asynchronous or incremental computations without relying on operating system threads. Coroutines preserve state across suspensions, typically by maintaining a separate call stack or a state machine.

Coroutines differ from threads in scheduling: coroutines yield control cooperatively, rather than the operating system preemptively interrupting execution. They are generally lighter-weight and cheaper to switch between. Implementations split into stackful coroutines (which save the full call stack) and stackless coroutines (which save the continuation in heap-allocated state).

Common uses include asynchronous I/O, streams processing, generators, backtracking, and pipeline stages. They simplify writing certain kinds of concurrent code by avoiding explicit callback structures. Language support varies; examples include Lua (built-in coroutines), Python (generators and the async/await syntax), Kotlin (suspend functions and coroutines), and C++20 (coroutines). JavaScript environments provide coroutine-like behavior through generators and the async/await pattern, enabling asynchronous programming in browsers and servers.

The concept originated in early programming languages such as Simula and Lisp dialects in the 1960s to support cooperative multitasking and generator-style control flow.