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Coroutine

Functions whose execution can be paused

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They have been described as "functions whose execution you can pause".

Melvin Conway coined the term coroutine in 1958 when he applied it to the construction of an assembly program. The first published explanation of the coroutine appeared later, in 1963.

Definition and types

There is no single precise definition of coroutine. In 1980 Christopher D. Marlin summarized two widely-acknowledged fundamental characteristics of a coroutine:

  1. the values of data local to a coroutine persist between successive calls;
  2. the execution of a coroutine is suspended as control leaves it, only to carry on where it left off when control re-enters the coroutine at some later stage.

Besides that, a coroutine implementation has 3 features:

  1. the control-transfer mechanism. Asymmetric coroutines usually provide keywords like yield and resume. Programmers cannot freely choose which frame to yield to. The runtime only yields to the nearest caller of the current coroutine. On the other hand, in symmetric coroutines, programmers must specify a yield destination.
  2. whether coroutines are provided in the language as first-class objects, which can be freely manipulated by the programmer, or as constrained constructs;
  3. whether a coroutine is able to suspend its execution from within nested function calls. Such a coroutine is a stackful coroutine. One to the contrary is called stackless coroutines, where unless marked as coroutine, a regular function can't use the keyword yield.

The paper "Revisiting Coroutines" published in 2009 proposed the term full coroutine to denote one that supports first-class coroutine and is stackful. Full Coroutines deserve their own name in that they have the same expressive power as one-shot continuations and delimited continuations. Full coroutines are either symmetric or asymmetric. Importantly, whether a coroutine is symmetric or asymmetric has no bearing on how expressive it can be, though full coroutines are more expressive than non-full coroutines. While their expressive power is the same, asymmetrical coroutines more closely resemble routine based control structures in the sense that control is always passed back to the invoker, which programmers may find more familiar.

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