Coroutine

Coroutines are computer program components that allow execution to be suspended and resumed, generalizing subroutines for cooperative multitasking. Coroutines are well-suited for implementing familiar program components such as cooperative tasks, exceptions, event loops, iterators, infinite lists and pipes. 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. There is no single precise definition of coroutine. In 1980 Christopher D. Marlin summarized two widely-acknowledged fundamental characteristics of a coroutine: the values of data local to a coroutine persist between successive calls; 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: 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 coroutine, programmers must specify a yield destination. whether coroutines are provided in the language as first-class objects, which can be freely manipulated by the programmer, or as constrained constructs; whether a coroutine is able to suspend its execution from within nested function calls. Such a coroutine is stackful. One to the contrary is called stackless coroutine, where unless marked as coroutine, a regular function can't use keyword yield. Revisiting Coroutines published in 2009 proposed 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.