The semantics of many control constructs (such as exceptions and coroutines) can be expressed in terms of reified control stacks, a representation of a control stack as an ordinary value. This is achieved by allowing a stack to be passed as a value within a program and to be restored at a later point, even if control has long since returned past the point of reification. Reified control stacks of this kind are called continuations; they are values that can be passed and returned at will in a computation. Continuations never “expire,” and it is always sensible to reinstate a continuation without compromising safety. Thus continuations support unlimited “time travel”—we can go back to a previous point in the computation and then return to some point in its future, at will.

Why are continuations useful? Fundamentally, they are representations of the control state of a computation at a given point in time. Using continuations we can “checkpoint” the control state of a program, save it in a data structure, and return to it later. In fact, this is precisely what is necessary to implement threads (concurrently executing programs)—the thread scheduler must be able to checkpoint a program and save it for later execution, perhaps after a pending event occurs or another thread yields the processor.