Continuations are different than concurrency. A continuation is useful for modelling a process that operates independently. You may or may not want the contin

Continuations are different than concurrency. A continuation is useful for modelling a process that operates independently. You may or may not want the continuations to be running concurrently. One of the earliest uses of continuations in programming languages was in simulation and modelling. Imagine that you want to model a quarry. You have a pile of rocks, a rock crusher, a pile of gravel and a whole bunch of trucks. A truck takes a rock from the rock pile. it gives it to the crusher. The crusher works on the rock for a while and outputs some gravel. A truck takes the gravel to the gravel pile. You want to simulate different ways of scheduling the trucks so as to be optimal. You could write your simulation so that you update every object in your system at every time stamp, but that's really complicated and it also makes it really difficult to collect statistics about processes. Another way is to create a kind of "process" for each action in the system and a "resource" for each thing that could be used in the system. You then write the code for each process as a continuation and you request resources, blocking when they aren't available. Your overall simulation loop is very simple: it runs a single continuation until it requests a resource, and then it blocks. It then runs the next continuation until it requests a resource. Then it blocks. You just keep doing this, unblocking continuations when the resources become available (and all other continuations' simulation clocks are passed its current clock). The nice thing about this is that you need no concurrency at all -- or you can have a concurrent thread of execution for each continuation. The latter will obviously run faster, but that's not necessarily the point. The point is that the code was dramatically easier to write. For some kinds of simulation, a million continuations is not unreasonable at all. In fact, for a lot of applications it would be great if you could have a lot more.