The Parallel Patterns Library (PPL) relies on a continuation-passing style for asynchronous programming, where tasks are invoked and then linked to callable objects that process their results. This approach was the primary method for handling asynchronous operations before the introduction of await and co_await keywords in C#, JavaScript, and C++, making it essential to understand for developers working with older code or scenarios that still employ this style.

On Writing Loops in PPL and Continuation-passing Style, Part 1

By Raymond Chen

From the article:

The Parallel Patterns Library (PPL) is based on a continuation-passing style, where you invoke a task, and then attach a callable object that will be given the result. Prior to the introduction of the await and co_await keywords to C#, JavaScript, and C++, this was your only real choice for asynchronous programming.

Sequential calculations are fairly straightforward in continuation-passing style because you just pass the next step as the continuation.

// Synchronous version
auto widget = find_widget(name);
auto success = widget.toggle();
if (!success) report_failure();

// Asynchronous version
find_widget(name).then([=](auto widget) {
    return widget.toggle();
}).then([=](auto success) {
    if (!success) report_failure();
});

Iteration is harder to convert to continuation-passing style because you need to restart the task chain, which means you have recursion.