Mastering Functional Programming

Delay and asynchronous computations

Imagine your program performs an HTTP call. For example, it tries to retrieve some JSON object from a web API. It can take some time; it may even take several seconds to complete.

Imagine that you want to make some competition within a function and return some value based on the result of the API request. You run into a problem here because the result of the API call was not available immediately. The fact that you need to wait for a specific long-running operation to complete is also a side effect.

You can block on this long-running operation and resume your competition once the result arrives. However, this function will also block any functions that are calling it. In the context of a performance-critical ...