Futures
Parallel collections offer a simple, yet powerful, framework for parallel operations. However, they are limited in one respect: the total amount of work must be known in advance, and each thread must perform the same function (possibly on different inputs).
Imagine that we want to write a program that fetches a web page (or queries a web API) every few seconds and extracts data for further processing from this web page. A typical example might involve querying a web API to maintain an up-to-date value of a particular stock price. Fetching data from an external web page takes a few hundred milliseconds, typically. If we perform this operation on the main thread, it will needlessly waste CPU cycles waiting for the web server to reply.
The solution is to wrap the code for fetching the web page in a future. A future is a one-element container containing the future result of a computation. When you create a future, the computation in it gets off-loaded to a different thread in order to...
