Mastering JavaScript Functional Programming

Unexpected Monads - Promises

Let's finish this section on monads, by mentioning yet another one that you may have used, though under a different name: Promises! We already commented, earlier in this chapter, that functors (and remember, monads are functors) had at least something in common with promises: using a method in order to access the value. However, the analogy is greater than that!

• Promise.resolve() corresponds with Monad.of() -- if you pass a value to .resolve(), you'll get a promise resolved to that value, and if you provide a promise, you will get a new promise, whose value will be that of the original one (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/resolve for more on this). ...