Mastering JavaScript Functional Programming - Second Edition

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! Previously, we mentioned 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, the value of which 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). This is an unwrapping ...