Basically, think of a monad as a Promise. Rather, a Promise is more-or-less an example of a monad. (Yes, I know that due to some technicalities it isn't, but it

Basically, think of a monad as a Promise. Rather, a Promise is more-or-less an example of a monad. (Yes, I know that due to some technicalities it isn't, but it behaves like one for the purposes of this comment) Mapping a monad is equivalent to Promise#then - if there's a value in the monad, then it calls the function you passed to map and returns the result wrapped in a monad. If there isn't a value in the monad, then it returns itself. For example, with the Maybe monad, if you have x = Maybe.Some(y), then x.map(f) = Maybe.some(f(y)). If you have x = Maybe.None, then x.map(f) = Maybe.None. With Promises, if you have x = Promise.resolve(1234), then x.then(x => x * 2) = Promise.resolve(1234 * 2). If you have x = Promise.reject(new Error('abcd')), then x.then(x => x * 2) = Promise.reject(new Error('abcd')). The IO monad is extremely similar to Promises - you call an IO function and get an IO monad as a result, then map that monad in much the same way you'd then a promise.