Function andThen

Apply a function to the wrapped value if Just and return a new Just containing the resulting value; or return Nothing if Nothing.

This differs from map in that thenFn returns another Maybe. You can use andThen to combine two functions which both create a Maybe from an unwrapped type.

You may find the .then method on an ES6 Promise helpful for comparison: if you have a Promise, you can pass its then method a callback which returns another Promise, and the result will not be a nested promise, but a single Promise. The difference is that Promise#then unwraps all layers to only ever return a single Promise value, whereas Maybe.andThen will not unwrap nested Maybes.

This is sometimes also known as bind, but not aliased as such because bind already means something in JavaScript.

Example

(This is a somewhat contrived example, but it serves to show the way the function behaves.)

import Maybe, { andThen, toString } from 'true-myth/maybe';

// string -> Maybe<number>
const toMaybeLength = (s: string) => Maybe.of(s.length);

// Maybe<string>
const aMaybeString = Maybe.of('Hello, there!');

// Maybe<number>
const resultingLength = andThen(toMaybeLength, aMaybeString);
console.log(toString(resultingLength)); // 13
Copy

Note that the result is not Just(Just(13)), but Just(13)!