The result of the thenFn (a new Maybe) if maybe is a
Just, otherwise Nothing if maybe is a Nothing.
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.
(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
Note that the result is not Just(Just(13)), but Just(13)!
The result of the thenFn (a new Maybe) if maybe is a
Just, otherwise Nothing if maybe is a Nothing.
Apply a function to the wrapped value if
Justand return a newJustcontaining the resulting value; or returnNothingifNothing.This differs from
mapin thatthenFnreturns anotherMaybe. You can useandThento combine two functions which both create aMaybefrom an unwrapped type.You may find the
.thenmethod on an ES6Promisehelpful for comparison: if you have aPromise, you can pass itsthenmethod a callback which returns anotherPromise, and the result will not be a nested promise, but a singlePromise. The difference is thatPromise#thenunwraps all layers to only ever return a singlePromisevalue, whereasMaybe.andThenwill not unwrap nestedMaybes.This is sometimes also known as
bind, but not aliased as such becausebindalready means something in JavaScript.Example
(This is a somewhat contrived example, but it serves to show the way the function behaves.)
Note that the result is not
Just(Just(13)), butJust(13)!