Apply a function to the wrapped value if Ok and return a new Ok
containing the resulting value; or if it is Err return it
unmodified.
This differs from map in that thenFn returns another Result.
You can use andThen to combine two functions which both create a Result
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 Result.andThen will not unwrap
nested Results.
This is is sometimes also known as bind, but not aliased as such because
bind already means something in JavaScript.
import { ok, err, andThen, toString } from 'true-myth/result';
const toLengthAsResult = (s: string) => ok(s.length);
const anOk = ok('just a string');
const lengthAsResult = andThen(toLengthAsResult, anOk);
console.log(toString(lengthAsResult)); // Ok(13)
const anErr = err(['srsly', 'whatever']);
const notLengthAsResult = andThen(toLengthAsResult, anErr);
console.log(toString(notLengthAsResult)); // Err(srsly,whatever)
Apply a function to the wrapped value if
Okand return a newOkcontaining the resulting value; or if it isErrreturn it unmodified.This differs from
mapin thatthenFnreturns anotherResult. You can useandThento combine two functions which both create aResultfrom 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, whereasResult.andThenwill not unwrap nestedResults.This is is sometimes also known as
bind, but not aliased as such becausebindalready means something in JavaScript.Examples