Function andThen

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.

Examples

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)
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Typeparam

T The type of the value wrapped in the Ok of the Result.

Typeparam

U The type of the value wrapped in the Ok of the Result returned by the thenFn.

Typeparam

E The type of the value wrapped in the Err of the Result.