[Javascript Crocks] Recover from a Nothing with the `coalesce` Method

Posted 2020-11-05 Answer1215

The alt method allows us to recover from a nothing with a default Maybe, but sometimes our recovery efforts might need to enlist some logic to recover properly. In this lesson, we’ll see how we can use the coalesce method on the Maybe to recover from a Nothing by calling a function.

 

When one would like to option a Maybe but would like to remain within a Maybe type, coalesce can be used. coalesce expects (2) functions for it‘s inputs.

The first function is used when invoked on a Nothing and will return a Justinstance wrapping the result of the function. The second function is used when coalesce is invoked on a Just and is used to map the original value, returning a new Just instance wrapping the result of the second function.

 

The use case of ‘coalesce‘ is similar to ‘alt‘, but instead wrting two function as ‘alt‘ does:

const getArticleName = obj => prop(‘name‘, obj)
    .chain(safe(isNonEmptyString)) // If return Nothing then use alt value
    .alt(Maybe.of(‘page not found‘));

const getArticleUrl = obj => getArticleName(obj)
    .map(createUrlFromName)
    .option(‘default‘);

const url = getArticleUrl(article);

In ‘coalesce‘, we just combine in one single function:

const getArticleUrl = obj => prop(‘name‘, obj)
    .chain(safe(isNonEmptyString)) // If return Nothing then use first function of coalesce
    .coalesce(() => ‘page not found‘, createUrlFromName)
    .option(‘default‘);

const url = getArticleUrl(article);

 

const crocks = require(‘crocks‘)
const { identity ,and, compose, isEmpty, isString, Maybe, not, prop, safe } = crocks
const { join, split, toLower } = require(‘ramda‘)


const article = {
    id: 1,
    _name: ‘Learning crocksjs functional programming library‘
};

const createUrlSlug = compose(join(‘-‘), split(‘ ‘), toLower);
const createUrl = slug => `https://egghead.io/articles/${slug}`;
const createUrlFromName = compose(createUrl, createUrlSlug);
const isNonEmptyString = and(not(isEmpty), isString);


const getArticleUrl = obj => prop(‘name‘, obj)
    .chain(safe(isNonEmptyString)) // If return Nothing then use first function of coalesce
    .coalesce(() => ‘page not found‘, createUrlFromName)
    .option(‘default‘);

const url = getArticleUrl(article);

console.log(url)