[Javascript Crocks] Recover from a Nothing with the `coalesce` M
Thealt
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 thecoalesce
method on the Maybe to recover from a Nothing by calling a function.
When one would like tooption
aMaybe
but would like to remain within aMaybe
type,coalesce
can be used.coalesce
expects (2) functions for it's inputs.
The first function is used when invoked on aNothing
and will return aJust
instance wrapping the result of the function. The second function is used whencoalesce
is invoked on aJust
and is used to map the original value, returning a newJust
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: , _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)