[Javascript Crocks] Recover from a Nothing with the `alt` method

Once we’re using Maybes throughout our code, it stands to reason that at some point we’ll get a Maybe and want to continue applying transformations. We might get a Nothing and want to perform those same transformations on some default value. In this lesson, we’ll do exactly that! We’ll see how we can use the alt method on the Maybe to recover from a Nothing and continue applying transformations with a Just.

 

The use case is that you get a Nothing from previous transformation, but you wish not to stop with Nothing, you want oto provide a default Maybe value and continue the process.

 

const crocks = require('crocks')
const { 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 getArticleName = obj => prop('name', obj)
    .chain(safe(isNonEmptyString)) // If previous return Just(""), we want to continue check make sure it is not empty, other return Nothing
    .alt(Maybe.of('page not found')); // If return Nothing then use alt value

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

const url = getArticleUrl(article);

console.log(url)

 

We are also able to chain multi 'alt' together, it will return first Just():

const bad = Nothing()
const good = Just(42)
const anotherGood = Just(10)
console.log(Nothing()
    .alt(bad)) // Nothing

console.log(Nothing()
    .alt(bad)
    .alt(good)) // Just 42

console.log(Nothing()
    .alt(bad)
    .alt(good)
    .alt(anotherGood)) // Just 42

 

More example:

const Maybe = require('crocks/Maybe')
const alt = require('crocks/pointfree/alt')
const converge = require('crocks/combinators/converge')
const prop = require('crocks/Maybe/prop')

const {Just} = Maybe;

// maybeGetDisplay :: a -> Maybe b
const maybeGetDisplay = prop('display')

// maybeGetFirst :: a -> Maybe b
const maybeGetFirst = prop('first')

// maybeGetLast :: a -> Maybe b
const maybeGetLast = prop('last')

// maybeConcatStrings :: Maybe String -> Maybe String -> Maybe String
const maybeConcatStrings = x => y => Just(x => y => x + ' ' + y).ap(x).ap(y).alt(x).alt(y)

// maybeMakeDisplay :: a -> Maybe String
const maybeMakeDisplay = converge(maybeConcatStrings, maybeGetFirst, maybeGetLast)

// maybeGetName :: a -> Maybe b
const maybeGetName = converge(alt, maybeGetDisplay, maybeMakeDisplay)

console.log(maybeMakeDisplay({ display: 'Jack Sparrow' }))
//=> Just('Jack Sparrow')

console.log(maybeMakeDisplay({ first: 'J', last: 'S' }))
//=> Just('J S')

console.log(maybeMakeDisplay({ display: 'Jack Sparrow', first: 'J', last: 'S' }))
//=> Just('Jack Sparrow')

console.log(maybeMakeDisplay({ first: 'J' }))
//=> Just('J')

console.log(maybeGetName({ last: 'S' }))
//=> Just('S')