Maybe

Maybe is a type that wraps optional values. It can either be a Just (has some value) or a Nothing (has no value).

It's defined like this: type Maybe<A> = Just<A> | Nothing

In JavaScript, it is a better way of handling null and undefined. Instead of writing an if statement, ternary expression, or && shorthand to check if a value is present you can just map over it instead and assign the result of this expression or return it. You can also chain operations together, often leading to much cleaner code.

It's inspired by Haskell's Maybe, Swift's Optional and Scala's Option.

Installation

Using yarn

$ yarn add maybes

or npm

$ npm install maybes

Usage

Import the library:

import { maybe } from 'maybes'

or if you want everything:

import { maybe, just, nothing } from 'maybes'

Use the maybe function to create a Maybe from a value.

const value = maybe(1) // Just(1)

value.isJust() // true
value.isNothing() // false

Use map to transform the value inside the Maybe.

value.map(v => v + 1) // Just(2)

Force unwrap the value with just() if it is present. Warning: this will throw an Error if it is a Nothing (has no value).

value.just() // 1 (or throws Error)

Use the maybe function to wrap a possibly empty value.

const empty = maybe(null)

empty.isJust() // false
empty.isNothing() // true

empty.map(v => v + 1) // noop (No Operation)
empty.just() // throws error

Use orJust() to provide a default value.

empty.map(v => v.toUpperCase()).orJust('hello') // 'hello'

Use orElse() to provide a default already wrapped in a Maybe. This can be useful if you want to combine two or more Maybe's together.

const hello = maybe('hello')
empty.map(v => v.toUpperCase()).orElse(hello) // Maybe('hello')

Chain operations together using map:

const m = maybe('Maybe  ')
const result = m
  .map(v => v.trim())
  .map(v => v.toUpperCase()) // Just('MAYBE')

Use flatMap if you need to return a Maybe in your closure instead of the value. For example, when you want to explicitly return Nothing in a particular case.

const a = maybe('hi')
const result = a.flatMap(v => {
  if (v === 'hi') {
    return just('world')
  } else {
    return nothing
  }
})

just is a function like maybe that takes a value. nothing is a reference to Nothing.

Using filter is usually the best way to return a Nothing given a predicate. It returns Just only if there is a value and applying the predicate function to the Maybe's value returns true.

const name = maybe('  ')
const upper = name
  .map(v => v.trim())
  .filter(v => v.length != 0)
  .map(v => v.toUpperCase())

Use forEach when you would otherwise use map but can't or don't want to return a value. This is usually when you are causing a side effect. forEach returns void and so enforces it's the last in a chain. It runs only if there is a non empty value.

maybe('effect').forEach(s => console.log(s))

Types

This library uses Flowtype so you can also import the Maybe type and use its definition:

import { maybe } from 'maybes'
import type { Maybe } from 'maybes'

function getSomething(): Maybe<string> {
  return maybe('something')
}

Don't worry if you don't use Flowtype though as it gets stripped by Babel.

Comparison with vanilla JavaScript

Ternary function that optionally calls another function called transform.

Without Maybe

(value) ? transform(value) : null

(Safe version in case value is falsy, e.g. 0)

(value != null) ? transform(value) : null

With Maybe (handles falsy values like 0 and '' automatically).

maybe(value).map(transform)

&& and || shorthand

Without Maybe

const object: ?Object = {
  value?: 'hello'
}

object && object.value && object.value.toUpperCase() || ''

With Maybe

maybe(object).flatMap(o => maybe(o.value).map(v => v.toUpperCase())).orJust('')

With types already converted to Maybe's.

const object = maybe({
  value: maybe('hello')
})

object.flatMap(o => o.value.map(v => v.toUpperCase())).orJust('')

License

Maybe is available under the MIT license. See LICENCE.