duck-fp

Maybe Examples

.map()

const double = (num: number) => num * 2

M.of(42).map(double).map(console.log) // Output: 84

.mapNullable()

const getOrNothing = (num) => num > 2 ? null : num

const a = M.of(4).map(getOrNothing) // type is Maybe<number | null>
const a = M.of(4).mapNullable(getOrNothing) // type is Maybe<number>

.asyncMap() deprecated

const sleep = (ms: number): Promise<number> =>
  new Promise((res) => setTimeout(() => res(ms), ms));

M.of(1000)
  .asyncMap((ms) => sleep(ms))
  .then((maybeTime) => {
    maybeTime.map(console.log); // 1000 after 1 second
  });

.flatMap()

M.of(42).flatMap((num) => maybe(num + 5)).map(console.log) // 47

const getNumber = () => Math.random() > 0.5 ? 1 : undefined

maybe(42)
  .flatMap((num) =>
    fromNullable(getNumber()).map((secondNum) => secondNum + num),
  )
  .map(console.log) // 43 or nothing

.equals()

const a = M.of(24)
const b = M.of(24)
const c = M.of('Hello!')

console.log(a.equals(b)) // true
console.log(a.equals(c)) // false

.getOrElse()

const value = M.none<string>().getOrElse('default string') // value is 'default string'
const value1 = M.of<string>('some string').getOrElse('default string') // value is 'some string'

.orElse()

const value = M.none<number>().flatGetOrElse(M.of(42)) // value is 42
const value = M.of(53).flatGetOrElse(M.of(42)) // value is 42

.merge()

const a = M.of('something')
const b = M.of('something again')

const merged = a.merge(b).map(console.log) // { left: "something", right: "something again" }

if something doesn’t have value

const a = M.none<string>()
const b = M.of('something again')

const merged = a.merge(b).map(console.log) // nothing happens

fromNullable()

const getNumberOrUndefined = () => Math.random() > 0.5 ? 5 : undefined

const number = M.fromNullable(getNumberOrUndefined())

number.map(console.log) // 5 or nothing

merge()

const a = M.of(5)
const b = M.of('hello')
const c = M.of({someKey: 'someValue'})

const merged = M.merge(a, b, c) // type is Maybe<[number, string, {someKey: 'someValue'}]>

merged.map(console.log) // [5, 'hello', { someKey: 'someValue' }] or if some maybe is nothing, nothing going to happen

mergeMap()

const add = (a: number, b: number) => a + b

const a = M.of(5)
const b = M.of(100)

const final = M.mergeMap(a, b, add)

final.map(console.log) // 105 or if some maybe is nothing, nothing going to happen

call()

const mfn = M.of(() => console.log('hello world'))

mfn.map(call) // hello world

or()

const a = M.none<number>()
const b = M.of(5)
const c = M.of(6)

const final = M.or(a, b, c)

final.map(console.log) // 5

fromThrowable()

const throwable = (num: number) => {
  if (num > 0.5) {
    throw new Error('number is greater than 0.5')
  }

  return ':)'
}

const something = M.fromThrowable(throwable)

something(Math.random()).map(console.log) // :) or nothing

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