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This library provides means for representing values that may or may not exist. This can come handy for representing optional arguments, error handling, and records with optional fields.


Library represents values that may or may not exist via Maybe<a> type. Existing value is repsented as Just<a> and absense as Nothing:

type Maybe <a> = 
  | Nothing
  | Just <a>

Given javascript / typescript semantics Nothing is just a type alias for union of primitives representing absence of value:

type Nothing = null | undefiend | void

For the same reason Just<a> is just a type alias for a:

type Just<a> = a

Chosen approach makes optional function argument & optional record fields of type a be fully compatible with Maybe<a>. There for all of the functions exposed are applicable.

This also avoids necessity to box existing values for the sake of typing.


import * as Maybe from "maybe.ts"

toValue(fallback:a, maybe:Maybe<a>):a

Provide a fallback value, turning an optional value into a normal value.

Maybe.toValue(5, Maybe.just(9)) // => 9
Maybe.toValue(6, Maybe.nothing) // => 6

map(f:(input:a) => b, maybe:Maybe<a>):Maybe<b>

Transform a Maybe value with a given function:, Maybe.just(9)) // => Maybe.just(3), Maybe.nothing) // => Maybe.nothing

chain(then:(input:a) => Maybe<b>, maybe:Maybe<a>):Maybe<b>

Utility to chain together two computations that may fail (return Nothing).

const makeGreeting = (name:string):string =>
  `Hello ${name}!`
const greet = (name:Maybe<string>):Maybe<string> =>
  Maybe.chain(makeGreeting, name)
greet(Maybe.nothing) // => Maybe.nothing
greet(Maybe.just('world')) // => Maybe.just('Hello world!')

and(left:Maybe<a>, right:Maybe<b>):Maybe<b>

Returns Nothing if the left Maybe is Nothing, otherwise returns the right Maybe.

Maybe.and(Maybe.just(2), Maybe.nothing) // => Maybe.nothing
Maybe.and(Maybe.nothing, Maybe.just('foo')) // => Maybe.nothing
Maybe.and(Maybe.just(2), Maybe.just('foo')) // => Maybe.just('foo')
Maybe.and(Maybe.nothing, Maybe.nothing) // => Maybe.nothing

or(left:Maybe<a>, right:Maybe<a>):Maybe<a>

Returns the left Maybe if it is a Just value, otherwise returns right Maybe.

Maybe.or(Maybe.just(2), Maybe.nothing) // => Maybe.just(2)
Maybe.or(Maybe.nothing, Maybe.just('foo')) // => Maybe.just('foo')
Maybe.or(Maybe.just(2), Maybe.just(100)) // => Maybe.just(100)
Maybe.or(Maybe.nothing, Maybe.nothing) // => Maybe.nothing

isJust(maybe:Maybe<a>):maybe is Just<a>

Predicate that refines given Maybe. If returns true given Maybe is refined to Just, otherwise refined to Nothing.

const name:Maybe<string> = Maybe.nothing
if (isJust(name)) {

isNothing = <a> (maybe:Maybe<a>):maybe is Nothing

Predicate that refines given Maybe. If returns true given Maybe is refined to Nothing, otherwise refined to Just

const name:Maybe<string> = Maybe.nothing
if (isNothing(name)) {
    console.error('No name')
} else {

just(value:a) => Maybe<a>

Turns arbitrary value of type a into Maybe<a>. Not that in most cases you dont need to do this and value a can be passed in place for Maybe<a>. Still sometimes it's convinent to downcast Just<a> (which is alias for a) to Maybe<a> and this function does just that:

Maybe.just(4) // => 4


This is just an exported null and you can use null, or undefined or null|undefined|void|a in place for Maybe<a> but sometimes code will read and communiacte intent a lot better when you do use explicit names.



yarn add --save maybe.ts


npm install --save maybe.ts

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