Space-monad

Option and Result monads for TypeScript.

Option

• Option()
• Option.all()
• Option.isOption
• None
• map
• flatMap
• filter
• fold
• orElse
• isDefined
• get
• getOrElse
• forEach
• contains
• exists
• toArray

Creating an Option

Option(x)

Creates an Option from a value. If the value is null or undefined, it will create a None, else a Some.

const some = Option(33) // some === Some(33)
const none = Option(null) // none === None

If you already know the value is defined for sure (not nullable) or not, you can create a Some or None directly:

const some = Some(33) // Some(null | undefined) wouldn't compile.
const none = None

Option.all([...optionsOrValues])

Creates a new Option holding the tuple of all the values contained in the passed array if they were all Some or non null/undefined values, else returns None

const some = Option.all([
  Option(10),
  20,
  Option(5)
])
// some === Some([10, 20, 5])

const none = Option.all([
  Option(10),
  None,
  Option(5),
  null
])
// none === None

Option.isOption

Returns whether the passed instance in an Option, and refines its type

import { Option, Some } from 'space-monad'
Option.isOption(Some(33)) // true

None

The Option constant representing no value.

import { None } from 'space-monad'

Transforming an Option

map

Maps the value contained in this Some, else returns None. Depending on the map function return value, a Some could be tranformed into a None, as a Some is guaranteed to never contain a null or undefined value.

const some = Option(33).map(x => x * 2)
// some === Some(66)

flatMap

Maps the value contained in this Some to a new Option, else returns None.

const some = Option(33).flatMap(_ => Option(44))
// some === Some(44)

filter

If this Option is a Some and the predicate returns true, keep that Some. In all other cases, return None.

const some = Option(33).filter(x => x > 32)
// some === Some(33)

fold

Applies the first function if this is a None, else applies the second function. Note: Since this method creates 2 functions everytime it runs, don't use in tight loops; use isDefined() instead.

const count = Option(10).fold(
  () => 100, // None
  count => count * 10 // Some
)

toArray

Transforms this option into an Array or either 1 or 0 element.

orElse

Returns this Option unless it's a None, in which case the provided alternative is returned.

const some = Option(null).orElse(() => Option(33))
// some === Some(33)

Misc

get

Some instances return their value, whereas None always return undefined. This method never throws.

const value = Some(33).get()
// value === 33

isDefined

Returns whether this Option has a defined value (i.e, it's a Some(value)) Note: this refines the type of the Option to be a Some so it's guaranteed its value is not null/undefined.

getOrElse

Returns this Option's value if it's a Some, else return the provided alternative

const value = Option(undefined).getOrElse(33)

// value === 33

forEach

Applies the given procedure to the option's value, if it is non empty.

Option(33).forEach(x => console.log(x))

contains

Returns whether this option is a Some that contain a specific value, using ===

Option(30).contains(30) // true

exists

Returns whether this option is a Some with a value satisfying the predicate.

Option(30).exists(n => n > 10) // true

Result

• Result, Ok, Err
• Result.isResult
• Result.all
• isOk
• map
• mapError
• flatMap
• fold

A Result is the result of a computation that may fail. An Ok represents a successful computation, while an Err represent the error case.

Importing Result

Here's everything that can be imported to use Results:

import { Result, Ok, Err } from 'space-monad'

const ok = Ok(10)
const err = Err('oops')

Result.isResult

Returns whether this instance is a Result (either an Ok or a Err) and refines its type

import { Result, Ok } from 'space-monad'

Result.isResult(Ok(10)) // true

Result.all

Creates a new Ok Result holding the tuple of all the values contained in the passed array if they were all Ok, else returns the first encountered Err.

import { Result, Ok, Err } from 'space-monad'

const result = Result.all([
  Ok(20),
  Err('nooo'),
  Ok(200),
  Err('oops')
]) // Err('nooo')

isOk

Returns whether this is an instance of Ok

import { Result, Ok, Err } from 'space-monad'

Ok(10).isOk() // true

map

Maps the value contained in this Result if it's an Ok, else propagates the Error.

import { Result, Ok, Err } from 'space-monad'

Ok(10).map(x => x * 2) // Ok(20)
Err(10).map(x => x * 2) // Err(10)

mapError

Maps the Error contained in this Result if it's an Err, else propagates the Ok.

import { Result, Ok, Err } from 'space-monad'

Ok(10).mapError(x => x * 2) // Ok(10)
Err(10).mapError(x => x * 2) // Err(20)

flatMap

Maps the value contained in this Result with another Result if it's an Ok, else propagates the Error. Note: It is allowed to return a Result with a different Error type.

import { Result, Ok, Err } from 'space-monad'

Ok(10).flatMap(x => Ok(x * 2)) // Ok(20)
Ok(10).flatMap(x => Err(x * 2)) // Err(20)

fold

Applies the first function if this is an Err, else applies the second function. Note: Don't use in tight loops; use isOk() instead.

import { Result, Ok, Err } from 'space-monad'

Ok(10).fold(
  err => console.error(err),
  num => num * 2
) // 20