RMS Typescript Monads

OneMany monad for TypeScript.

OneMany

I wrote the OneMany monad to work in conjunction with the Result monad, found in the npm rms-ts-monad package package.

I use the Result monad to capture the results of CRUD operations:

export type ResultOk = {
	code: number, // a REST/HTTP status code (200 series)
	payload: TBD // element or an element collection
}

export type CrudError = {
	code: number,
	content: string
}

export type CrudResult = Result<CrudError, ResultOk>;

In the above example, ResultOk.payload can be a single or multiple instances of an entity

• to hold the results of a CRUD operation to create a single entity, create(entity: Entity): Entity, ResultOk.payload must represent a single Entity;
• to hold the results of CRUD operations to read all entities, read(): Entity[], ResultOk.payload must represent an Entity collection.

The OneMany monad comes to the rescue, enabling us to rewrite ResultOk:

export type ResultOk = {
	code: number, // a REST/HTTP status code (200 series)
	payload: OneMany // element or an element collection
}

• OneMany, One, Many
• OneMany.isOneMany
• OneMany.all
• isOne
• map
• mapMany
• flatMap
• fold

Importing OneMany

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

import { OneMany, One, Many } from 'rms-ts-monad'

const one = One(10)
const many = Many([10, 20, 30])

OneMany.isOneMany

Returns whether this instance is a OneMany (either an One or a Many).

import { OneMany, One } from 'rms-ts-monad'

OneMany.isOneMany(One(10)) // true

OneMany.all

Creates a new One OneMany holding the tuple of all the values contained in the passed array if they were all One, else returns the first encountered Many.

import { OneMany, One, Many } from 'rms-ts-monad'

const result = OneMany.all([
  One(20),
  Many([10, 20, 30]),
  One(200),
  Many([40, 50, 60])
]) // Many([10, 20, 30]),

isOne

Returns whether this is an instance of One

import { OneMany, One, Many } from 'rms-ts-monad'

One(10).isOne() // true

map

If OneMany is a One, maps it content, else propagates the Many.

import { OneMany, One, Many } from 'rms-ts-monad'

One(10).map(x => x * 2) // One(20)
Many([10, 20, 30]).map(x => x * 2) // Many([10, 20, 30])

mapMany

If OneMany is a Many, maps it content, else propagates the One.

import { OneMany, One, Many } from 'rms-ts-monad'

One(10).mapMany(x => x * 2) // One(10)
Many([10, 20, 30]).mapMany(x => x * 2) // Many([20, 40, 60])

flatMap

Maps the value contained in this OneMany with another OneMany; if it's a One it propagates a One, otherwise a Many;

import { OneMany, One, Many } from 'rms-ts-monad'

One(10).flatMap(x => One(x * 2)) // One(20)
Many([10, 20, 30]).flatMap(x => Many(x * 2)) // Many([20, 40, 60])

fold

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

import { OneMany, One, Many } from 'rms-ts-monad'

One(10).fold(
  many => console.error(JSON.stringfy(many)),
  num => num * 2
) // 20


Many([10, 20, 30]).fold(
  many => console.error(JSON.stringfy(many)),
  num => num * 2
) // 10, 20, 30