monas

Monas (from Greek μονάς - "singularity") is a monad library for JavaScript apps. It's been inspired by Scala and developed with TypeScript. Monas introduces two fundamental monads: Option<A> and Either<A, B>.

$ yarn add monas

Option<A>

Represents optional values. Instances of Option are either an instance of Some or the object None.

The idea is to get rid of null and undefined and, thus, eliminate null pointer exceptions, reduce branching (if statement) and produce better code.

Exported	Description
Option<A>	the base abstract class that implements utility functions for instances of classes Some and None. It's primarily used to indicate that a type has an optional value.
Some<A>	one of the possible implementations of Option<A> that wraps a value. The incapsulated value is available by the get() method.
None<A>	another implementation of Option<A> indicating absence of value.
some<A>(x: A): Option<A>	a helper function instantiating objects of type Some or None based on a provided value.
none: Option<A>	a single instance of None<A>.

let greeting: Option<string> = some('Hello world');
greeting = some(null); // will be none
assert(greeting === none);

some() will return none if a given argument is null or undefined:

Reading data from Option:

a) getOrElse():

let str: string = greeting.getOrElse(''); 
// Returns greeting or empty string if none.

b) get():

let str: string = greeting.get(); 
// Returns greeting or throws an exception.

c) Using Symbol.Iterable:

let [str]: string = [...greeting]; 
// returns ["something"] or empty array [] if none.

OR

for(let str: string of greeting) {
    assert(str, "something");
}

Examples

The most idiomatic way to use an Option instance is to treat it as a collection or monad and use map, flatMap, filter, or foreach.

Let's consider an example where for a given country code we need to find the country name or print "N/A" if it's not found.

import { Option, none, some } from './Option';

type Country = { name: string, code: number };
let countries: Country[] = [{ name: 'United States', code: 1 }, { name: 'United Kingdom', code: 44 }];

function getNameByCode(code: number): string {
    // find a country by code
    const country = countries.find(c => c.code === code);

    // if the country is not null return the name or N/A
    return some(country).map(_ => _.name).getOrElse('N/A');
    //     ^^^           ^^^ select name   ^^^ get a value if exists
    //     create Option<Country>              otherwise use 'N/A'
}

More examples could be found here.

Either<A, B>

Represents a value of one of two possible types (a disjoint union). An instance of Either is an instance of either Left or Right. Convention dictates that Left is used for failure and Right is used for success.

Exported	Description
Either<A, B>	the base abstract class that implements utility functions for instances of classes Left and Right.
Right<A, B>	a right "good" part.
Left<A, B>	a left "fail over" part, e.g. Error.
right<A, B>(x: B): Either<A, B>	a helper function instantiating Right objects.
left<A, B>(x: B): Either<A, B>	a helper function instantiating Left objects.

Generally Either can be considered as an alternative to Option where instead of None a useful information could be encapsulated into Left. It turns out that Either is a power-full type for validations since it can return either a successfully parsed value, or a validation error.

Either<A, B> can be instantiated by calling right(something) or left(error):

let eitherNumber: Either<Error, number> = right(42); // equivalent to new Right(42)

OR

let eitherNumber: Either<Error, number> = left(Error('Not a number')); // equivalent to new Left('Not a number')

Either is a right-biased monad:

let eitherNum: Either<number, Error> = right<number, Error>(42);
eitherNum.map(num => num * 2).getOrElse(-1);
// returns 84

Another example:

let eitherNum: Either<number, Error> = left<number, Error>(Error('Not a number.'));
eitherNum.map(_ => _ * 2).getOrElse(-1);
// returns -1

Use mapLeft(), foreachLeft() to handle Left values:

function print(numberOrError: Either<Error, number>) {
    numberOrError
        .map(num => num)
        .foreach(printNumber)
        .mapLeft(err => err.message)
        .foreachLeft(printError);
}

Use swap() function to swap Left and Right.

left<number, string>('Not a number').swap().getOrElse('');
// returns 'Not a number'

Either implements Symbol.Iterable:

let eitherNumber = right<number, Error>(42).map(_ => _ * 2);

let [n] = [...eitherNumber]; // n === 42

for(let num of eitherNumber) {
    assert(num, 42);
}

More examples could be found here.

Docs

• Medium Monas — Scala monads for javascript

License

MIT