monads

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Option, Result, and Either types for JavaScript

Inspired by Rust

Zero dependencies • Lightweight • Functional

Install

npm install @thames/monads

Getting started

The Option<T> type

Option represents an optional value: every Option is either Some and contains a value, or None, and does not.

Note

Full documentation here: Option

import { Option, Some, None } from '@thames/monads';const divide = (numerator: number, denominator: number): Option<number> => {
  if (denominator === 0) {return None;
  } else {return Some(numerator / denominator);
  }};// The return value of the function is an optionconst result = divide(2.0, 3.0);// Pattern match to retrieve the valueconst message = result.match({
  some: (res) => `Result: ${res}`,
  none: 'Cannot divide by 0',});console.log(message); // "Result: 0.6666666666666666"

The Result<T, E> type

Result represents a value that is either a success (Ok) or a failure (Err).

Note

Full documentation here: Result

import { Result, Ok, Err } from '@thames/monads';const getIndex = (values: string[], value: string): Result<number, string> => {
  const index = values.indexOf(value);

  switch (index) {case -1:      return Err('Value not found');default:      return Ok(index);
  }};const values = ['a', 'b', 'c'];getIndex(values, 'b'); // Ok(1)getIndex(values, 'z'); // Err("Value not found")

The Either<L, R> type

Either represents a value that is either Left or Right. It is a powerful way to handle operations that can result in two distinctly different types of outcomes.

Note

Full documentation here: Either

import { Either, Left, Right } from '@thames/monads';const divide = (numerator: number, denominator: number): Either<string, number> => {
  if (denominator === 0) {return Left('Cannot divide by 0');
  } else {return Right(numerator / denominator);
  }};const result = divide(2.0, 3.0);const message = result.match({
  left: (err) => `Error: ${err}`,
  right: (res) => `Result: ${res}`,});console.log(message); // "Result: 0.6666666666666666"