@thames/monads

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---

Option, Result, and Either types for JavaScript

🦀 Inspired by Rust

Zero dependencies • Lightweight • Functional

---

Install

``sh
npm install @thames/monads
`

Getting started

$3

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

> [!NOTE]
> Full documentation here: Option

`ts
import { Option, Some, None } from '@thames/monads';

const divide = (numerator: number, denominator: number): Option => {
if (denominator === 0) {
return None;
} else {
return Some(numerator / denominator);
}
};

// The return value of the function is an option
const result = divide(2.0, 3.0);

// Pattern match to retrieve the value
const message = result.match({
some: (res) => Result: ${res},
none: 'Cannot divide by 0',
});

console.log(message); // "Result: 0.6666666666666666"
`

$3

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

> [!NOTE]
> Full documentation here: Result

`ts
import { Result, Ok, Err } from '@thames/monads';

const getIndex = (values: string[], value: string): Result => {
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")
`

$3

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

`ts
import { Either, Left, Right } from '@thames/monads';

const divide = (numerator: number, denominator: number): Either => {
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"
``