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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
### The `Option` 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](https://thames-technology.github.io/monads/interfaces/Option.html)
```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"
```
### The `Result` type
Result represents a value that is either a success (Ok) or a failure (Err).
> [!NOTE]
> Full documentation here: [Result](https://thames-technology.github.io/monads/interfaces/Result.html)
```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")
```
### The `Either` 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](https://thames-technology.github.io/monads/interfaces/Either.html)
```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"
```