@snowfrog/result

Type-safe error handling without exceptions.

Install

npm install @snowfrog/result

Usage

Errors are a fact of life in software development. In JavaScript, errors are usually thrown using the throw keyword and this approach makes sense when something bad happens in your code and there's nothing you can do about it. This most commonly occurs when a bug of some kind has been detected and it's not clear to the programmer how to handle the error.

Most errors, though, aren't serious enough to require the program to stop entirely. Sometimes, when a function fails, it's for a reason that you can easily interpret and respond to. That's where Result comes in. It makes handling these common errors type-safe and explicit.

Result<T, E> is a type that represents a value that may or may not have succeeded. It's an abstract class with the implementations,Ok<T>, representing success and containing a value, and Err<E>, representing an error and containing an error value.

Functions should return Result whenever errors are expected and recoverable.

Method overview

Result comes with some convenience methods.

import { Ok, Err, Result} from '@snowfrog/result';

let goodResult: Result<number, number> = new Ok(10);
let badResult: Result<number, number> = new Err(10);

// The `isOk` and `isErr` methods do what they say.
expect(goodResult.isOk() && !goodResult.isErr()).toBe(true);
expect(badResult.isErr() && !badResult.isOk()).toBe(true);

// `map` consumes the `Result` and produces another.
goodResult = goodResult.map((x) => x + 1);
badResult = badResult.map((x) => x - 1);

// Use `andThen` to continue the computation.
const evenBetterResult: Result<boolean, number> = goodResult.andThen((x) => new Ok(x === 11));

// Use `orElse` to handle the error.
badResult = badResult.orElse((x) => new Ok(x + 20));

// Return the contents with `unwrap`.
const finalAwesomeResult = evenBetterResult.unwrap();

Querying the variant

The isOk and isErr methods return true if the Result is Ok or Err, respectively.

Extracting contained values

These methods extract the contained value in a Result<T, E> when it is the Ok variant. If the Result is Err:

• expect throws with a provided custom message
• unwrap throws with a generic message
• unwrapOr returns the provided default value
• unwrapOrElse returns the result of evaluating the provided function

The throwing methods expect and unwrap rely on E's toString() implementation.

These methods extract the contained value in a Result<T, E> when it is the Err variant. They use T's toString() implementation. If the Result is Ok:

• expectErr throws with a provided custom message
• unwrapErr throws with a generic message

Transforming contained values

This method transforms the contained value of the Ok variant:

• map transforms Result<T, E> into Result<U, E> by applying the provided function to the contained value of Ok and leaving Err values unchanged

This method transforms the contained value of the Err variant:

• mapErr transforms Result<T, E> into Result<T, F> by applying the provided function to the contained value of Err and leaving Ok values unchanged

These methods transform a Result<T, E> into a value of a possibly different type U:

• mapOr applies the provided function to the contained value of Ok, or returns the provided default value if the Result is Err
• mapOrElse applies the provided function to the contained value of Ok, or applies the provided default fallback function to the contained value of Err

Boolean operators

These methods treat the Result as a boolean value, where Ok acts like true and Err acts like false. There are two categories of these methods: ones that take a Result as input, and ones that take a function as input (to be lazily evaluated).

The and and or methods take another Result as input, and produce a Result as output. The and method can produce a Result<U, E> value having a different inner type U than Result<T, E>. The or method can produce a Result<T, F> value having a different error type F than Result<T, E>.

method	this	input	output
and	Err(e)	(ignored)	Err(e)
and	Ok(x)	Err(d)	Err(d)
and	Ok(x)	Ok(y)	Ok(y)
or	Err(e)	Err(d)	Err(d)
or	Err(e)	Ok(y)	Ok(y)
or	Ok(x)	(ignored)	Ok(x)

The andThen and orElse methods take a function as input, and only evaluate the function when they need to produce a new value. The andThen method can produce a Result<U, E> value having a different inner type U than Result<T, E>. The orElse method can produce a Result<T, F> value having a different error type F than Result<T, E>.

method	this	function input	function result	output
andThen	Err(e)	(not provided)	(not evaluated)	Err(e)
andThen	Ok(x)	x	Err(d)	Err(d)
andThen	Ok(x)	x	Ok(y)	Ok(y)
orElse	Err(e)	e	Err(d)	Err(d)
orElse	Err(e)	e	Ok(y)	Ok(y)
orElse	Ok(x)	(not provided)	(not evaluated)	Ok(x)

API Documentation

https://snowfrogdev.github.io/snowfrogdev/result/

License

MIT License

Copyright (c) 2022 Philippe Vaillancourt

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.