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@utx/result

Forked from @vultix's work

A typescript implementation of Rust's Result and Option objects.

Brings compile-time error checking and optional values to typescript.

Contents

Installation

NPM

npm install @utx/result

Yarn

yarn add @utx/result

Example

Result Example

Convert this:

import { existsSync, readFileSync } from 'fs';

function readFile(path: string): string {
    if (existsSync(path)) {
        return readFileSync(path);
    } else {
        // Callers of readFile have no way of knowing the function can fail
        throw new Error('invalid path');
    }
}

// This line may fail unexpectedly without warnings from typescript
const text = readFile('test.txt');

To this:

import { existsSync, readFileSync } from 'fs';
import { Ok, Err, Result } from '@utx/result';

function readFile(path: string): Result<string, 'invalid path'> {
    if (existsSync(path)) {
        return new Ok(readFileSync(path)); // new is optional here
    } else {
        return new Err('invalid path'); // new is optional here
    }
}

// Typescript now forces you to check whether you have a valid result at compile time.
const result = readFile('test.txt');
if (result.ok) {
    // text contains the file's content
    const text = result.val;
} else {
    // err equals 'invalid path'
    const err = result.val;
}

Option Example

Convert this:

declare function getLoggedInUsername(): string | undefined;

declare function getImageURLForUsername(username: string): string | undefined;

function getLoggedInImageURL(): string | undefined {
    const username = getLoggedInUsername();
    if (!username) {
        return undefined;
    }

    return getImageURLForUsername(username);
}

const stringUrl = getLoggedInImageURL();
const optionalUrl = stringUrl ? new URL(stringUrl) : undefined;
console.log(optionalUrl);

To this:

import { Option, Some, None } from '@utx/result';

declare function getLoggedInUsername(): Option<string>;

declare function getImageForUsername(username: string): Option<string>;

function getLoggedInImage(): Option<string> {
    return getLoggedInUsername().andThen(getImageForUsername);
}

const optionalUrl = getLoggedInImage().map((url) => new URL(stringUrl));
console.log(optionalUrl); // Some(URL('...'))

// To extract the value, do this:
if (optionalUrl.some) {
    const url: URL = optionalUrl.val;
}

Usage

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

Creation

let okResult: Result<number, Error> = Ok(10);
let errorResult: Result<number, Error> = Err(new Error('bad number!'));

Type Safety

Note: Typescript currently has a bug, making this type narrowing only work when strictNullChecks is turned on.

let result: Result<number, Error> = Ok(1);
if (result.ok) {
    // Typescript knows that result.val is a number because result.ok was true
    let number = result.val + 1;
} else {
    // Typescript knows that result.val is an `Error` because result.ok was false
    console.error(result.val.message);
}

if (result.err) {
    // Typescript knows that result.val is an `Error` because result.err was true
    console.error(result.val.message);
} else {
    // Typescript knows that result.val is a number because result.err was false
    let number = result.val + 1;
}

Stack Trace

A stack trace is generated when an Err is created.

let error = Err('Uh Oh');
let stack = error.stack;

Unwrap

let goodResult = new Ok(1);
let badResult = new Err(new Error('something went wrong'));

goodResult.unwrap(); // 1
badResult.unwrap(); // throws Error("something went wrong")

Expect

let goodResult = Ok(1);
let badResult = Err(new Error('something went wrong'));

goodResult.expect('goodResult should be a number'); // 1
badResult.expect('badResult should be a number'); // throws Error("badResult should be a number - Error: something went wrong")

Map and MapErr

let goodResult = Ok(1);
let badResult = Err(new Error('something went wrong'));

goodResult.map((num) => num + 1).unwrap(); // 2
badResult.map((num) => num + 1).unwrap(); // throws Error("something went wrong")

goodResult
    .map((num) => num + 1)
    .mapErr((err) => new Error('mapped'))
    .unwrap(); // 2
badResult
    .map((num) => num + 1)
    .mapErr((err) => new Error('mapped'))
    .unwrap(); // throws Error("mapped")

Else

Deprecated in favor of unwrapOr

UnwrapOr

let goodResult = Ok(1);
let badResult = Err(new Error('something went wrong'));

goodResult.unwrapOr(5); // 1
badResult.unwrapOr(5); // 5

Empty

function checkIsValid(isValid: boolean): Result<void, Error> {
    if (isValid) {
        return Ok.EMPTY;
    } else {
        return new Err(new Error('Not valid'));
    }
}

Combining Results

results has two helper functions for operating over n Result objects.

Result.all

Either returns all of the Ok values, or the first Err value

let pizzaResult: Result<Pizza, GetPizzaError> = getPizzaSomehow();
let toppingsResult: Result<Toppings, GetToppingsError> = getToppingsSomehow();

let result = Result.all(pizzaResult, toppingsResult); // Result<[Pizza, Toppings], GetPizzaError | GetToppingsError>

let [pizza, toppings] = result.unwrap(); // pizza is a Pizza, toppings is a Toppings.  Could throw GetPizzaError or GetToppingsError.
Result.any

Either returns the first Ok value, or all Err values

let url1: Result<string, Error1> = attempt1();
let url2: Result<string, Error2> = attempt2();
let url3: Result<string, Error3> = attempt3();

let result = Result.any(url1, url2, url3); // Result<string, Error1 | Error2 | Error3>

let url = result.unwrap(); // At least one attempt gave us a successful url

Usage with rxjs

resultMap

Allows you to do the same actions as the normal rxjs map operator on a stream of Result objects.

import { of, Observable } from 'rxjs';
import { Ok, Err, Result } from '@utx/result';
import { resultMap } from '@utx/result/rxjs-operators';

const obs$: Observable<Result<number, Error>> = of(Ok(5), Err('uh oh'));

const greaterThanZero = obs$.pipe(
    resultMap((number) => number > 0), // Doubles the value
); // Has type Observable<Result<boolean, 'uh oh'>>

greaterThanZero.subscribe((result) => {
    if (result.ok) {
        console.log('Was greater than zero: ' + result.val);
    } else {
        console.log('Got Error Message: ' + result.val);
    }
});

// Logs the following:
// Got number: 10
// Got Error Message: uh oh

resultMapErr

import { resultMapErr } from '@utx/result/rxjs-operators';

Behaves exactly the same as resultMap, but maps the error value.

resultMapTo

import { resultMapTo } from '@utx/result/rxjs-operators';

Behaves the same as resultMap, but takes a value instead of a function.

resultMapErrTo

import { resultMapErrTo } from '@utx/result/rxjs-operators';

Behaves the same as resultMapErr, but takes a value instead of a function.

elseMap

Allows you to turn a stream of Result objects into a stream of values, transforming any errors into a value.

Similar to calling the else function, but works on a stream of Result objects.

import { of, Observable } from 'rxjs';
import { Ok, Err, Result } from '@utx/result';
import { elseMap } from '@utx/result/rxjs-operators';

const obs$: Observable<Result<number, Error>> = of(Ok(5), Err(new Error('uh oh')));

const doubled = obs$.pipe(
    elseMap((err) => {
        console.log('Got error: ' + err.message);

        return -1;
    }),
); // Has type Observable<number>

doubled.subscribe((number) => {
    console.log('Got number: ' + number);
});

// Logs the following:
// Got number: 5
// Got error: uh oh
// Got number: -1

elseMapTo

import { elseMapTo } from '@utx/result/rxjs-operators';

Behaves the same as elseMap, but takes a value instead of a function.

resultSwitchMap and resultMergeMap

Allows you to do the same actions as the normal rxjs switchMap and rxjs switchMap operator on a stream of Result objects.

Merging or switching from a stream of Result<T, E> objects onto a stream of <T2> objects turns the stream into a stream of Result<T2, E> objects.

Merging or switching from a stream of Result<T, E> objects onto a stream of Result<T2, E2> objects turn the stream into a stream of Result<T2, E | T2> objects.

import { of, Observable } from 'rxjs';
import { Ok, Err, Result } from '@utx/result';
import { resultMergeMap } from '@utx/result/rxjs-operators';

const obs$: Observable<Result<number, Error>> = of(new Ok(5), new Err(new Error('uh oh')));

const obs2$: Observable<Result<string, CustomError>> = of(new Ok('hi'), new Err(new CustomError('custom error')));

const test$ = obs$.pipe(
    resultMergeMap((number) => {
        console.log('Got number: ' + number);

        return obs2$;
    }),
); // Has type Observable<Result<string, CustomError | Error>>

test$.subscribe((result) => {
    if (result.ok) {
        console.log('Got string: ' + result.val);
    } else {
        console.log('Got error: ' + result.val.message);
    }
});

// Logs the following:
// Got number: 5
// Got string: hi
// Got error: custom error
// Got error: uh oh

filterResultOk

Converts an Observable<Result<T, E>> to an Observble<T> by filtering out the Errs and mapping to the Ok values.

import { of, Observable } from 'rxjs';
import { Ok, Err, Result } from '@utx/result';
import { filterResultOk } from '@utx/result/rxjs-operators';

const obs$: Observable<Result<number, Error>> = of(new Ok(5), new Err(new Error('uh oh')));

const test$ = obs$.pipe(filterResultOk()); // Has type Observable<number>

test$.subscribe((result) => {
    console.log('Got number: ' + result);
});

// Logs the following:
// Got number: 5

filterResultErr

Converts an Observable<Result<T, E>> to an Observble<T> by filtering out the Oks and mapping to the error values.

import { of, Observable } from 'rxjs';
import { Ok, Err, Result } from '@utx/result';
import { filterResultOk } from '@utx/result/rxjs-operators';

const obs$: Observable<Result<number, Error>> = of(new Ok(5), new Err(new Error('uh oh')));

const test$ = obs$.pipe(filterResultOk()); // Has type Observable<number>

test$.subscribe((result) => {
    console.log('Got number: ' + result);
});

// Logs the following:
// Got number: 5