ts-results

A typescript implementation of Rust's Result object. Brings compile-time error checking to typescript.

Note: This is the documentation for the newly released ts-results@2.0.0 with breaking changes. To see breaking changes, go to CHANGELOG.md

Installation
Example
Usage
- Creation
- Type Safety
- Unwrap
- Expect
- Map, MapErr
- Else
- Combining Results
Usage with rxjs

Installation

$ npm install ts-results

$ yarn install ts-results

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 'ts-results';

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

// 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;
}

Usage

import { Result, Err, Ok, Results } from 'ts-results';

Creation

let okResult: Result<number, Error> = new Ok(10);
let okResult2 = Ok<number, Error>(10); // Exact same as above

let errorResult: Result<number, Error> = new Ok(new Error('bad number!'));
let errorResult2 = Ok<number, Error>(new Error('bad number!')); // Exact same as above

Type Safety

let result = Ok<number, Error>(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;
}

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<number, Error>(1);
let badResult = Err<number, Error>(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 = new Ok(1);
let badResult = new 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

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

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

Combining Results

There may be some cases where we have two or more separate Result objects and we want to do something with both values. This is handled by using the Results function to combine them.

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

let result = Results(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.

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 'ts-results';
import {resultMap} from 'ts-results/rxjs-operators';

const obs$: Observable<Result<number, Error>> = of(new Ok(5), new 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 'ts-results/rxjs-operators';

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

resultMapTo

import {resultMapTo} from 'ts-results/rxjs-operators';

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

resultMapErrTo

import {resultMapErrTo} from 'ts-results/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 'ts-results';
import {elseMap} from 'ts-results/rxjs-operators';

const obs$: Observable<Result<number, Error>> = of(new Ok(5), new 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 'ts-results/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 'ts-results';
import {resultMergeMap} from 'ts-results/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 'ts-results';
import {filterResultOk} from 'ts-results/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 'ts-results';
import {filterResultOk} from 'ts-results/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

lmortimer / ts-results