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rusty-core

Option and Result as inspired by https://doc.rust-lang.org/stable/core

Usage (Deno)

import type { Option, OptionPromise, Result, ResultPromise } from "https://deno.land/x/rusty_core@v2.0.0/mod.ts";
import { Err, None, Ok, optionFrom, resultFrom, Some } from "https://deno.land/x/rusty_core@v2.0.0/mod.ts";

Table of contents

Introduction

Inspired by the Rust core, which defines the Option and Result types that greatly improve the execution safety by preventing null pointer exceptions.

Also, great inspiration for the implementation came from the Monads library

This implementation supports the in-place modifiers of Option (insert, take), as well as combining of Promises to either type.

A note on Promises and async/await

Both Option and Result have several combinator methods, like andThen, orElse and map. Those methods accept one or more callback functions that can be async.

The examples below demonstrates how Promises and async callbacks can be combined with the andThen and mapOrElse.

Example

function doFetch(url: string): ResultPromise<Response, string> {
  return resultFrom(
    fetch(url)
      .then(
        Ok<Response, string>,
        (err) => Err(err.toString()),
      ),
  );
}

function fetchJson(url: string) {
  return doFetch(url)
    .andThen(async (response) => {
      if (response.ok) {
        return Ok(await response.json());
      } else return Err(`${response.status}: ${await response.text()}`);
    });
}

fetchJson("https:///jsonplaceholder.typicode.com/todos/1")
  .mapOrElse(
    (err) => console.error("Failed", err),
    (todo) => console.log("Success", todo),
  );

Example with non-Rust helper functions

function pauseIfNeeded(ms: Option<number>) {
  return ms.mapOrElse(
    () => Some(0),
    async (ms) => Some(await sleep(ms)),
  );
}

Due to the second callback being async, the return type for that callback will be Promise<Option> But mapOrElse requires both callbacks to have the same result type. 

error: TS2740 [ERROR]: Type 'Promise<Option<number>>' is missing the following properties from type 'Option<number>': getOrInsert, getOrInsertWith, insert, replace, and 19 more.
    async (ms) => Some(await sleep(ms)),
                  ~~~~~~~~~~~~~~~~~~~~~
    at file:///projects/rusty-core/cli/doc.ts:10:19

    The expected type comes from the return type of this signature.
        fn: (some: T) => U,
            ~~~~~~~~~~~~~~
        at file:///projects/rusty-core/src/option/combinators.ts:76:9

So one could surround the Some(0) with Promise.resolve. But then the return type would be Promise<Option>. Combining multiple Option and Result together would be cumbersome and error prone, as this requires constructs like

(await pauseIfNeeded(Some(2))).map( ... );

To improve the usability, mapOption and mapResult were added to return an OptionPromise or ResultPromise, in order to allow direct use of the combinators

function pauseIfNeeded(ms: Option<number>) {
  return ms.mapOption(
    () => SomePromise(0),
    async (ms) => Some(await sleep(ms)),
  );
}

At first, it might be confusing when to use mapOrElse and when mapOption or mapResult. Conditional types are used to assist a little here. When mapOrElse would return something like Promise<Option> or Promise<Result>, the compiler will complain when the combining methods are used:

error: TS2339 [ERROR]: Property 'map' does not exist on type '"To return a Promise to an Option, use mapOption"'.
pauseIfNeeded(Some(9)).map(None);

See the combinators test for more examples.

Option

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

You could consider using Option for:

  • Nullable pointers (undefined in JavaScript)
  • Return value for otherwise reporting simple errors, where None is returned on error
  • Default values and/or properties
  • Nested optional object properties

Options are commonly paired with pattern matching to query the presence of a value and take action, always accounting for the None case.

function divide(numerator: number, denominator: number): Option<number> {
  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"

Original implementation: https://doc.rust-lang.org/core/option

A note on None()

The Rust core library defines None to be assignable to any Option<T>. Since the below example also comes from the core library, we had to implement None as a function: None()

const x = None<number>();
const y = x.getOrInsert(5);
assertEquals(y, 5);
assertEquals(x, Some(5));

A note on null and undefined

Both null and undefined are considered a value which can be used with Some() and are not to be confused with None().

isSome() => boolean

Returns true if the option is a Some value.

isNone() => boolean

Returns true if the option is a None value.

unwrapOr(optb: T) => T

Returns the contained value or optb.

map(fn: (val: T) => U) => Option<U>

Maps an Option<T> to Option<U> by applying a function to a contained value.

mapOrElse(def: () => U, fn: (val: T) => U) => U

Computes a default function result (if None), or applies a different function to the contained value (if Some).

andThen(fn: (val: T) => Option<U>) => Option<U>

Returns None if the option is None, otherwise calls fn with the wrapped value and returns the result.

Some languages call this operation flatmap.

or(optb: Option<T>) => Option<T>

Returns the option if it contains a value, otherwise returns optb.

and(optb: Option<T>) => Option<T>

Returns None if the option is None, otherwise returns optb.

insert(value: T): T

Inserts value into the option, then returns a mutable reference to it.

If the option already contains a value, the old value is dropped.

replace(value: T): Option<T>

Replaces the actual value in the option by the value given in parameter, returning the old value if present, leaving a Some in its place without deinitializing either one.

take(): Option<T>

Takes the value out of the option, leaving a None in its place.

Result

Type Result<T,E> represents an result value: every Result is either Ok and contains a value, or Ok, which holds an error value.

class CannotDivideByZero {}

function divide(
  numerator: number,
  denominator: number,
): Result<number, CannotDivideByZero> {
  if (denominator === 0) {
    return Err(new CannotDivideByZero());
  } else {
    return Ok(numerator / denominator);
  }
}

// The return value of the function is always a result
for (const result of [divide(7, 0), divide(2.0, 3.0)]) {
  result.mapOrElse(
    (_) => console.error("Cannot divide by zero"),
    (ok) => console.log(`Result: ${ok}`),
  );
}
// "Cannot divide by zero"
// "Result: 0.6666666666666666"

A note on Ok()

When a Result has a void Ok type (Result<void,unknown>), Ok must be called as Ok(undefined)

Original implementation: https://doc.rust-lang.org/core/result

isOk() => boolean

Returns true if the result is a Ok value.

isErr() => boolean

Returns true if the result is an Err value.

unwrapOr(res: T) => T

Returns the contained value when it is an Ok, otherwise res.

map(fn: (val: T) => U) => Result<U,E>

Maps an Result<T,E> to Result<U,E> by applying a function to a contained Ok value, otherwise the Err.

mapOrElse(fn: (val: T) => U) => U

Maps a Result<T, E> to U by applying fallback function default to a contained Err value, or function fn to a contained Ok value.

This function can be used to unpack a successful result while handling an error.