oxide.ts

Rust's Option<T> and Result<T, E>, implemented for TypeScript. Zero dependencies, full test coverage and complete in-editor documentation.

Installation

$ npm install oxide.ts --save

New features in 1.1.0

• Added intoTuple to Result
• Added flatten to Option and Result

Usage

Core Features

• Importing
• Option
• Result
• Converting
• Nesting
• Iteration
• Safe
• All
• Any

Advanced

• Match
• Combined Match
• Match Chains
• Compiling

Importing

You can import the complete oxide.ts library:

import { Option, Some, None, Result, Ok, Err, match, Fn, _ } from "oxide.ts";

Or just the core library, which exclues the match feature:

import { Option, Some, None, Result, Ok, Err } from "oxide.ts/core";

Option

An Option represents either something, or nothing. If we hold a value of type Option<T>, we know it is either Some<T> or None. Both types share a common API, so we can chain operations without having to worry whether we have Some or None until pulling the value out:

import { Option, Some, None } from "oxide.ts";

function divide(x: number, by: number): Option<number> {
   return by === 0 ? None : Some(x / by);
}

const val = divide(100, 20);

// Pull the value out, or throw if None:
const res: number = val.unwrap();
// Throw a custom error message in the case of None:
const res: number = val.expect("Don't divide by zero!");
// Pull the value out, or use a default if None:
const res: number = val.unwrapOr(1);

// Map the Option<T> to Option<U> by applying a function:
const strval: Option<string> = val.map((num) => `val = ${num}`);
// Unwrap the value or use a default if None:
const res: string = strval.unwrapOr("val = <none>");
// Map, assign a default and unwrap in one line:
const res: string = val.mapOr("val = <none>", (num) => `val = ${num}`);

The type annotations applied to the const variables are for information - the correct types would be inferred.

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Result

A Result represents either something good (T) or something not so good (E). If we hold a value of type Result<T, E> we know it's either Ok<T> or Err<E>. You could think of a Result as an Option where None has a value.

import { Result, Ok, Err } from "oxide.ts";

function divide(x: number, by: number): Result<number, string> {
   return by === 0 ? Err("Division by zero") : Ok(x / by);
}

const val = divide(100, 20);

// These are the same as Option (as are many of the other methods):
const res: number = val.unwrap();
const res: number = val.expect("Don't divide by zero!");
const res: number = val.unwrapOr(1);

// Map Result<T, E> to Result<U, E>
const strval: Result<string, string> = val.map((num) => `val = ${num}`);
const res: string = strval.unwrapOr("val = <err>");
const res: string = val.mapOr("val = <err>", (num) => `val = ${num}`);

// Unwrap or expect the Err (throws if the Result is Ok):
const err: string = val.unwrapErr();
const err: string = val.expectErr("Expected division by zero!");

// Or map the Err, converting Result<T, E> to Result<T, F>
const errobj: Result<string, Error> = val.mapErr((msg) => new Error(msg));

Converting

These methods provide a way to jump in to (and out of) Option and Result types. Particularly these methods can streamline things where:

• A function returns T | null, T | false or similar.
• You are working with physical quantities or using an indexOf method.
• A function accepts an optional argument, T | null or similar.

Note: Converting to a Result often leaves you with a Result<T, null>. The null value here is not very useful - consider the equivalent Option method to create an Option<T>, or use mapErr to change the E type.

into

Convert an existing Option/Result into a union type containing T and undefined (or a provided falsey value).

function maybeName(): Option<string>;
function maybeNumbers(): Result<number[], Error>;
function printOut(msg?: string): void;

const name: string | undefined = maybeName().into();
const name: string | null = maybeName().into(null);

// Note that the into type does not reflect the E type:
const numbers: number[] | undefined = maybeNumbers().into();
const numbers: number[] | false = maybeNumbers().into(false);

// As a function argument:
printOut(name.into());

intoTuple

Convert a Result<T, E> into a tuple of [null, T] if the result is Ok, or [E, null] otherwise.

function getUsername(): Result<string, Error>;

const query = getUsername();
const [err, res] = query.intoTuple();

if (err) {
   console.error(`Query Error: ${err}`);
} else {
   console.log(`Welcome: ${res.toLowerCase()}`);
}

from

Convert to an Option/Result which is Some<T>/Ok<T> unless the value is falsey, an instance of Error or an invalid Date.

The T is narrowed to exclude any falsey values or Errors.

const people = ["Fry", "Leela", "Bender"];
// Create an Option<string> from a find:
const person = Option.from(people.find((name) => name === "Fry"));
// or shorter:
const person = Option(people.find((name) => name === "Bender"));

In the case of Result, the E type includes:

• null (if val could have been falsey or an invalid date)
• Error types excluded from T (if there are any)

function randomName(): string | false;
function tryName(): string | Error;
function randomNumbers(): number[] | Error;

// Create a Result<string, null>
const person = Result.from(randomName());
// Create a Result<string, Error | null>
const name = Result(tryName());
// Create a Result<number[], Error>
const num = Result(randomNumbers());

nonNull

Convert to an Option/Result which is Some<T>/Ok<T> unless the value provided is undefined, null or NaN.

function getNum(): number | null;
const num = Option.nonNull(getNum()).unwrapOr(100); // Could be 0

const words = ["express", "", "planet"];
const str = Option.nonNull(words[getNum()]);
str.unwrapOr("No such index"); // Could be ""

qty

Convert to an Option/Result which is Some<number>/Ok<number> when the provided val is a finite integer greater than or equal to 0.

const word = "Buggalo";

const g = Option.qty(word.indexOf("g"));
assert.equal(g.unwrap(), 2);

const z = Option.qty(word.indexOf("z"));
assert.equal(z.isNone(), true);

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Nesting

You can nest Option and Result structures. The following example uses nesting to distinguish between found something, found nothing and database error:

function search(query: string): Result<Option<SearchResult>, string> {
   const [err, result] = database.search(query);
   if (err) {
      return Err(err);
   } else {
      return Ok(result.count > 0 ? Some(result) : None);
   }
}

const result = search("testing");
const output: string = match(result, {
   Ok: {
      Some: (result) => `Found ${result.count} entries.`,
      None: () => "No results for that search.",
   },
   Err: (err) => `Error: ${err}.`,
});

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Iteration

An Option or Result that contains an iterable T type can be iterated upon directly. In the case of None or Err, an empty iterator is returned.

The compiler will complain if the inner type is not definitely iterable (including any), or if the monad is known to be None or Err.

const numbers = Option([1.12, 2.23, 3.34]);
for (const num of numbers) {
   console.log("Number is:", num.toFixed(1));
}

const numbers: Option<number[]> = None;
for (const num of numbers) {
   console.log("Unreachable:", num.toFixed());
}

It's also possible to iterate over nested monads in the same way:

const numbers = Option(Result(Option([1, 2, 3])));
for (const num of numbers) {
   console.log("Number is:", num.toFixed(1));
}

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Safe

Capture the outcome of a function or Promise as an Option<T> or Result<T, E>, preventing throwing (function) or rejection (Promise).

Safe Functions

Calls the passed function with the arguments provided and returns an Option<T> or Result<T, Error>. The outcome is Some/Ok if the function returned, or None/Err if it threw. In the case of Result.safe, any thrown value which is not an Error is converted.

function mightThrow(throws: boolean) {
   if (throws) {
      throw new Error("Throw");
   }
   return "Hello World";
}

const x: Result<string, Error> = Result.safe(mightThrow, true);
assert.equal(x.unwrapErr() instanceof Error, true);
assert.equal(x.unwrapErr().message, "Throw");

const x = Result.safe(() => mightThrow(false));
assert.equal(x.unwrap(), "Hello World");

Note: Any function which returns a Promise (or PromiseLike) value is rejected by the type signature. Result<Promise<T>, Error> or Option<Promise<T>> are not useful types - using it in this way is likely to be a mistake.

Safe Promises

Accepts a Promise and returns a new Promise which always resolves to either an Option<T> or Result<T, Error>. The Result is Some/Ok if the original promise resolved, or None/Err if it rejected. In the case of Result.safe, any rejection value which is not an Error is converted.

async function mightThrow(throws: boolean) {
   if (throws) {
      throw new Error("Throw");
   }
   return "Hello World";
}

const x = await Result.safe(mightThrow(true));
assert.equal(x.unwrapErr() instanceof Error, true);
assert.equal(x.unwrapErr().message, "Throw");

const x = await Result.safe(mightThrow(false));
assert.equal(x.unwrap(), "Hello World");

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All

Reduce multiple Options or Results to a single one. The first None or Err encountered is returned, otherwise the outcome is a Some/Ok containing an array of all the unwrapped values.

function num(val: number): Result<number, string> {
   return val > 10 ? Ok(val) : Err(`Value ${val} is too low.`);
}

const xyz = Result.all(num(20), num(30), num(40));
const [x, y, z] = xyz.unwrap();
assert.equal(x, 20);
assert.equal(y, 30);
assert.equal(z, 40);

const err = Result.all(num(20), num(5), num(40));
assert.equal(err.isErr(), true);
assert.equal(err.unwrapErr(), "Value 5 is too low.");

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Any

Reduce multiple Options or Results into a single one. The first Some/Ok found (if any) is returned, otherwise the outcome is None, or in the case of Result - an Err containing an array of all the unwrapped errors.

function num(val: number): Result<number, string> {
   return val > 10 ? Ok(val) : Err(`Value ${val} is too low.`);
}

const x = Result.any(num(5), num(20), num(2));
assert.equal(x.unwrap(), 20);

const efg = Result.any(num(2), num(5), num(8));
const [e, f, g] = efg.unwrapErr();
assert.equal(e, "Value 2 is too low.");
assert.equal(f, "Value 5 is too low.");
assert.equal(g, "Value 8 is too low.");

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Match

Mapped matching is possible on Option and Result types:

const num = Option(10);
const res = match(num, {
   Some: (n) => n + 1,
   None: () => 0,
});

assert.equal(res, 11);

You can nest mapped matching patterns and provide defaults. If a default is not found in the current level it will fall back to the previous level. When no suitable match or default is found, an exhausted error is thrown.

function nested(val: Result<Option<number>, string>): string {
   return match(val, {
      Ok: { Some: (num) => `found ${num}` },
      _: () => "nothing",
   });
}

assert.equal(nested(Ok(Some(10))), "found 10");
assert.equal(nested(Ok(None)), "nothing");
assert.equal(nested(Err("Not a number")), "nothing");

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Combined Match

Mapped Matching and Chained Matching can be combined. A match chain can be provided instead of a function for Some, Ok and Err.

function matchNum(val: Option<number>): string {
   return match(val, {
      Some: [
         [5, "5"],
         [(x) => x < 10, "< 10"],
         [(x) => x > 20, "> 20"],
      ],
      _: () => "none or not matched",
   });
}

assert.equal(matchNum(Some(5)), "5");
assert.equal(matchNum(Some(7)), "< 10");
assert.equal(matchNum(Some(25)), "> 20");
assert.equal(matchNum(Some(15)), "none or not matched");
assert.equal(matchNum(None), "none or not matched");

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Match Chains

Chained matching is possible on any type. Branches are formed by associating a condition with a result (with an optional default at the end). The first matching branch is the result.

More detail about chained matching patterns is available in the bundled JSDoc.

Examples

function matchArr(arr: number[]): string {
   return match(arr, [
      [[1], "1"],
      [[2, (x) => x > 10], "2, > 10"],
      [[_, 6, 9, _], (a) => a.join(", ")],
      () => "other",
   ]);
}

assert.equal(matchArr([1, 2, 3]), "1");
assert.equal(matchArr([2, 12, 6]), "2, > 10");
assert.equal(matchArr([3, 6, 9]), "other");
assert.equal(matchArr([3, 6, 9, 12]), "3, 6, 9, 12");
assert.equal(matchArr([2, 4, 6]), "other");

interface ExampleObj {
   a: number;
   b?: { c: number };
   o?: number;
}

function matchObj(obj: ExampleObj): string {
   return match(obj, [
      [{ a: 5 }, "a = 5"],
      [{ b: { c: 5 } }, "c = 5"],
      [{ a: 10, o: _ }, "a = 10, o = _"],
      [{ a: 15, b: { c: (n) => n > 10 } }, "a = 15; c > 10"],
      () => "other",
   ]);
}

assert.equal(matchObj({ a: 5 }), "a = 5");
assert.equal(matchObj({ a: 50, b: { c: 5 } }), "c = 5");
assert.equal(matchObj({ a: 10 }), "other");
assert.equal(matchObj({ a: 10, o: 1 }), "a = 10, o = _");
assert.equal(matchObj({ a: 15, b: { c: 20 } }), "a = 15; c > 10");
assert.equal(matchObj({ a: 8, b: { c: 8 }, o: 1 }), "other");

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Compiling

Match patterns can also be compiled into a function. More detail about compiling is available in the bundled JSDoc.

const matchSome = match.compile({
   Some: (n: number) => `some ${n}`,
   None: () => "none",
});

assert.equal(matchSome(Some(1)), "some 1");
assert.equal(matchSome(None), "none");

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