Rust's Option<T> and Result<T, E>, implemented
for TypeScript. Zero dependencies, full test coverage and complete in-editor documentation.
$ npm install oxide.ts --saveResultflatten to Option and ResultYou 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";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.
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));These methods provide a way to jump in to (and out of) Option and Result
types. Particularly these methods can streamline things where:
T | null, T | false or similar.indexOf method.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.
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());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()}`);
}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());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 ""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);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}.`,
});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));
}Capture the outcome of a function or Promise as an Option<T> or
Result<T, E>, preventing throwing (function) or rejection (Promise).
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.
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");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.");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.");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");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");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.
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");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");