@snowfrog/option

Type-safe optional value representation.

Install

npm install @snowfrog/option

Usage

Option<T> is an abstract class that represents an optional value: every Option is either Some and contains a value, or None and does not. Option types can have a number of uses:

• Initial values
• Return values for function that are not defined over their entire input range (partial functions)
• Return value for otherwise reporting simple errors, where None is returned on error
• Optional properties of an object
• Optional function arguments

Method overview

Option provides a wide variety of different methods.

import { Some, None, Option } from '@snowfrog/option';

let something: Option<number> = new Some(10);
let nothing: Option<number> = new None();

// The `isSome` and `isNone` methods do what they say.
expect(something.isSome() && !something.isNone()).toBe(true);
expect(nothing.isNone() && !nothing.isSome()).toBe(true);

// `map` consumes the `Option` and produces another.
something = something.map((x) => x + 1);
nothing = nothing.map((x) => x - 1);

// Use `andThen` to continue the computation.
const somethingDifferent: Option<boolean> = something.andThen((x) => new Some(x === 11));

// Use `orElse` to handle the None.
const somethingFromNothing = nothing.orElse((x) => new Some(x + 20));

// Return the contents with `unwrap`.
const finalSomething = somethingDifferent.unwrap();

Querying the variant

The isSome and isNone methods return true if the Option is Some or None, respectively.

Extracting the contained value

These methods extract the contained value in an Option<T> when it is the Some variant. If the Option is None:

• expect throws with a provided custom message
• unwrap throws with a generic message
• unwrapOr returns the provided default value
• unwrapOrElse returns the result of evaluating the provided function

Transforming contained values

These methods transform the Somme variant:

• filter calls the provided predicate function on the contained value t if the Option is Some(t), and returns Some(t) if the function returns true; otherwise, returns None
• map transforms Option<T>> to Option<U> by applying the provided function to the contained value of Some and leaving None values unchanged

These methods transform Option<T> to a value of a possibly different type U:

• mapOr applies the provided function to the contained value of Some and returns the provided default value if the Option is None
• mapOrElse applies the provided function to the contained value of Some, or returns the result of evaluating the provided fallback function if the Option is None

These methods combine the Some variants of two Option values;

• zip returns Some([s, o]) if this is Some(s) and the provided Option is Some(o); otherwise, returns None
• zipWith calls the provided function fn and returns Some(fn(s, o)) if this is Some(s) and the provided Option value is Some(o); otherwise, returns None

Boolean operatiors

These methods treat the Option as a boolean value, where Some acts like true and None acts like false. There are two categories of these methods: ones that take an Option as input, and ones that take a function as input (to be lazily evaluated).

The and, or, and xor methods take another Option as input, and produce an Option as output. Only the and method can produce an Option<U> value having a different inner type U than Option<T>.

| method | this | input | output | |---------|---------|-----------|-----------| | and | None | (ignored) | None | | and | Some(x) | None | None | | and | Some(x) | Some(y) | Some(y) | | or | None | None | None | | or | None | Some(y) | Some(y) | | or | Some(x) | (ignored) | Some(x) | | xor | None | None | None | | xor | None | Some(y) | Some(y) | | xor | Some(x) | None | Some(x) | | xor | Some(x) | Some(y) | None |

The andThen and orElse methods take a function as input, and only evaluate the function when they need to produce a new value. Only the andThen method can produce an Option<U> value having a different inner type U than Option<T>.

| method | this | function input | function result | output | |-----------|-----------|----------------|-----------------|-----------| | andThen | None | (not provided) | (not evaluated) | None | | andThen | Some(x) | x | None | None | | andThen | Some(x) | x | Some(y) | Some(y) | | orElse | None | (not provided) | None | None | | orElse | None | (not provided) | Some(y) | Some(y) | | orElse | Some(x) | (not provided) | (not evaluated) | Some(x) |

Example

enum Kingdom {
  Plant,
  Animal,
}

class Thing {
  constructor(public kingdom: Kingdom, public size: number, public name: string) {}
}

// A list of data to search through.
const allTheBigThings = [
    new Thing(Kingdom.Plant, 250, "redwood"),
    new Thing(Kingdom.Plant, 230, "noble fir"),
    new Thing(Kingdom.Plant, 229, "sugar pine"),
    new Thing(Kingdom.Animal, 25, "blue whale"),
    new Thing(Kingdom.Animal, 19, "fin whale"),
    new Thing(Kingdom.Animal, 15, "north pacific right whale"),
];

// We're going to search for the name of the biggest animal,
// but to start with we've just got `None`.
let nameOfBiggestAnimal: Option<string> = new None();
let sizeOfBiggestAnimal = 0;
for (const bigThing of allTheBigThings) {
  if (bigThing.kingdom === Kingdom.Animal && bigThing.size > sizeOfBiggestAnimal) {
    sizeOfBiggestAnimal = bigThing.size;
    nameOfBiggestAnimal = new Some(bigThing.name);
  }
}

const message = nameOfBiggestAnimal.mapOr("There are no aminals 😢", (name) => `The biggest animal is ${name}`);
console.log(message)

Documentation

https://snowfrogdev.github.io/snowfrogdev/option/

License

MIT License

Copyright (c) 2022 Philippe Vaillancourt

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.