Introduction to Generics

generics.ts

// 1. Example: get last element of any array

function last_element<X>(arr: Array<X>): X {
	return arr[arr.length - 1];
}

// 2. Example: get id of an object which has an id

type person = {
	name: string;
	age: number;
	id: string;
};

type city = {
	name: string;
	location: string;
	id: string;
};

function get_id<T extends { id: string }>(object: T): string {
	return object.id;
}

// 3. Example: add an id to an object

function add_id<T>(obj: T): T & { id: string } {
	const id = crypto.randomUUID();
	return { ...obj, id };
}

// 4. Example: unify database entries

type database_entry<X> = {
	id: string;
	created: Date;
	updated: Date;
} & X;

type user = database_entry<{
	name: string;
	age: number;
	role: "admin" | "editor" | "guest";
}>;

type post = database_entry<{
	title: string;
	text: string;
	likes: number;
}>;

// 5. Example: utilities

function is_equal<X>(a: X, b: X) {
	return a === b;
}

function identity<X>(a: X): X {
	return a;
}

function compose<X, Y, Z>(
	f: (x: X) => Y,
	g: (y: Y) => Z
): (x: X) => Z {
	return function (x: X): Z {
		const y = f(x);
		const z = g(y);
		return z;
	};
}

// 6. Example: Promises

async function get_answer(): Promise<number> {
	return 42;
}

async function get_user(): Promise<user> {
	return {
		id: "u591y",
		created: new Date("2023-04-02"),
		updated: new Date("2023-05-26"),
		name: "John Doe",
		age: 21,
		role: "editor",
	};
}

// 7. Example: cache decorator function

async function expensive_api_call(): Promise<string> {
	return "This is the result";
}

function cache_function<T>(fun: () => T, duration: number): () => T {
	let cache: T;
	let saved: Date;

	return function () {
		const now = new Date();

		if (cache && now.getTime() - saved.getTime() < duration) {
			console.log("read from cache 😎");
			return cache;
		} else {
			console.log("need to compute 🙄");
			const result = fun();
			cache = result;
			saved = new Date();
			return result;
		}
	};
}

const cached_api_call = cache_function(expensive_api_call, 10000);

setInterval(async () => {
	await cached_api_call();
}, 1000);

// 8. Example: Generic classes

// See the Server class in
// https://github.com/ScriptRaccoon/svelte-chat-app/blob/main/server.ts

// 9. Example: Function evaluation and currying

function evaluate<X, Y>(fun: (x: X) => Y, x: X): Y {
	return fun(x);
}

function curry<X, Y, Z>(
	fun: ([x, y]: [X, Y]) => Z
): (x: X) => (y: Y) => Z {
	return (x: X) => (y: Y) => fun([x, y]);
}