Java Either class

Today, at work, I had the dire need of discriminated unions in Java, which of course doesn’t support it. Then I found the idea of the Either type, which I implemented myself using the Optional class from Java 8:

import java.util.Optional;
import java.util.function.Consumer;
	
/**
 * Guarda 3 elementos mutuamente exclusivos, isto é, apenas 1 dos 3 pode existir
 * dentro do objeto.
 */
public class Either3<T1, T2, T3> {
	
	private Optional<T1> v1 = Optional.empty();
		
	private Optional<T2> v2 = Optional.empty();
		
	private Optional<T3> v3 = Optional.empty();
	
	/** Cria um objeto com o valor do tipo 1. */
	public static <<1, T2, T3> Either3<T1, T2, T3> of1(T1 v1) {
		Either3<T1, T2, T3> obj = new Either3<>();
		obj.v1 = Optional.of(v1);
		return obj;
	}
	
	/** Cria um objeto com o valor do tipo 1. */
	public static <T1, T2, T3> Either3<T1, T2, T3> of2(T2 v2) {
		Either3<T1, T2, T3> obj = new Either3<>();
		obj.v2 = Optional.of(v2);
		return obj;
	}
	
	/** Cria um objeto com o valor do tipo 1. */
	public static <T1, T2, T3> Either3<T1, T2, T3> of3(T3 v3) {
		Either3<T1, T2, T3> obj = new Either3<
		obj.v3 = Optional.of(v3);
		return obj;
	}
	
	/** Retorna o valor do tipo 1; caso não exista, lança {@code NoSuchElementException}. */
	public T1 get1() {
		return v1.get();
	}
	
	/** Retorna o valor do tipo 2; caso não exista, lança {@code NoSuchElementException}. */
	public T2 get2() {
		return v2.get();
	}
	
	/** Retorna o valor do tipo 3; caso não exista, lança {@code NoSuchElementException}. */
	public T3 get3() {
		return v3.get();
	}
	
	/** Diz se o objeto possui um valor do tipo 1. */
	public boolean has1() {
		return v1.isPresent();
	}
	
	/** Diz se o objeto possui um valor do tipo 2. */
	public boolean has2() {
		return v2.isPresent();
	}
	
	/** Diz se o objeto possui um valor do tipo 3. */
	public boolean has3() {
		return v3.isPresent();
	}
	
	/** Executa a função caso haja um valor do tipo 1. */
	public void if1(Consumer<? super T1> consumer) {
		v1.ifPresent(consumer);
	}
	
	/** Executa a função caso haja um valor do tipo 2. */
	public void if2(Consumer<? super T2> consumer) {
		v2.ifPresent(consumer);
	}
	
	/** Executa a função caso haja um valor do tipo 3. */
	public void if3(Consumer<? super T3> consumer) {
		v3.ifPresent(consumer);
	}
}

Since Java doesn’t have variadic generic parameters, having one class to each amount is needed. Nonetheless, it’s doable and remarkably ergonomic.

Using Zustand, TypeScript and Immer together

In React land, Redux nightmare finally came to an end with the inception of Zustand, which throws away all the chores we had to previously do, leaving us with a simple workflow of minimal code. On top of that, TypeScript allows us to relax our brains with some degree of strong typing.

When dealing with our Zustand store, just like Redux, we have to manipulate a chunk of immutable data, which becomes more and more annoying as we deepen the object structure. And, when it comes to dealing immutable data, Immer is everyone’s best friend – just slap a produce call and you can go home earlier.

Now here comes the question: how to use Immer inside our Zustand store?

Turns out Zustand provides a few middlewares – functions which lie between the data and your manipulation functions –, and among them an Immer middleware. Using the middleware has two advantages:

• you don’t need to explicitly call produce, because the state argument already comes to you as a WritableDraft;
• you don’t need to explicitly type the state argument, because it already comes to you properly typed.

To put everything together, with automatic type inference, we also need the combine middleware, so we don’t need to write the store type by hand. Here is the full template that can be used as the starting point of your store:

import {create} from 'zustand';
import {combine} from 'zustand/middleware';
import {immer} from 'zustand/middleware/immer';

interface Person {
	name: string;
	age: number;
}

const useStore = create(immer(
	combine({
		people: [] as Person[],
	},
	(set, get) => ({
		add(name: string, age: number) {
			set(state => {
				state.people.push({name, age});
			});
		},
		remove(name: string) {
			set(state => {
				state.people = state.people.filter(p => p.name === name);
			});
		},
	})),
));

export default useStore;

Notice how you can simply mutate the state inside the actions.

TypeScript map for objects

In another round of React experiments, after another Vue frustration with Volar not typing event arguments, I was again into the state normalization land. When dealing with objects with IDs as keys, every operation requires a tedious reduce, in contrast to the intuitive map we use with arrays.

So I wrote a map for objects:

function mapObj<T>(obj: Record<string, T>, callback: (elem: T, key: string) => T): Record<string, T> {
	return Object.keys(obj).reduce((accum, key) => {
		accum[key] = callback(obj[key], key);
		return accum;
	}, {} as Record<string, T>);
}

It’s fully typed, and it was surprisingly easy to write. TypeScript is a great language.

Testing GitHub pull requests before merging

My personal projects on GitHub oftentimes. It’s very flattering when other people get interested in your personal work, and actively use it. GitHub pull request is how other people suggest changes to our code.

Pull requests can be merged in GitHub with just one click, but it’s also possible – and recommended – trying the changes locally first. The usual way to do this is creating a local branch, then pulling the modified code onto it.

As an example, let’s take pull request #75 of WinSafe. We create another branch called foo and check the pull request onto it by using:

git fetch origin pull/75/head:foo
git checkout foo

Simple enough, and for some reason it took me years to try this for the first time today.

Fetch calls with global Jotai object

Zustand is a great React state management library, but it has the drawback of not having a standard way to define getters, which must be defined as ordinary hooks.

Written by the same author, Jotai seems to be the answer to this situation. However, while Zustand provides a way to change the state outside React components, Jotai does not. Today I was able to devise a way to work around this, by simply applying the hooks concept, answering my own question:

const errorAtom = atom(''); // storage

const errorReadAtom = atom(get => { // read-only (computed)
	return 'Error: ' + get(errorAtom);
});

const writeErrorAtom = atom(null, (get, set, arg: string): void => { // setter
	set(errorAtom, arg);
});

function useGet(): (url: string) => Promise {
	const [, setError] = useAtom(errorAtom);
	return async (url: string): Promise => {
		const resp = await fetch(url);
		const json = await resp.json();
		setError('Hello');
		return json as T;
	};
}

Usage:

function App() {
	const doGet = useGet();

	async function click(): void {
		const data = await doGet('/foo');
	}

	return <button onClick={click}>Click</button>;
}

So, with Jotai I finally have the complete solution to state management in React.

The three rules of lifetime elision in Rust

While reviewing some Rust fundamentals, I stumbled across this excellent video about lifetimes. What caught my attention the most was the “three rules” of lifetime elision – a topic I had some idea about, but I’ve never seen clearly explained.

For reference, they are:

• Each parameter that is a reference gets its own lifetime parameter;
• If there is exactly one input lifetime parameter, that lifetime is assigned to all output lifetime parameters;
• If there are multiple input lifetime parameters, but one of them is &self or &mut self, the lifetime of self is assigned to all output lifetime parameters.

Zustand computed values

This week I’ve faced some situations with Vue’s reactive that alarmed me. I’m finding the hard way what I’ve read a couple times: reactive proxies can behave unpredictably in some situations. Too bad I’m in the middle of a project which began in Vue 3. I’m strongly considering rewriting it in React now – yes, it will be an insane amount of work, including back-end changes to return normalized objects.

But React brings its own problems. The biggest of all is certainly the raw state management.

Among all state management tools I’m evaluating, Zustand is showing to be the most promising. It’s ticking all the boxes, and the only open question so far is computed state. The best I could do was to use custom hooks, but they look rather ugly and verbose:

import create from 'zustand';
import {combine} from 'zustand/middleware';

/**
 * The store, which holds the state and the actions.
 */
const useBearStore = create(
	combine({
		bears: 0,
	},
	(set, get) => ({
		increasePopulation(): void {
			set(state => ({ bears: state.bears + 1 }));
		},
		removeAllBears(): void {
			set({ bears: 0 });
		},
	})),
);

export default useBearStore;

/**
 * Custom hook that returns a computed value.
 */
export function useBearCountPlusOne(): number {
	const bears = useBearStore(s => s.bears);
	return bears + 1;
}

Usage example in a component:

import useBearStore, {useBearCountPlusOne} from './useBearStore';

function App() {
	const increasePopulation = useBearStore(s => s.increasePopulation);
	const populationPlusOne = useBearCountPlusOne();

	return <>
		<div>Population + 1: {populationPlusOne}</div>
		<button onClick={() => increasePopulation()}>
			Increase population
		</button>
	</>;
}

If I find a way to rework these loose hooks, I think I found my way.

Multiple className values in React

React offers basically zero support for CSS. Personally I’m fond of CSS Modules along with SCSS instead of the slow CSS-in-JS solutions out there. Still, we need a way to manage multiple classes in className property.

In order to mitigate this problem, I wrote a TypeScript function to deal with sequential or conditional class names situations:

/**
 * Generates the className attribute with the given class names.
 */
export function cls(...names: (string | undefined)[]): string;
export function cls(names: (string | undefined)[]): string;
export function cls(names: Record<string, boolean>): string;
export function cls(names: any): string {
	if (typeof names === 'string' && arguments.length === 1) { // just 1 string
		return names === undefined ? '' : names;
	} else if (typeof names === 'string' && arguments.length > 1) { // multiple strings
		let s = '';
		for (const name of arguments) {
			if (name !== undefined) s += name + ' ';
		}
		return s;
	} else if (Array.isArray(names)) { // string array
		let s = '';
		for (const name of names) {
			if (name !== undefined) s += name + ' ';
		}
		return s;
	} else if (typeof names === 'object') {
		let s = '';
		for (const name in names) {
			if (names[name]) s += name + ' ';
		}
		return s
	} else {
		throw 'Invalid class name input!';
	}
}

Example using the 4 argument possibilities:

import {cls} from '@/funcs';
import c from './App.module.scss';
	
function App() {
	return <>
		<div className={cls( c.first )} />
		<div className={cls( c.first, c.second )} />
		<div className={cls( [c.first, c.second] )} />
		<div className={cls({
			[c.first]: true,
			[c.second]: false,
		})} />
	</>;
}

This covers all situations I ever faced.

The spread operator version was added in February 9, 2023.

Sizes of Windows integral types

While developing WinSafe, it’s very common to convert the Windows integral data types to their Rust equivalent. Care must be taken, however, when it comes to pointer size, which varies according to the architecture. Since WinSafe is aimed to both 32 and 64-bit Windows, I must pay attention.

For reference, below is the table I’m using to figure out the sizes:

Signed C	Signed Rust	Unsigned C	Unsigned Rust	32-bit	64-bit
CHAR	i8	UCHAR BYTE BOOLEAN	u8	8 bit (1 byte)
SHORT	i16	USHORT WCHAR WORD	u16	16 bit (2 byte)
BOOL INT LONG	i32	UINT ULONG DWORD	u32	32 bit (4 byte)
INT_PTR LONG_PTR LPARAM	isize	UINT_PTR ULONG_PTR WPARAM SIZE_T	usize	32 bit (4 byte)	64 bit (8 byte)
LARGE_INTEGER LONG64 LONGLONG	i64	ULARGE_INTEGER ULONG64 ULONGLONG DWORD64 DWORDLONG QWORD	u64	64 bit (8 byte)

The table above is an extension of this one.

Default props in React function components

Having default props in a React component is a rather common situation. The most popular way to accomplish this is to pass the default values to a defaultProps property on the function component. However, this property will be deprecated in the future.

Spoiler: due to the sheer amount of code written with it, it never will be deprecated. It’s more likely that a warning will show in the console.

Anyway, in order to keep things clean and guard from this future warning, I came up with a clean pure TypeScript solution to this problem:

interface Props {
	name: string;
	surname?: string;
	age?: number;
}

const defaultProps = {
	surname: 'Doe',
};

function MyComponent(props0: Props) {
	const props = {...defaultProps, ...props0};

	return <div>{props.surname}</div>;
}

The code above provides the correct behavior and proper TypeScript validation. It ended becoming an answer on StackOverflow.

Since the spread order may be a a bit hard to remember, this function does the trick:

export function defProp<P, D>(props: P, defaultProps: D): P & D {
	return {...defaultProps, ...props};
}

function MyComponent(props0: Props) {
	const props = defProp(props0, defaultProps);

	// ...
}

Extending built-in TypeScript objects in Vite

Checking whether an array is empty is a rather common operation. The usual way to do this in TypeScript is by checking the length property, but it’s very verbose:

if (myArray.length === 0) {
	// ...
}

For some reason, the JavaScript standard doesn’t specify an isEmpty() method. Fortunately, we can extend native JavaScript objects to add any methods we want. While this is a great feature, I was unable to make it work in a Vue + TypeScript + Vite project. I want the autocomplete of my VSCode to properly display it.

To make the TypeScript compiler recognize it, you must declare your method extensions as global by wrapping the declaration in a declare global block.

At first I tried to write the declaration in the “src/env.d.ts” file, but it didn’t work. Turns out you need to place them in a different file, as explained here. So I finally wrote them in a “src/extensions.d.ts” for the typings, and then the implementations themselves must be in a file that will be actually called in your application:

I tested the code above in a Vue project, but I believe it will work accordingly in a React project as well.

Checking if user passed a slot in Vue

In Vue 3 with the Composition API, there is no this.$slot entry to programmatically poke on the slots. You must summon the slots by calling useSlots() – yes, that’s a React hook right there.

The returned object has one entry for each slot. If your component has only one unnamed slot, it will be named 'default'. So, in order to check whether the user didn’t pass the slot, you simply check whether the entry exists:

<script setup lang="ts">
import {computed, useSlots} from 'vue';

const slots = useSlots();
const hasSlot = computed(() => slots['default'] !== undefined);
</script>