ADT, Decorators, Pattern Matching

option.js

// This is a decorated version of <https://github.com/tc39/proposal-pattern-matching#custom-matcher-protocol-interpolations>

// A predefined ADT decorator.
// Use it on static "constructor" methods only.
// Pass an extractor function, to produce a pattern-matching value.
// The instance must have an [ADT.kind] property
// set to the constructor method used.
const ADT = (valExtractor=()=>null) => (_, {methodName, isStatic, kind, addInitializer}) => {
	if(kind == "method" && isStatic) {
		addInitializer(function(){
			// `this` is the class
			const cls = this;
			const method = cls[methodName];
			method[Symbol.matcher] = (matchable) => {
				if(matchable instanceof cls && matchable[ADT.kind] == method) {
					return {matched: true, value:valExtractor(matchable)};
				}
				return {matched:false};
			}
		});
	} else {
		throw new Exception("only usable on static methods");
	}
}
ADT.kind = new Symbol("ADT kind");

class Option {
	constructor(kind, val) {
		this[ADT.kind] = kind;
		if(this[ADT.kind] == Option.Some) {
			this._value = val;
		}
	}
	
	get value() {
		if(this.isSome()) return this._value;
		throw new Exception("Can't get the value of an Option.None.");
	}
	
	get isSome() {
		return this[ADT.kind] == Option.Some;
	}

	@ADT(x=>x.value)
	static Some(val) {
		return new Option(Option.Some, val);
	}
	
	@ADT()
	static None() {
		return new Option(Option.None);
	}
}

match(result) {
	when (${Option.Some} with val) -> console.log(val);
	when (${Option.None}) -> console.log("none");
}