class BaseFunction {
  static #allowInstantiation = false;

  constructor(...args) {
    if (!BaseFunction.#allowInstantiation) {
      throw new Error(
        "Why are you trying to use 'new'? Classes are so 2015! Use our fancy 'run' method instead!"
      );
    }
    for (const [name, validator] of this.parameters()) {
      this[name] = validator(args.shift());
    }
  }

  parameters() {
    return [];
  }

  body() {
    return undefined;
  }

  static run(...args) {
    BaseFunction.#allowInstantiation = true;
    const instance = new this(...args);
    BaseFunction.#allowInstantiation = false;
    return instance.body();
  }
}

class Add extends BaseFunction {
  parameters() {
    return [
      ["a", (x) => Number(x)],
      ["b", (x) => Number(x)],
    ];
  }

  body() {
    return this.a + this.b;
  }
}

console.log(Add.run(5, 3)); // 8
    • Ephera
      21•1 month ago

      Yep, some code examples from the official documentation. This:

      printPersons(
    roster,
    (Person p) -> p.getGender() == Person.Sex.MALE
        && p.getAge() >= 18
        && p.getAge() <= 25
);

      …is syntactic sugar for this:

      interface CheckPerson {
    boolean test(Person p);
}

printPersons(
    roster,
    new CheckPerson() {
        public boolean test(Person p) {
            return p.getGender() == Person.Sex.MALE
                && p.getAge() >= 18
                && p.getAge() <= 25;
        }
    }
);

      …which is syntactic sugar for this:

      interface CheckPerson {
    boolean test(Person p);
}

class CheckPersonEligibleForSelectiveService implements CheckPerson {
    public boolean test(Person p) {
        return p.gender == Person.Sex.MALE &&
            p.getAge() >= 18 &&
            p.getAge() <= 25;
    }
}

printPersons(roster, new CheckPersonEligibleForSelectiveService());

      The printPersons function looks like this:

      public static void printPersons(List<Person> roster, CheckPerson tester) {
    for (Person p : roster) {
        if (tester.test(p)) {
            p.printPerson();
        }
    }
}

      Basically, if you accept a parameter that implements an interface with only one method (CheckPerson), then your caller can provide you an object like that by using the lambda syntax from the first example.

      They had to retrofit lambdas into the language, and they sure chose the one hammer that the language has.

      Source: https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html

      • @mbtrhcs@feddit.org
        15•1 month ago

        That’s not quite right. In bytecode, lambdas are significantly more efficient than anonymous class instances. So while the lambda implementation is semantically equivalent, characterizing it like you have is reductive and a bit misleading.

    • @ornery_chemist@mander.xyz
      5•1 month ago

      Hence, Clojure. It’s not just functions that implement IFn… as the string of “cannot cast to clojure.lang.IFn” errors that I get because I couldn’t be bothered to validate my data’s shape is eager to inform me.

    • Kairos
      3•1 month ago

      Golang also does this, but it’s not classes.

        • Kairos
          1•30 days ago

          Golang uses modules, not classes. Each of which may have its own main function.

          • EthanEnglish
            1•1 month ago

            Huh? Main file? Do you mean main package? A module can contain an arbitrary number of main packages but I don’t see how that has anything to do with this post. Also are you saying modules are equivalent to classes? That may be the strangest take I’ve ever heard about Go.