scoped-private.md (original) (raw)

Sketch: scoped private fields

So far, private fields are introduced as follows:

class MyClass { #privateField = 123; }

I’d like to propose a small extension: scoped private fields.

Functional style (the proposal)

The following style is becoming popular in the JavaScript world. It benefits from scoped privacy:

private #data;

function createStringBuilder() { return { #data: '', }; }

function add(sb, str) { sb.#data += str; }

function toString(sb) { return sb.#data; }

New in this code:

• The keyword private in the first line.
• The ability to use private fields in object literals.

OOP style

As a slight downside, you now always need to use the keyword private:

class StringBuilder { private #data = ''; // keyword is required add(str) { this.#data += str; } toString() { return this.#data; } }

On the upside, this gives you the freedom to widen the scope of privacy:

private #data;

class StringBuilder { #data = ''; // no keyword! add(str) { this.#data += str; } } function toString(stringBuilder) { return stringBuilder.#data; }

Alternative syntax

Alternative syntax has been proposed:

• Keyword private not needed for “normal” private fields.
• Two keywords if you want to widen the scope of privacy: private and outer.

Example:

private #data;

class StringBuilder { outer #data = ''; // keyword is now required add(str) { this.#data += str; } } function toString(stringBuilder) { return stringBuilder.#data; }

FP example:

private #data;

function createStringBuilder() { return { outer #data: '', // keyword is now required }; }

function add(sb, str) { sb.#data += str; }

function toString(sb) { return sb.#data; }