less minimalism, richer leather

minimalist-classes.js

// A response to jashkenas's fine proposal for minimalist JavaScript classes.

// Harmony always stipulated classes as sugar, so indeed we are keeping current
// JavaScript prototype semantics, and classes would only add a syntactic form
// that can desugar to ES5. This is mostly the same assumption that Jeremy
// chose, but I've stipulated ES5 and used a few accepted ES.next extensions.

// Where I part company is on reusing the object literal. It is not the syntax
// most classy programmers expect, coming from other languages. It has annoying
// and alien overhead, namely colons and commas. For JS community members who
// don't want classes, either proposal is "bad", so let's focus on the class
// fans who will use this feature.
//
// Therefore I propose giving classes their own new kind of body syntax, which
// is neither an object literal nor a function body. The main advantage is that
// syntax for class elements can be tuned to usability, even as it desugars to
// kernel semantics drawn from ES5 extended by both the super and private names
// proposals.

// Not shown below is the super proposal at
//
// http://wiki.ecmascript.org/doku.php?id=harmony:object_initialiser_super
//
// which composes as expected when 'super' is used in methods defined in a
// class instead of in an object literal.
//
// In contrast, you will see hints of the private name objects proposal at
//
// http://wiki.ecmascript.org/doku.php?id=harmony:private_name_objects
//
// Also visible throughout: the method definition shorthand syntax proposed at
//
// http://wiki.ecmascript.org/doku.php?id=harmony:object_literals#object_literal_property_shorthands
//
// For programmers who want classes as sugar for constructors and prototype, I
// hope this proposal wins.
//
// To be perfectly honest, I personally would be happy with Allen Wirfs-Brock's
// "Exemplars" approach and no classes. But for programmers coming from classy
// languages, I believe "minimalist classes" will tend to be too minimal to be
// usable out of the box. They'll want more.
//
// Brendan Eich, 1-Nov-2011

// First, basic usage from a real-world library (Three.js)

class Color {

  // Commas are a curse in class syntax, not a blessing as in current object
  // literal syntax. But with method definitions as proposed for ES6 object
  // literals, commas could be optional. Allen proposed this at the last TC39
  // meeting.
  //
  // Problems if commas are optional after data properties, but after methods
  // should be ok.

  constructor(hex) {
    ...
  }

  // DUBIOUS: prototype data properties are a footgun, Alex Russell points to
  // hazards involving mutable objects. I observe that non-writable prototype
  // data properties are unheard of, likely because you can't shadow them by
  // assignment.
  //
  // But if we must have them, we could use object literal syntax for writable
  // data properties as follows:
  //
  r: 1, g: 1, b: 1,

  copy(color) {
    ...
  }

  setRGB(r, g, b) {
    ...
  }

  setHSV(h, s, v) {
    ...
  }

}


// To create a class with its prototype chain set correctly:

class Fox extends Animal {
  ...

  // The special syntax for defining class-level properties and methods uses a
  // prefix keyword. The 'static' keyword is reserved and somewhat hated (or at
  // least considered a misnomer), but I went with it instead of 'class' to
  // avoid confusion in overloading the c-word, and to be more future-friendly
  // in case nested classes are wanted (right now I do not want them, since a
  // class at class-body level would declare a prototype inner class and not a
  // static inner class).
  //
  // Class-side properties inherit, just as constructor-side properties do with
  // the <| proposal:
  //
  // http://wiki.ecmascript.org/doku.php?id=harmony:object_literals#individual_extension_summary

  static const CONSTANT: 42,

  static classMethod() {
    ...
  }
}

console.log(Fox.CONSTANT);

Fox.classMethod();


// Classes are expression forms as well as declaration forms. As with function
// expressions, the name is optional in a class expression.

const frozenClass = Object.freeze(class {...});

// Named class expressions are supported too. The name is bound only in the
// scope of the class body, as for named function expressions.

animals.push(class Fox {});

// An anonymous class expression with superclass specification, after Jeremy's
// gist, but with an explicit and required body.

var subclass = function(parent) {
  return class extends parent {
    ...
  };
};


// Unlike Jeremy's proposal, classes cannot be built up programmatically by
// abutting an expression to a class head. That's too dynamic, it doesn't work
// with the 'super' proposal.  So any synthesis of a class body must use eval
// or equivalent. At this time, I do not propose adding a Class constructor
// analogous to Function, but I believe it could be added without issue.


// As in Jeremy's gist, here's the Monster class from the harmony: proposal
// (http://wiki.ecmascript.org/doku.php?id=harmony:classes#the_proposal_in_a_nutshell)
// ... sans unnecessary 'public' keyword prefixing!

class Monster {

  constructor(name, health) {
    this.name = name;
    this.health = health;
  }

  attack(target) {
    log("The monster attacks " + target);
  }

  isAlive() {
    return this.health > 0;
  }

  setHealth(value) {
    if (value < 0) {
      throw new Error("Health must be non-negative.");
    }
    this.health = value;
  }

  numAttacks: 0,

  const attackMessage: "The monster hits you!"

}

// Now there's one more problem to address, which could be deferred, but which
// falls under "batteries included" to some (not all) of the cohorts of classy
// programmers using JS or coming to it fresh in the future. And that is the
// cost of this[kPrivateName] given const kPrivateName = Name.create(...) where
// module Name from "@name" has been declared.
//
// Instead I propose we support private kPrivateName, ...; as a special form
// only in class bodies (for now) that both creates a private name object and
// binds it to a lexical const binding that may be accessed on the right of @
// in methods defined in a class.
//
// For class-private instance variables, obj@foo is supported as well (with no
// LineTerminator to the left of @ in this case, or it's short for this@foo).
// See the new sameName method for an example of infix and prefix @ in action.
//
// David Flanagan suggests keeping the unary-prefix @foo shorthand, but making
// the long-hand obj.@foo. Breaks E4X but no matter -- but the win is that the
// [no LineTerminator here] restriction on the left of unary-prefix @ is not
// needed. Also the references don't grep like email addresses, which counts
// with David and me. So I've incorporated David's suggestion. See other.@name
// etc. below.
//
// There is no const instance variable declaration. Non-writable instance vars
// (properties to most people) are quite rare. Use ES5's Object.defineProperty
// if you must. This avoids ugly fights about read before constant instance var
// initialization too.
//
// OOP often requires factoring common subroutines into private methods, so the
// 'private' keyword should be supported as a method prefix, just as 'static'
// is. Should we allow 'static private' methods too? Same rationale applies if
// you have two static (public) methods with a large common subroutine.

class Monster {

  private { name, health }  // can group a prefix, same for const and static
                            // note comma optional after closing brace, as for
                            // method definitions

  public flair,             // you should have 37 pieces of public flair

  // Yes, shorthand for @name = name given formal param name, etc. -- just as
  // in CoffeeScript!
  constructor(@name, @health) {
    @flair = 0;
  }

  sameName(other) {
    return @name === other.@name;
  }

  private attackHelper(target) {
    ...
  }

  attack(target) {
    log("The monster attacks " + target);
    return @attackHelper(target);
  }

  isAlive() {
    return @health > 0;
  }

  setHealth(value) {
    if (value < 0) {
      throw new Error("Health must be non-negative.");
    }
    @health = value;
  }

  numAttacks: 0,

  const attackMessage: "The monster hits you!"

}

// As in the harmony:classes proposal, 'const class' freezes all of
//  1. the class binding, if named declaration rather than expression,
//  2. the class prototype induced by the const class,
//  3. the constructor method of the const class.
//
// 'const class' also seals instances constructed by the constructor, as if the
// last line of the constructor method were Object.seal(this), called using the
// original values of Object and Object.seal.

const class Frosty {
}