Automatic creation of constructors.

valueobject.js

// TODO: Automatically create a useful isEqual function.
function valueObject() {
    var names = arguments;
    function wrapped() {
        var o = this instanceof wrapped ? this : {}, i;
        if(arguments.length != names.length) {
            throw new TypeError("Expected " + names.length + " arguments, got " + arguments.length);
        }
        for(i = 0; i < names.length; i++) {
            o[names[i]] = arguments[i];
        }
        return o;
    }
    return wrapped;
}

var Let = valueObject('name', 'params', 'left', 'right');
var StrLit = valueObject('value');
var NumLit = valueObject('value');
var BoolLit = valueObject('value');
var UndefinedLit = valueObject();

console.log(new Let('brian', [], StrLit('Brian McKenna'), NumLit(22)) instanceof Let); // true
console.log(new BoolLit(true) instanceof BoolLit); // true