Objects are a poor man’s closures. Closures are a poor man’s objects.

Some people say everything is an object. I used to too but these days I think everything is actually a function.

  1. A map? Isn’t that just a function that maps from one value to another?

  2. An array? Well that can be thought of as a map which can be thought of as a function.

  3. An object? Those are just partial functions that return functions.

Lets take a closer look. Here is a pair implemented as a function. It is implemented in Javascript for illustration purposes.

var pairCreator = function(first, second) {
  var first, second;
  return function(which) {
    return which == 0 ? first : second;
  }
}
var pair = pairCreator('hello', 'there');
console.log(pair(0) + ' ' + pair(1));

How do we implement an object as a function? Well it is pretty similar to the pair example.

var MyObjConstructor = function(greeting) {
  var greet = function() {
    console.log(greeting);
  }
  var leave = function() {
    ...
  }
  return function(method) {
    switch(method) {
      case 'greet':
        return greet;
      case 'leave':
        return leave;
    }
  }
}
var obj = MyObjConstructor("hello there");
obj('greet')(); // call the greet method
var leave = obj('leave'); // save a pointer to the leave method

Of course you wouldn’t ever really want to implement an object like that in Javascript ;)

It is an interesting thought experiment to try to look at everything as a function. It’ll get you to design your software in new ways and to find entirely new applications for closures.

The venerable master Qc Na was walking with his student, Anton. Hoping to prompt the master into a discussion, Anton said** “Master, I have heard that objects are a very good thing — is this true?”** Qc Na looked pityingly at his student and replied, “Foolish pupil — objects are merely a poor man’s closures.”

Chastised, Anton took his leave from his master and returned to his cell, intent on studying closures. He carefully read the entire “Lambda: The Ultimate…” series of papers and its cousins, and implemented a small Scheme interpreter with a closure-based object system. He learned much, and looked forward to informing his master of his progress.

On his next walk with Qc Na, Anton attempted to impress his master by saying “Master, I have diligently studied the matter, and now understand that objects are truly a poor man’s closures.” Qc Na responded by hitting Anton with his stick, saying “When will you learn? Closures are a poor man’s object.” At that moment, Anton became enlightened.

http://people.csail.mit.edu/gregs/ll1-discuss-archive-html/msg03277.html