Is this a monad?

Question

I'm trying to understand the concept of monads and I want to know if this code is an implementation of this concept (in JavaScript).

I have function M which return new object that have set method which create wrapper method

var foo = M().set('getX', function() { 
  return this.x; 
}).set('setX', function(x) { 
  this.x = x;
}).set('addX', function(x) { 
  this.x += x;
});

And then I can chain method of foo

foo.setX(10).addX(20).addX(30).getX()

will return 60

and the same if I have object with methods and call M with this object.

var foo = {
  x: 10,
  add: function(x) {
    this.x += x;
  }
};

M(foo).add(10).add(20).add(30).x

will return 70

Functions are wrapped inside M object so the this context inside method is always that M object.

f = M({x: 20}).set('getX', function() {
   return this.x; 
}).set('addX', function(x) {
   this.x += x;
}).addX(10).getX

so f is function with context of object wrapped by M — if I call f() it will return 30.

Am I understand this correctly? Is M a monad?

EDIT modified code is on github https://github.com/jcubic/monadic

Answer 1

This is a monoid pattern. Each state-updating operation, such as .setX(10), .addX(20), and so forth, is a computation that transforms one object. (To be syntactically valid, you would have to write it as a one-parameter function function(x) {x.addX(20);}, but I think it's clearer if I use the short form.)

Two things make this a monoid. First, there is an identity element: .addX(0) does nothing to its object. Second, any two operations can be combined. For example, .setX(10).addX(20) is also a computation that transforms one object.

It is not a monad. The computations supported by your methods are limited to writing and updating this.x. (.getX() is not a member of the monoid because you can't chain anything after it). For example, with a monad you can have one member of a chain of operations execute an if-then-else to decide what comes next in the chain. Your methods can't do that.