Functional JavaScript - What is a Functor?

When you first start looking into functional programming, you hear a lot of scary terms such as functors, monads, monoids, lift, fold, etc. Some of those things are sort of complicated to understand if you don't have a master's degree in Math, others are not that complex. In any case, those things are generally very useful.

Many times you don't even have to know what the terminology means, we just use it and follow with our life. For instance, in languages that are purely functional or have a functional first approach, things like Functors are part of the language, and we don't have to worry about them.

That's not the case of JavaScript as we have to write our Functors, or use a library that does it for us.

Why would you care?

Well, because functors help us solve a lot of problems when writing code in a functional style. We can use functors to manage null or undefined, to handle errors, to make async calls, among other things. They are an important piece of any functionally styled application.

So what is it?

And please, not the math explanation...

Simply put, a functor is a value (in JS it's an object) that:

1. Has a map method that expects a function
2. The expected function can return any kind of value
3. The map function will return another functor of the same type (as the original functor)

I still don't get it, give me an example

Let's see one of the most simple Functors, the Maybe.

Maybe is used to handle null and undefined values, so our code won't break if we have one of those. Let's see an example of how to use Maybe:

const movie = new Maybe({ title: "Star Wars" });

Also, it's common to have an of function to help us create the Maybe value.

const movie = Maybe.of({ title: "Star Wars" });

Ok, that's good! Now the rule number one says that a Functor must have a method called map that expects a function as a parameter. And rule number two says that function must return any kind of value. So let's put that to use:

const getMovieTitle = movie => movie.title;

const movie = Maybe.of({ title: "Star Wars" });

const movieTitle = movie.map(getMovieTitle); // Maybe('Star Wars');

Here we have a getMovieTitle function that receives a movie and returns its title. We pass that function to the map method. And the map method will call the given function if inside the Maybe value (in this case the variable movie), there's a value that is not null nor undefined.

Finally, the map function will return another Maybe value containing the result of the getMovieTitle function, which will be a Maybe with the value 'Star Wars'.

That's it. That's what a Functor is.

Here's how one could write the Maybe Functor:

const Maybe = function (val) {
	this.__value = val;
};

Maybe.of = function (val) {
	return new Maybe(val);
};

Maybe.prototype.map = function (f) {
	return this.__value !== null && this.__value !== undefined
		? Maybe.of(f(this.__value))
		: Maybe.of(null);
};

What we have here is a constructor that will save a value, in a "private" variable. Then the of function will be used to help us create new Maybe values. And finally, the real purpose of the Maybe functor that is its map function, which will check for null or undefined and call the function passed in as the argument.

See how the map function respects the third rule? It will always return another Maybe, be that with the returned value from the function or with null.

But I've heard that a Maybe is a Monad

Well, Maybe could be a Monad as well. It only has to implement its interface and follow its rules. If you know a language like C# or Java, it's the same as having a class that implements two different interfaces. So yeah, you can have a Maybe value that is a Functor and also a Monad.

A note about classes in JavaScript

I'm about to commit a functional heresy here. In JavaScript a class is just some syntactic sugar over a constructor function. So one could write the Maybe Functor using a class, like this:

class Maybe {
	constructor(val) {
		this.__value = val;
	}

	map(f) {
		return this.__value !== null && this.__value !== undefined
			? Maybe.of(f(this.__value))
			: Maybe.of(null);
	}

	static of(val) {
		return new Maybe(val);
	}
}

The results of this and the previous example are the same.

And why is that a functional heresy you might ask? Well, because in functional programming is about functions, and classes are from another paradigm, they're from OOP and, although we can mix both and the results are the same, we should write code for the next person reading it. If you agree with your teammates that those types can be written using classes, then it's OK.

What's next

Next, I'll be talking about how to put a few Functors into practice.