Introduction to monads in JavaScript

http://blog.jcoglan.com/2011/03/05/translation-from-haskell-to-javascript-of-selected-portions-of-the-best-introduction-to-monads-ive-ever-read/

by Isofarro • 14 years ago

155 34 14 years ago

blog.jcoglan.com

crazygringo• 14 years ago

Can anyone tell me, in what real-world practical situation, this would be a better design model than just normal IO-based logging?

Would there ever be a reason to do this in the browser? Or is this something that would be more useful in a Node.js environment, and if so, when? Or is it more theoretical than practically useful?

gridaphobe• 14 years ago

The Writer Monad probably isn't too useful in any impure language, and IMO was a poor choice by the author. If you read the last few paragraphs though, you're introduced to the List Monad (the bit about HTML nodes), which is useful in impure languages.

Another example I would have liked to see is the Maybe Monad, which allows composition of functions that might return a value, or null/undefined.

tmhedberg4• 14 years ago

If we look at an impure language like JavaScript through Haskell-colored glasses, essentially every line of code in a JS program is automatically in the IO monad. Haskell's bonus is that it gives you the frequently-exercised option of writing code that's explicitly not capable of doing I/O, simply by omitting IO from its type, and having this constraint enforced by the compiler. This self-imposed restriction helps avoid the numerous problems that come along with uncontrolled side effects. But since you can't "escape" the IO monad in JavaScript, there's not much point in making it explicit by doing output in a visibly monadic way.

That being said, there are plenty of uses for the monad pattern outside of the I/O domain. That's where a lot of emphasis is placed in Haskell, because Haskell couldn't do I/O without monads, but there are tons of other useful monads that have nothing to do with side effects. As the article illustrates, two of these are Writer and List (written as [] in Haskell). Another massively useful one is Maybe, which represents nullability, but by making it explicit in a value's type we can avoid a lot of the problems that arise in "nullable by default" languages.

In some sense, jQuery is an example of a monadic library, wherein the unit function is $, which lifts its argument into the jQuery monad, and bind is represented by the chain of method calls applied to that lifted value, each of which returns a different value, still wrapped in the jQuery monad. I've been told that LINQ in the C# world works similarly (in fact, there is supposedly a lot of bleed-over of ideas from Haskell into .NET because Simon Peyton-Jones works alongside some of the .NET language designers at Microsoft Research).

jlarocco2• 14 years ago

This is one of the reasons why I don't use Haskell.

Every concept you mention is made more complicated thinking about it with monads. Every time I use Haskell I feel like too much mental energy is wasted on silly stuff like this.

bluekeybox• 14 years ago

> In some sense, jQuery is an example of a monadic library, wherein the unit function is $, which lifts its argument into the jQuery monad, and bind is represented by the chain of method calls applied to that lifted value, each of which returns a different value, still wrapped in the jQuery monad.

Aha. Thank you. Ever since I first learned about monads in Haskell (which is not so long ago), I kept noticing examples of such patterns in some libraries and was wondering whether it's only me who thinks there is a similarity.

pash• 14 years ago

> ... LINQ in the C# world works similarly (in fact, there is supposedly a lot of bleed-over of ideas from Haskell into .NET because Simon Peyton-Jones works alongside some of the .NET language designers at Microsoft Research).

Erik Meijer, the creator of LINQ, is himself a functional-programming guru. A couple of years ago, he gave a series of lectures on Haskell for MSDN's Channel 9 [0] that are a great introduction to Haskell and FP generally. Throughout the series, he points out intriguing connections between the ways of doing things in Haskell and C# that at first seem like they could not be more different.

0. http://channel9.msdn.com/Series/C9-Lectures-Erik-Meijer-Func...

ufo• 14 years ago

The problem with monads is that its hard to justify the whole concept until you start wanting abstract some code to work with more then one monad.

That said, specific monads actually show up pretty often. A very important example is the Promise monad used by some Javascript async libraries:

(for example, see https://github.com/kriskowal/q or http://dojotoolkit.org/documentation/tutorials/1.6/promises/)

Basically, creating a new promise is the monadic "return" and the "promise.then" method is the monadic "bind" (or "fmap", if the callback returns a normal value instead). I found that knowing a bit about monads helped me work better with the promises from the Dojo framework but I guess your mileage may vary...

Peaker4• 14 years ago

Generalizing Monad in a language without return-type polymorphism is hard. How do you implement "return" (unit)?

tikhonj5• 14 years ago

The problem is that the tutorial was transliterated from Haskell. So while it's useful for understanding the idea behind monads, the examples are not very motivating in a language like JavaScript. The one interesting example in the article for actual JavaScript use is probably the list monad which lets you compose functions returning an arbitrary number of results rather than just one (these functions are often called "nondeterministic").

However, it's possible to imagine other useful monads for JavaScript use. For example, I think we can abstract over CPS (continuation-passing style, like asynchronous functions in Node.js) functions using a monad.

This will involve changing our idea of CPS a little bit: instead of having a function f(x, k) where x is the argument and k is the callback, let's have a function f(x) which returns a function g(k) with x and k being the argument and callback respectively. Now we have to define the basic monad functions on it.

First, let's start with unit (which is called "return" in Haskell). The idea is simple: we want to go from some value to a function accepting a callback. What is a reasonable way to do this? Let's define it in a trivial way: we go from x to a function that, given a callback, immediately calls it with x. So:

    CPS.unit = function (x) {
      return function (k) {
        k(x);
      };
    };

Next, we want to define bind. Now, what will bind do? The core idea is this: given a wrapped value m x and a function f: x → m y, we need to apply f to the wrapped value. Now, in this context, the "wrapped value" is a function accepting a callback. Essentially, this wrapped value is a promise to provide x at some point in the future. So we want to take this "promise" and get a new promise for the result of calling another CPS function on it. Here is a way to do this:

    CPS.bind = function (f, x) { // f: x → m y, x: m x
      return function (k) {
        x(function (x_val) {
          f(x_val)(k);
        });
      };
    };

Admittedly, this code is a little confusing; I will try to explain it. The basic idea is simple: we create a new promise (the outermost returned function) that will trigger after x triggers and will then pass both the result of x (x_val) and the given callback (k) into f. That is, we get a promise of the result of f called with the eventual value of x.

This lets us bind together all these asynchronous values so that the result is still asynchronous. This makes managing "callback hell" simpler: instead of having a ton of nested callbacks, we can have a string of calls to bind. We also get a bunch of functions for free. For example, let's say you have a deferred x and want to apply a function to the result. This is essentially mapping the function over the deferred value. You could write it like this:

    CPS.map = function (f, x) {
      return CPS.bind(compose(CPS.unit, f), x);
    }

However, the true power of monads is that this map function works exactly the same way on every monad. So you could rewrite it like this:

    Monad.prototype.map = function (f, x) {
      return this.bind(compose(this.unit, f), x);
    }

then make all your monad "types" (for lack of a better word) inherit from Monad. So the CPS type I showed here would get map for free, but so would an analogous List type.

Moreover, this is true for a whole bunch of useful functions. For example, you would be able to generalize reduce to accept a reducer function that behaves like a monad instead of like a normal function (you could call it reduceM). So you would then be able to reduce using an asynchronous function! I think that's pretty cool.

Note that it isn't enough just to write unit and bind to get a monad--the functions also have to follow some laws. If these laws aren't followed, you get all sorts of weird behavior with your monad. I think my CPS instance follows these laws, but I'm too lazy to check just this moment. I'll leave it as an exercise to the reader (I've always wanted to say that).

The reason we have to do some extra legwork (like creating the CPS object) is that JavaScript is not good at differentiating between types. Particularly, there is no way to consider only CPS functions. Additionally, there is a valid monad instance for all functions which behaves differently from the CPS one, and there would be no way to differentiate between the two without making it explicit. Now, I'm sure there is a neat way to make bind and unit methods of appropriate types, but that would make the examples even more complicated and I don't want to think about it right now :P.

In summary: monads let you generalize over a bunch of different concepts, not just IO. In particular, you can use monads to write code that works generically over CPS functions and nondeterministic functions and a bunch of others.

EDIT: I forgot to note: promises already have a well-known meaning in JavaScript. I don't actually know that meaning because I've never used them. The only reason I use the word is because I couldn't think of anything better to call them. It's likely the semantics of existing promises and the semantics of the objects I described are actually different, but they do serve the same purpose.

ibotty• 14 years ago

re the comment in the original article about monadic javascript: take a look at roy. http://roy.brianmckenna.org

plainOldText• 14 years ago

Wow, I now understand monads. This article nailed it.

tikhonj5• 14 years ago

If you like this sort of thing, check out the Arrowlets[1] library for JavaScript. It uses "arrows", an abstraction in a similar vein to monads, to make CPS functions and event handling neater (among other things).

[1] http://www.cs.umd.edu/projects/PL/arrowlets/

From playing around with it, I think the arrows from that library are more useful to practical JavaScript programming than monads.

Ralith• 14 years ago

Considering that arrows are a generalization of monads, that shouldn't be a surprise.

luciferous2• 14 years ago

Since you're familiar with arrowlets, would you mind having a look at samsara?

[samsara]: http://lcfrs.org/samsara/docs/dragndrop.html

tikhonj5• 14 years ago

That seems cool--CoffeeScript's syntax is definitely better-suited for this sort of thing than JavaScript's!

The main page says the library's in CoffeeScript, but the drag and drop example uses JavaScript. Do you have a similar (or even the same) example in CoffeeScript? I'd like to see what it looks like.

doublerebel• 14 years ago

    ...it helps you spot accidental complexity...Being able to spot and extract
    such boilerplate can radically improve the clarity of your code.

I think a second version of this tutorial in CoffeeScript would really shine a light on how CS is the best of JavaScript. Monads and FP are a breeze -- practically encouraged -- in CoffeeScript. It really removes the cruft of JS syntax and lets me focus on well-designed code, and I've found myself using similar patterns since I switched to CS. I code faster and (contrary to what I expected) debugging is much easier.

tripzilch• 14 years ago

I've read many attempts to explain Monads by now, but this is the first time I think I actually "got" it. Well done!

Introduction to monads in JavaScript

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