OCaml to JavaScript buzzwords

There has been a lot of change in the OCaml -> JS landscape over the last couple of years, and I regularly see questions about what all the names mean. This is my attempt to sort out the world as I know it.

As you navigate the world of compiling OCaml to JS, most if not all of the following will come up at some point:

That’s a lot to remember, but each works in their own similar yet completely distinct way. They can also be mixed and matched as projects see fit. I’m going to use diagrams to attempt to explain all of this and hopefully provide some clarity.

With apologies to those who understand how over-simplified this is, here is a rough overview of how compilers work, specifically in the case of OCaml.

OCaml Compiler.png
The OCaml compiler

Code flows from top to bottom in three distinct phases:

  1. Syntax is read into an Abstract Syntax Tree
  2. Checks are done to confirm the AST has valid semantics in the OCaml language
    • For example, type checking
    • Optimisation is performed at this level too
  3. The AST is written out to a machine executable
    • OCaml supports both native output, like a C compiler, and platform-independent bytecode output like a Java compiler (although not using JVM bytecode, OCaml has its own unique VM).

Are you with me so far? Good. Next we’re looking at the development that happened in 2011 with the creation of js_of_ocaml.

js_of_ocaml runs after the compiler

This is very simple, on the surface of it. Leave the compiler alone and machine translate the bytecode to JavaScript. This makes JSOO able to handle literally any pure OCaml code, including the diverse ecosystem of libraries, but there are two major downsides:

  • It can be a bit slow, JSOO is effectively a second compiler
  • The resulting JS is mostly unreadable machine code (since that’s more or less what JSOO had to start with). Source maps help here, but they aren’t a silver bullet.

In early 2016, Bloomberg open sourced their answer to this process. Instead of treating the compiler as a black box and working with the result, they dug in and replaced the output phase with Bucklescript.

Bucklescript replaces the compiler backend

This no doubt took a lot of effort to achieve, but the result has some unquestionable benefits:

  • By working with the compiler internals, their output retains the structure of the original code
  • Raw JavaScript types are used (i.e. OCaml strings are JS strings) producing clean readable JavaScript – at least until advanced types are used (here’s the mapping).
  • Most if not all of the compiler speed is retained

Really the only downside I can see is that by working so deep in the compiler they have to chase new compiler releases (as I write this it is still based on the OCaml compiler released in July 2015). Not that this is a particularly bad thing as the OCaml compiler is fairly stable – and it has since been further mitigated as we’ll see in a moment.

A few months after Bloomberg opened their project to the world, Facebook came along and announced Reason. I watched on with great interest as OCaml seemed poised to win the hearts and minds of JavaScript developers.

Reason replaces the compiler frontend.

Things are moving a lot faster at this point. Because reason is a 1:1 syntax mapping to OCaml, they’re able to keep up with compiler releases quite easily. This allows them to focus on what they really want to bring to the JavaScript ecosystem; reliable and easy-to-use tooling that leverages the power of OCaml to tempt JS programmers with mostly familiar syntax. And thanks to leaving the rest of the compiler untouched, the same familiar syntax can compile to native binaries that perform better than JS could ever dream of.

But here is where the real fun begins. If you compare the reason diagram to earlier diagrams, you’ll begin to wonder if it could be combined with with either js_of_ocaml or bucklescript. And that’s exactly what happened. When I first looked at this, efforts were made to support both as ways to compile reason source code to JS. That’s still possible, but like all good communities a single recommended approach is appearing and the tools are moving in that direction.

In the last few months the community has settled on the solution that is very fast and produces output JS that can be very similar to the input reason code. Native compilation and tooling still uses the reason compiler, but bucklescript has added first-class support for reason syntax directly into their compiler.

By their powers combined…

Wow. Check out where we are now.

  • The syntax is familiar to JavaScript developers, and there is a groundswell of effort building to bring react developers in particular on board.
  • The full power of OCaml is available. I won’t sit here and argue it’s the best type system around, I freely admit it has flaws, but OCaml still has tangible benefits over JavaScript. The choice of a battle-tested compiler with a mature and diverse ecosystem is hard to pass up.
  • The output JavaScript is clean, readable, and easy enough to follow that you could check it into source control for team members that don’t want to learn reason yet. I have literally seen recommendations to do this.

Three years ago, I announced my belief that OCaml was the next big thing for JavaScript. Thanks to reason and bucklescript, that future looks to be fast approaching. It’s an exciting time to be a JavaScript developer.

Come join the fun in the reasonml discord!

A look back at “why OCaml”

My post describing why I was interested in OCaml was originally intended as an internal document at my office arguing that we should invest in an AltJS project. The failure of that effort was unfortunate, but the post opened up a gold mine of support. Not just once, but twice (we’ll get to that in a moment).

I saw confusion from people who hadn’t heard of OCaml, and some attackers, but plenty of people defending it. It was a fairly normal programming language discussion, to be honest, which struck me as impressive for a language that wasn’t mainstream and vindicated my choice.

Writing up my thoughts on this would’ve been a good idea three years ago, and certainly two years ago after the second traffic bump, but despite the probable lack of value in these links now I still want to get these thoughts out on record.

So let’s start with the morning after the post went up (due to time zones most of the discussion happened while I was asleep). I saw wordpress notifications that my stats were booming, most referrals pointing to this reddit thread. A coworker captured a screen shot of the hacker news front page for me, with discussion happening in this Hacker News thread. More than 15000 views on that single day.

hacker news blog post pos5.png

That was unbelievably cool. I found a discussion on lobsters, Erik Meijer tweeted about it, I enjoyed my 15 minutes of fame and participated in what remained of the discussion as best I could.

But almost a year later, in February 2015, it happened again:

blog view stats.png

Thanks to someone else posting it to Hacker News:

hacker news pos7.png

The total stats are what I really want to highlight. 18309 views in March 2014, plus another 15101 in February 2015, have raised this post to great exposure. It has legs, too, continuing to drive 100-200 views a month.

And yet I’ve been silent since then. Looking back at the two hacker news threads now, I don’t think I even read the 2015 comments in great detail. The failure of my efforts in 2014 pretty much lead to burnout on all coding outside of work hours. I didn’t even celebrate the 10th anniversary of creating this blog (November 2015). It was all just… nothing.

I was very excited about all of this in 2014. Thanks to some prompting from Jordan Walke on twitter, who gave me a really interesting gist link to the ocaml-based API that he was hoping to use for react, I embarked on creating a good quality ocaml-js wrapper. One that maintained pure ocaml as much as possible, unlike later efforts. I have kept it private all this time due to the embarrassing lack of progress since burning out, but it’s relevant now so here we go.


Note the distinct lack of commits after about a month. I am very proud however that the MyComponent file makes no reference to js_of_ocaml, and the example code only uses it to pass a DOM element to the render function. This is how web coding should be, and I’m very happy to see that it is the direction we’re headed.

So why am I surfacing now? In a similar parallel to last time, it’s because discussions at work have finally returned to AltJS. I have a new post brewing in my head about the current state of OCaml and JS, but I wanted to get this one out first. I’m doing it for my own record and for posterity, but feel free to get the discussions rolling again 😀

Why OCaml, why now?

OCaml first hit my radar in November 2013. I had just learnt SML, a similar but older language, in the excellent Programming Languages Coursera course. Dan Grossman is one of the best lecturers I’ve ever seen, I found his explanations hit all the right notes and made learning easy. The simplicity of the SML syntax, and the power of the language while still producing code that is readable with minimal training immediately appealed to me.

Over the last 3 years I have tried, and failed, to learn Haskell. The combination of minimalist syntax, pure functional programming style and lazy evaluation is like a 3-hit sucker punch that is very hard to grasp all at once. Having now learnt SML and OCaml, which like Haskell are based on the ML language, that has changed. I have yet to put any more effort into learning Haskell, but it is now clear to me that the syntax is only a small leap from ML and the pure functional style has similarities to SML.

I still don’t want to write production code in Haskell, but the fact that I find it less scary than I used to indicates I have made a significant jump in my knowledge and, arguably, career in the last 6 months.

Dynamic typing

Before I go any further, I need fans of dynamic typing to exit the room. My 12 years in the industry have set my camp firmly on the static typing side of the fence, and discussions about static vs dynamic will not be productive or welcome here.

So, why OCaml?

Smarter people than me have written about this, but I’ll give it a shot.

I have found OCaml to be a refreshing change of pace. Most of my favourite things are derived from the ML base language; variants, records, and pattern matching combine to create elegantly powerful code that is still easy to follow (unlike most Haskell code I’ve seen).

Ocaml takes the expression-based ML style and incorporates enough imperative features to make it comfortable for someone learning Functional Programming. Don’t know how to use recursion to solve a problem? Drop into a for loop in the middle of your expression. Need some debug output? Add it right there with a semicolon to sequence expressions.

Throw in almost perfect static type inference, a compiler that gives useful error messages and immutable-by-default variables and I just can’t get enough. I won’t sit here and list every feature of the language, but hopefully that piques your interest as much as it did mine 😉

Industry acceptance

There is always an element of “I have a hammer, everything looks like a nail” when learning a new language but the evidence that OCaml is becoming more widely accepted is not hard to find.

In the middle of February, Thomas Leonard’s OCaml: what you gain post made waves; the reddit and hackernews discussions are fascinating. A lot of people using OCaml in the industry came out of the woodwork for that one. I’m still working my way through the series of 11 posts Thomas made, dating back to June 2013, about his process of converting a large Python codebase to OCaml.

Facebook have a fairly extensive OCaml codebase (more details below).

It doesn’t take much googling to find presentations by Skydeck in 2010 (they wrote ocamljs, the first ocaml to JS compiler) or a 2006 talk describing why OCaml is worth learning after Haskell.

OCamlPro appear to be seeing good business out of OCaml, and they have an excellent browser-based OCaml tutorial (developed using, of course, js_of_ocaml).

No list of OCaml developers would be complete without mentioning the immense amount of code at Jane Street.

There are plenty of other success stories.

The elephant in the room

The first question I usually get when I tell a Functional Programming guru that I’m learning OCaml is “Why not Haskell?”. It’s a fair enough question. Haskell can do a ton more than OCaml can, and there are only one or two things OCaml can do that Haskell can’t (I don’t know the details exactly, I would think it was zero). I see a lot of references to OCaml being a gateway drug for Haskell.

The answer is JavaScript. As much as I hate the language, JS is the only realistic way to write web apps. Included in the many and varied AltJS languages, both OCaml and Haskell can be compiled to JavaScript but the Haskell compilers aren’t mature enough yet (and I’m not convinced lazy evaluation in JavaScript will have good performance).

In fact, some study has revealed OCaml may be the most mature AltJS compiler of all by virtue of support for existing OCaml libraries.


Late last year I started hearing about OCaml at Facebook. Their pfff tool, which is a serious OCaml codebase all by itself, is already open source – but there was talk of an even larger project using js_of_ocaml (the link seems to be offline, try the video). That presentation by Julien Verlaguet is almost identical to the one he gave at YOW! 2013 and it really grabbed my attention. (Hopefully the YOW! video is online soon, as it’ll be better quality).

To cut a long story short, Facebook created a new language (Hack, a statically typed PHP variant) and wrote the compiler in OCaml. They then use js_of_ocaml to compile their entire type checker into JavaScript, as the basis of a web IDE (@19 minutes in the video) along the lines of cloud9. Due to the use of OCaml for everything, this IDE has client-side code completion and error checking. It’s pretty amazing.

Maturity of tools and js_of_ocaml

The more I dive into OCaml, and specifically js_of_ocaml, the more it amazes me how the maturity of the tools and information reached suitability for production use just as I need them.

  • The package manager OPAM is now a little over 12 months old and every library I’ve looked at is available on it. Wide community acceptance of a good package manager is a huge plus.

  • The Real World OCaml book was released in November and is an excellent read. The book is so close to the cutting edge they had features added to September’s 4.01.0 compiler release for them 🙂

  • OCaml Labs has been around for 12 months, and they’re helping to move the OCaml community forward into practical applications (see the 2013 summary).

  • Ocsigen are investing heavily in js_of_ocaml (among other things) with the next release including an improved optimiser (I can attest to the fact that it’s awesome) and support for FRP through the React library.

Moving forward

Is it perfect? No. Software development is not a one-size-fits-all industry. There are as many articles cursing the limitations of OCaml as there are singing its praises. But in the current market, and with the size of JavaScript applications we are starting to generate, I believe OCaml has a bright future.