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David Sancho
David Sancho

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query-json: A story of cross-compilation with Reason

What's query-json

query-json is a faster and simpler re-implementation of jq's language in Reason.

It's a CLI to run small programs against JSON, the same idea as sed but for JSON files. As a web engineer is an essential tool while developing APIs.

Started with the goal to create something useful and learn during the process. Was very interested in learning about how to write parsers and compilers using the OCaml stack: menhir and sedlex, compile it to any architecture and try to compile it to JavaScript.

This post explains the project, how it was made and which decisions were followed and some reflections.

Why learning parsers/compilers

I had a little idea about the theory, but very vague and useless and was the righ time to learn more since I created styled-ppx which is a
ppx (Pre*Processor Ex*tension) that allows CSS-in-Reason/OCaml. Needs to parse the CSS and have a backend that compiles to bs-emotion.

I asked for some help to @EduardoRFS about writing a CSS Parser that supports the entire CSS3 specification which I will want to understand, improve and maintain over time.

How it works

query-json ".store.books | filter(.price > 10)" stores.json

This reads stores.json and access to "store" field, access to "books" field (since it's an array) it will run a filter on each item and filter each item by it's "price" that's larger than 10 and finally, print the resultant list.

    "title": "War and Peace",
    "author": "Leo Tolstoy",
    "price": 12.0
    "title": "Lolita",
    "author": "Vladimir Nabokov",
    "price": 13.0

The first argument is called query and it's a jq expression. The second one is called json or payload, and it's a valid JSON file.

The semantics of jq consist of a set of piped operations, where each output is connected to an input where the first input it's the JSON itself. Some pseudo-code to illustate:

{ /* json */ } | filter | transform | count

In order to transform the query to a set of operations that run against a JSON, we will divide the problem into 3 steps: parse, compile and run.


Responsible for transforming a string into an AST(Abstract Syntax Tree) and provide a good error message when the input is malformed.

One of the beauties of jq is that all the expressions are piped by default, so .store | .books is equivalent to .store.books. I designed the AST to represent the pipe structure in its nature. If you want to know more about jq's language, check their wiki.

Let's dive into an example. When the parser recieves .store.books will return:


All the operations are transformed into these constructors from above (Pipe, Key). Those constructors are called Variants.

Variants models values that may assume one of many known variations. This feature is similar to enums in other languages, but each variant may optionally contain data that is carried inside. Variants belong to a large group of types called ADTs.

The entire query-json program representation it's one big recursive Variant.

Following with a more complex example, let's parse .store.books | filter(.price > 10):


You can see more examples in the parsing tests


The compilation step recieves the AST expression and transforms it to code. It's a big recursive pattern match, which is another great feature of Reason and looks something like this:

let rec compile = (expression, json) => {
  switch (expression) {
  | Empty => empty
  | Keys => keys(json)
  | Key(key, opt) => member(key, opt, json)
  | Index(idx) => index(idx, json)
  | Head => head(json)
  | Tail => tail(json)
  | Length => length(json)
  /* [...] */

On the left side are defined all the possible Variants and on the right side the operations. Those operations transforms the JSON. Here is where map, filter, reduce, index, etc... are implemented.


The easier part, the compile step give us back a curried function that expects a json as the only argument, we just apply the function to this JSON and print the result.

This example only describes the happy path, in reality, the parsing and compilation steps return a Result type which allows handling errors more composable.


Now we already covered how it works internally and a little overview of the architecture. Now let's dive on how developers can use it in their machines.

But first, let's recap on what's what.

OCaml is a programing language from the family of ML ("Meta Language") and is best known for their static type system and type-inference.
OCaml allows to compile to binary code which are often compared to C/C++ performance.

BuckleScript is a compiler (a fork of the OCaml compiler) that outputs JavaScript code, instead of binary. BuckleScript supports OCaml syntax and Reason syntax.

Reason is a language that can be compiled with the OCaml compiler or BuckleScript.

How to cross-compilation to binary

All of the build process and their tests runs on our CI in Github Actions, which allows running Mac, Windows and Linux images. Each build compiles with the OCaml compiler to each architecture.

Once the build and test succeeds it pushes the binary into Github Releases and npm registry.

How to compile to the Web

The compilation to JavaScript is the sweet section of this blog post since we are using OCaml (under the hood while writing Reason), and it allows us to compile directly to JavaScript using js_of_ocaml.

js_of_ocaml is a compiler that can be plugged to the OCaml's one and It makes it possible to run pure OCaml programs into JavaScript.
As you can see, query-json uses menhir, sedlex and yojson

In order to use it in esy, the package manager, I only needed to add:

esy add js_of_ocaml
esy add js_of_ocaml-compiler

and modify it's dune file, the build system:

 (name Js)
 (modes js)
 (libraries console.lib source yojson js_of_ocaml)

After running the build, it generates a bundle.js!

Made query-json's playground

After having a JavaScript artefact I was able to run it as a playground in the browser. To teach people how to use it without the binary install or to test new versions on each pull request, well, the possibilities of the web are endless!

Alt Text

I build a website using Reason and BuckleScript which uses query-json JavaScript's artefact to run it, you can try it yourself here:

The query-json computation runs synchronous since it's able to run on each key-stroke. Comparing this playground with that needs to hit a backend and respond with the result.

Having a playground as a serverless frontend app It's a massive improvement over backend-dependant ones. Actually, most of the REPL's from Reason, OCaml, Flow, ReScript uses js_of_ocaml.


This allows any OCaml backend being able to run in a browser without much hassle, sharing code between backend and frontend has been a dream for a lot of Engineering teams.

But not only share logic matters, here's a list of other upsides:

  • Portability, moving code from server to client or viceversa, sharing marshal/unmarshal code, easier contract testing.
  • Familiarity: Writing the same patterns would benefit new comers that need to learn less platform-specific rules.
  • Usage of OCaml's ecosystem: Access to many libraries and ppxs and latest ocaml's features.
  • Features that weren't possible: Some apps might benefit from Server-side rendering, others might benefit from moving stuff to offline, and many app-specific designs that are unblocked by this.


This solution has downsides as well, js_of_ocaml isn't the tool that solves all of your problems, actually, there's no such tool.


It's quite big compared to regular JavaScript applications.

The entire playground is about 660kb. Includes the Monaco Editor (~356kb) and the rest is query-json and js_of_ocaml runtime (300kb).

js_of_ocaml wasn't created with the same mentality as most Frontend developers solve their problems today and was born 10 years ago to run some OCaml into a browser.


One of my biggest complains about the OCaml community is the lack of quality documentation.

Coming from the JavaScript community (which have more than 9 million devs) there's plenty of tutorials, examples, manuals, many StackOverflow resolved questions and this culture of the copy-paste driven development.

That gives all sort of problems, but lowers the barrier to the language and the usage of many dependencies for new comers or not so passionate FP developers.

It's a non-sense compare OCaml and JavaScript, but one of the biggest missing pieces is good documentation at all levels.

Bridge between BuckleScript/Reason and js_of_ocaml

Js_of_ocaml lacks some of the basics to enable compatibility with JavaScript codebases, modern build systems such as webpack and many bindings to other libraries. So to build the online playground, I used BuckleScript for the UI and js_of_ocaml for the query-json browser build, and this required having to make them communicate to each other.

This isn't particular to js_of_ocaml, is more related to the mix of js_of_ocaml and BuckleScript. In order to run the js_of_ocaml artefact into the BuckleScript/Reason codebase, I needed to write bindings for it.

[@bs.module "../../_build/default/js/Js.bc.js"]
external queryJson: (string, string) => result(string, string) = "run";

Using the Result from OCaml in the query-json's JavaScript entry-point and the Result from BuckleScript in the Reason code I needed to write an unsafe bridge which transforms OCaml result to the internal representation of BuckleScript variant.

There're probably better ways of achieving it since I made the implementation very unsafe davesnx/query-json/js/

It's a tradeoff, as always...

You can't really compare BuckleScript/rescript with js_of_ocaml, since both tools try to solve the same problem in a very different fashion.

If you are writing an OCaml backend and your team is familiar with it. Your project would benefit from sharing code, don't require a hard need on a lower bundlesize and want to carry all OCaml dependencies, ppx and patterns, js_of_ocaml it's the best option.

Since most modern build tools such as Webpack or Rollup allows to lazy import chunks of your app, you might find no-issue with the big bundle, since most shared logic doesn't require to be present during load-time.

You won't have the facility to write frontend code with React, but I'm sure it will change soon since there's a current implementation of React in jsso jsoo-react.

For now, writing a website in ReasonReact and BuckleScript it's the most robust experience. You can find a lot of bindings to JavaScript libraries, there's a lot of usage in production, plenty of examples.

I truly believe that ReScript it's such a great project to target JavaScript developers that found TypeScript slow and liar or Flow abandoned. It improves the status quo for many issues regarding the Reason Community and keeps delivering in a fast pace the best tooling.

It makes me very sad that ReScript will diverge at some point from the OCaml ecosystem and won't spend time to build a cross-platform language with the advantatges of cross-compilation and future features from OCaml.


The future of query-json is to provide a better experience in running operations on top of JSON.

Providing better error messages and better performance has been the goal.

Currently jq has an issue, it's very powerful but confusing. The amount of questions of StackOverflow proves that there's a lot of problems without a solution on the language/compiler.

The other mission of query-json is to push performance forward, now we are implementing most of the missing functionality since I first release it and next is to explore performance optimizations, such as:

  • Replacing menhir with a written parser/lexer
  • Using OCaml multicore
  • Partial JSON parsing, only parse the needed parts of a json based on the user's query


Hope you liked the project and the story, let me know if you are interested in those topics I'm always happy to chat and help. I answer to all the DMs on Twitter.

See you next time 👋

Thanks everyone how reviewed this blog post, Javi, Enric and Gerard

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