Simple OCaml Development Workflow on CommonJS
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                      Easy Sharing of Buildable Projects
                    Automatically builds dependent projects
                        Generates Autocomplete Support
                           Generates Documentation
                                Just Hit Build
  • CommonML allows you to build OCaml programs on CommonJS/package.json. If you know CommonJS/package.json, there's not much to learn.
  • Dependencies are installed to the standard local directory node_modules sandbox.
  • Your project (and dependencies) are automatically built in the correct order, into a local artifact directory.
  • All of your dependencies' "exports" are automatically namespaced via the package name. If your package.json has two dependencies (Package1 and Package2), and each exports a Util module, CommonML automatically generates module aliases that requires you to refer to them as Package1.Util and Package2.Util. Internal modules within Package1 and Package2 may reference their internal util module via Util (without namespace).
  • Developing/depending on local packages is the same as developing against remote dependencies. (Just use the standard npm link command).

This is merely an experiment that explores an OCaml based compilation workflow based on the familiar CommonJS. OPAM is the official, high performance package manager for OCaml and you should use that instead of this project for real development. This is only intended for people who really want to try out OCaml development with their familiar CommonJS workflow/namespacing.

Automatically generates autocomplete for dependencies

Note: npm is a hosting service, in addition to a command line interface to CommonJS/package.json. You can use only the command line tools and host your dependencies at arbitrary git URLS (a reason why CommonJS is great). Nothing stops you from hosting OCaml code on npm's hosted service too. The npm command line is just a tool for installing files on your disk.


If you already know CommonJS, you already know CommonML. Most of this README is just a basic tutorial on package.json/npm.

00. Try Building an Example Project!

In ExampleProjects/MyProject/ there is an example project (which just means a src/ directory with OCaml files and a package.json listing its dependencies). Check out the package.json file to see that it depends on a couple of other CommonML projects that are hosted on github (not npm's service).

# Install the project's dependencies.
cd ./ExampleProjects/MyProject/
npm install   #Installs dependencies.

# Build and Run
node node_modules/CommonML/build.js

# If you have OCaml's Vim/Emacs Merlin plugin installed, editing
# any of the example files will now have working autocomplete.

1. Make A Package From Scratch:

To make your own package just like the example:

#1. Make a directory with a `package.json` file and list any modules
# that should be visible to *other* packages in the "exports" field.

mkdir MyProject && cd MyProject
echo '{
  "name": "MyProject",
  "dependencies": {"CommonML": "git://"},
  "CommonML": {
    "exports": ["MyPublicModule"],
    "compileFlags": [],
    "linkFlags": []
}' >> ./package.json

#2. Downloads dependencies including `CommonML` into `./node_modules`
npm install

#3. Place all your package's `.ml` files inside of a directly named `src/`.
mkdir ./src         # All .ml/.mli source files go here.
echo 'let _ = print_string "Hello World"' >> src/

#4. Build native executable
node node_modules/CommonML/build.js

#5. Run executable

2. Building

  • All CommonML dependencies automatically built in the correct order.
  • Every CommonML package now contains an autogenerated .merlin file for code-completion.
  • Source directories free of build artifacts, mirrored into MyProject/_build_byte.
  • Executable output for topmost project placed into MyProject/_build_byte/MyProject/myProject.out.

4. Depend On Other Packages

Depending on other CommonML packages is done exactly the same way as adding any other npm dependencies.

  1. Add any other valid CommonML packages to dependencies
  2. Now your package.json looks like Figure 1 below.
  3. Rerun npm install. This installs all dependencies into node_modules.
  4. Now your package's source files can simply refer to SomeOtherPackage.TheirExportedModules.blah.

Figure 1:

  "dependencies": {
     "CommonML": "git://",
     "SomeOtherPackage: "git://"


  1. Just push your package.json and src/ directory to any git URL.
  2. Other people may point to that git URL in their package.json's dependencies.
  3. Other people just run npm install and your package will be installed into their node_modules sandbox.

Resolving Duplicate Packages:

Duplicate modules are namespaced automatically, but duplicate packages can only be resolved by the package manager - npm. If two packages depend on two different versions of the same module - there is no solution yet. If two packages depend on compatible versions of the same module, then npm dedupe should take care of it.

Build Parameters:

Dedicated build directories (with their own resource caches) are created for each build flag combination. For example, if you build a root project YourProject "for debugging", with native compilation, then the resulting binary is placed at RootOfYourProject/_build_native_debug/YourProject/yourProject.out and intermediate build artifacts for dependencies are placed at RootOfYourProject/_build_native_debug/ADependency/....

Build bytecode (default):(From YourProject root)

node node_modules/CommonML/build.js --compiler=byte # To compile bytecode

# Then run

Build native binaries:(From YourProject root)

node node_modules/CommonML/build.js --compiler=native # To compile to native binaries

# Then run

Preprocess all .mly/mll for yacc and lex

# See example project TAPLArith
node node_modules/CommonML/build.js --compiler=byte --yacc=true

To enable stack traces:(From YourProject root)

# Your binary and dependencies must be compiled with --forDebug
node node_modules/CommonML/build.js --forDebug=true
# You must have a runtime parameter set
# Execute the binary that was built into the dedicated directory

Build JavaScript With Sourcemaps:(From YourProject root)

# Building JavaScript is a special case of building for debug, and building
# for bytecode (a final step converts the bytecode to JavaScript).
# Ensure you have `js_of_ocaml` installed and available on your PATH.
# Install via `opam install js_of_ocaml`.
node node_modules/CommonML/build.js --forDebug=true --jsCompile=true
# open a test html page that includes `./jsBuild/app.js'

Customizing JavaScript Build Location And Running With Server

If you have feedback about the actual debugging experience with source maps (breakpoints, stepping through etc), please file issues on the js_of_ocaml github page. Include screenshots, screencasts or detailed descriptions of how the debugging experience can be made intuitive and more similar to debugging JS.

--jsCompile will generate two artifacts: The symlink (jsBuild) to the actual js bundle and source maps and a fake directory structure to get source maps to work when served from a web server.

When building for JS, the root package's package.json's CommonML.jsPlaceBuildArtifactsIn field will determine where the js build artifacts will be placed. It is considered relative to package root. If not specified, it will default to the package root itself.

When running locally, source maps naturally work correctly because they specify absolute file paths to original source files. But when serving that web server root from a web server, the same origin policy restricts you from seeing the source maps. So (unfortunately) CommonML has to pollute that destination jsPlaceBuildArtifactsIn with a faked directory path that resembles the absolute path to where your original source files were located so that source maps will work even when running on a web server. You will see a generated path that symlinks to your original build directory, such as:

YourProject/myJsPlaceBuildArtifactsIn/Users/yourName/path/to/YourProject/_build_byte_debug ->  YourProject/_build_byte_debug/

This is just so that when you run a web server at YourProject/myJsPlaceBuildArtifactsIn (or whatever you configured jsPlaceBuildArtifactsIn) source maps will work correctly.

Integrating With JavaScript:

When you build for JavaScript --forDebug=true --jsCompile=true, you are simply compiling into JS. This doesn't give you the ability to do anything except print output. To actually interface with the containing JavaScript environment, you'll need to not only compile using js_of_ocaml, but also use the js_of_ocaml runtime libraries which should be compiled into JavaScript along with your application code. To ensure that it is included, and linked, add js_of_ocaml to your package.json file's CommonML.findlibPackages field. (There's also a syntax extension available to make interacting with JavaScript more sugary).

"CommonML": {
  "findlibPackages": [{"dependency": "js_of_ocaml"}],

You can also include a field in your package.json's CommonML section called jsResources. This may be set to a directory (relative to your project's root) whos contents should be copied into the js build directory. A generated app.js file will also be generated by js_of_ocaml and placed alongside them. See ExampleProjects/MyProject/.

"CommonML": {
  "jsResources": "jsResourcesDir",

CommonML ensures that source maps work whether or not you serve your files from a web server, or from local disk. It does this by creating the appropriate sym links that your browser's source maps will understand.


  1. CommonML is opinionated and only builds packages of a certain form.
  2. See the documentation for more details.
  3. Each package lists all of its dependencies in its package.json.
  4. CommonML ensures that each of your dependency\' packages are accessible and build before your package.
  5. Each of your dependencies must also be CommonML compatible.

Why NPM?:

  1. npm (command line tool, that CommonML uses) is not (service).
  2. npm (command line tool) is merely a way to organize and install dependencies - it has nothing to do with JS.
  3. npm do not depend on any central repository and is extremely popular.
  4. npm (and therefore CommonML) can work entirely based on github repos.
  5. npm allows local development as a special case of sharing (see npm link)

Package Structure

Any set of files/directories that have the following form are a valid CommonML project. To make a CommonML project, just make these files with the following:

  • Root project directory that matches the name of your project. In this case MyProject.
  • A src directory that may contain any files, but may not contain two .ml files with the same file name.
  • A package.json file directly inside of the project root directory (more on that later).

Typical project structure.

└── MyProject/
    ├── package.json
    └── src/
        ├── moduleOne.mli
        └── someDirectory/

Recall that in OCaml, a file named automatically becomes a module Foo. The only question is, which modules can see other modules? CommonML comes up with a reasonable convention that makes sharing easy.

Suppose you hae a package root directory named YourProject/

  • The module located at YourProject/src/any_path/ can access YourProject/src/any_other_path/ simply by typing ModuleY. The file paths don't effect visibility within a package.
  • Therefore no two OCaml files inside of YourProject/src may have the same name -> no two OCaml modules inside of YourProject/src may have the same name.
  • YourProject package allows other dependent packages to reference internal module YourInternalModule if and only if YourInternalModule name is listed in YourProject's package.json (CommonML.exports field).

The package.json:

Each package's package.json must provide information that instructs CommonML how to compile, run, and be depended on by other packages.

  "name": "MyProject",
  "version": "1.0",
  "description": "Simple My Project example in OCaml",
  "dependencies": {
    "CommonML": "ssh+git://",
    "YourProject": "1.0"
  "CommonML": {
    "exports": ["ExportedModule", "AnotherExportedModuleName"],
    "compileFlags": ["-g", "-w","-30","-w","-40"],
    "findlibPackages": [
      {"dependency": "comparelib"},
      {"syntax": "comparelib.syntax"}
  • There is also a place to put compiler flags and findlibPackages in the package.json.

Use CommonML To Tame Local Development:

You can use the familiar npm link command to have projects depend on other projects locally without publishing them publicly.

├── MyProject/
│   ├── package.json      // { "dependencies": {"YourProject": "1.0", "CommonUtility": "1.0"},
│   └── src/              //    "exports: ["MyProject", "Util"]
│       ├── myProject.mli // }
│       ├──
│       └── util/
│           └──
├── YourProject/
│   ├── package.json     // {"dependencies": "ObscureUtility": "1.0", "CommonUtility": "1.0"}
│   └── src/
│       ├── yourProject.mli
│       ├──    // Only visible to MyProject
│       └── util/
│           └──       // May only be observed by modules in MyProject - distinct from MyProject.util
├── CommonUtility/
│   ├── package.json     // {"dependencies": {}
│   └── src/
│       ├── yourProject.mli
│       ├──      // Only visible to MyProject
│       └──        // May only be observed by modules in YourProject
└── ObscureUtility/
    ├── package.json     // {"dependencies": {}}
    └── src/
        ├── obscureUtility.mli
        └──  // May only be observed by YourProject


  • Local development should be the same exact workflow as sharing and depending on published modules. Local development is merely sharing with yourself.

  • No README should ever contain the phrase: "Make sure you have package X installed globally on your system".

  • Installing a package should always be as simple as listing it as a dependency in package.json and running npm install. It should install everything you need to be able to build, run and (eventually) generate documentation. This isn't always possible for huge projects, but CommonML is for the subset of projects for which it is possible.

*Yes, CommonML currently relies on having OCaml/nodeJS installed on your system. One thing at a time.


Which module is the root of the executable? CommonML's build script automatically determines this by way of using ocamldep behind the scenes. In short, it's whichever one does not have any other dependencies.


You may supply a custom preprocessor program that should be executed on each file in order to parse the source into a canonical AST. The preprocessor must be available in your PATH and be able to accept an arbitrary file, and either invoke a custom parser, or invoke the standard parser. Your preprocessor likely would use file extensions as "hints" as to which to do.

Therefore, CommonML also allows you to indicate that particular "pairs" of extensions should be treated as interfaces/implementations respectively - otherwise the file extension "hints" that are used by your preprocessor will confuse the rest of the compilation toolchain.

You will often want to supply both a preprocessor and extensions. Below is an example of doing both, by populating the preprocessor and extensions fields in a package.json's CommonML field:

  "CommonML": {
    "exports": ["MyProjectMod"],
    "preprocessor": "myCustomPreprocessor",
    "extensions": [{
      "interface": ".heyoi",
      "implementation": ".heyo"

The preprocessor and extensions fields are separate concepts that are often used simultaneously.


  • Currently an "inconsistent interface" message can occur when an interface file does not exist for an implementation file. This happens because the arguments to a compilation command does not inlude an interface for obvious reasons, yet there is an automatically generated interface cmi sitting in the build directory. That cmi could be an outdated cmi. If there was a real existing interface file, the cmi would get overwritten by a fresh, non-conflicting build artifact. The quick solution is to remove all .cmi's for every implementation that does not have a corresponding interface file. (Or even more generally, remove all the artifacts that we didn't explicitly supply as an output.).
  • Replicate this development flow with OPAM/ocamlbuild to see how close we can get.
  • Different configurable compiler flags for byte vs native.
  • Warn when observing .ml/i files outside of src directory.
  • Build CommonML compatible packages from OPAM.
  • Generate ocamldebug startup script for last dependency seen in ocamldep, set to break on the first line.
  • An a.out is generated by ocamlfind when it shouldn't be: ocamlfind ocamlc -linkpkg -package js_of_ocaml -only-show is supposed to find transitive dependencies, but only show without compiling, but it compiles and generates a left-over a.out. This is easily reproducable.
  • The --forDebug flag should be examined to determine if the optimized js compilation should be used (and source maps omitted).
  • Find a better solution for sourcemaps than creating fake directory structures and/or have js_of_ocaml just inline the source contents into the sourcemaps so we don't have to deal with file paths at all.
  • js_of_ocaml should itself be turned into a CommonML dependency to remove any need to use the findlibPackages field (it's confusing that we even have the notion of "findlib packages") and package.json is fully sufficient to do everything we need. This is only temporary.


To show OCaml parsing errors :


Stylesheet for documentation borrowed from vim-awesome (MIT license).