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Real World OCaml v2

This is the source code for the Real World OCaml 2nd edition, which is still a work in progress. The original edition was written by Yaron Minsky, Anil Madhavapeddy and Jason Hickey, and the revised edition is being led by Yaron Minsky and Anil Madhavapeddy. There have been significant contributions to the revised tooling from Ashish Agarwal, Jeremy Yallop, Frederic Bour, and Sander Spies.

An online snapshot of the development book is available from There is a Feedback pane on each chapter which leads to a dedicated section on the OCaml discussion forum where you can register broader feedback. More specific issues such as typos can be reported on the issue tracker.

Repository layout

Each chapter of the book sits in a separate subfolder of the book/ directory. The file contains the text of the chapter, written in markdown. Each OCaml or shell code block in the chapter is validated using mdx. The more complex and structured examples live in an examples/ sub folder and mdx is used to keep the examples and the chapter's code block in sync.

The bin/ folder contains the OCaml scripts used to generate the books HTML and PDF versions.

All of the code and examples are built using OCaml 4.09.0.


Here are the commands to build the website:

Installing Dependencies

You can install system dependencies by running:

make depext

All OCaml dependencies are vendored in the duniverse/ directory except for the dune build system itself. It's preferable to use an empty opam switch with only dune installed to avoid conflicts between the opam and local libraries. To set up your RWO development environment you can run:

opam switch create rwo 4.13.1
opam install dune=3.0.2

Generating the HTML

To generate the HTML pages:


The HTML pages are created in _build/default/static/. Open _build/default/static/index.html to start browsing the freshly built version of the book.

WIP Chapters

It is possible to mark a chapter as WIP so that it won't be included in the main website or PDF. To do so, you can wrap your chapter in a (wip <chapter-folder>) in the table of content file, book/toc.scm, e.g.:

You can run dune build @site-wip or dune build @pdf-wip to generate the website or PDF versions of the book including all WIP chapters. You can find them respectivey in _build/default/static-wip/index.html and _build/default/static-wip/book.pdf.

Once the chapter is ready, you can simply replace (wip <chapter-folder>) with <chapter-folder> in book/toc.scm.

Testing the code examples

It is possible to automatically test that the the code examples files work fine. To check that shell scripts and .ml files do what they are expected:

make test

This will run all the tests in "determinitic mode", which is suitable for the CI and it will display the diff between what is expected and what is produced.

To accept the changes:

make promote

Testing non-deterministic examples

A few code examples are not deterministic: for instance benchmarks. In this case, there is a specific dune profile to use:

dune runtest --profile non-deterministic ...

To accept the changes:

make promote

Standalone examples

Examples in each chapter's examples/ folder are split between correct/ and erroneous/. Each individual example is a valid dune-project that lives in its own sub folder. Examples that contain errors on purpose, for instance to showcase some specific compile errors, go into the latter. All other examples should go in correct/.

Examples in the correct/ folder are automatically built and tested in the CI. It's possible to build and test them individually using the dune alias corresponding to the example folder name. For instance, to build and test the example in book/imperative-programming/examples/correct/dictionary, one can run:

dune build @dictionary

Note that the runtest alias will also build and test examples so running make test will build all of the book's examples.

When adding a new chapter, the example folder should have the following structure:

├── correct/
├── dune
└── erroneous/

With the following dune file:

(data_only_dirs correct erroneous)

  (source_tree ./))
  (with-stdout-to dune.gen
   (run rwo-examples-rules ./))))

 (alias runtest)
 (action (diff dune.gen)))


From that point forward, running dune runtest will generate the right dune rules for each folder in correct/. When the rules change, they must be accepted through promotion first.

Each example must explicitly define its external dependencies in a .rwo-example. For instance, if your example requires base and core, it must include the following .rwo-example file at its root:

(packages base core)


RWO's dependencies are managed using the opam-monorepo plugin. The dependencies are expressed in the rwo.opam opam file as they would be for any project. The plugin is used to generate a rwo.opam.locked lockfile from this deps specification using the opam monorepo lock command. Running opam monorepo pull will then fetch the sources locally into the duniverse/ folder so that rwo and its dependencies can all be built together in a single dune-workspace.

You can install it by running:

opam install opam-monorepo

Before running opam-monorepo lock it's important to have the proper opam configuration. We need to both add the opam-overlays repo which contains dune port of some of our dependencies. We also use a pinned version of ctypes until the dune-port is stable. To set these up, you can run:

opam repository add dune-opam-overlays git+
opam pin add ctypes.0.20.1+dune
opam pin add ctypes-foreign.0.20.1+dune

Upgrading or adding dependencies

Before upgrading or adding any dependency, you should make sure they are up-to-date according to the rwo.opam file by running:

opam monorepo lock
opam monorepo pull

and committing the resulting duniverse/ and rwo.opam.locked if they changed. This preliminary step will help distinguish how new dependency specifications actually impact the lockfile and duniverse by splitting out the unrelated updates.

Once the above is done, you can modify the dune-project package definition by adding a new dependency or modifying the bounds on an existing one. Then run:

dune build rwo.opam
opam monorepo lock
opam monorepo pull

to update the opam file, the lockfile and the duniverse folder.