OCaml flake example project

This is an overly elaborate example of building a ‘Hello World’ package with Nix flakes, OCaml, and Dune. The following things are demonstrated:

• A library and an executable that depends on that library
• Building documentation
• Expect tests and Cram tests

Disclaimer

I'm not a Nix expert, nor am I an OCaml expert so there might be better approaches to some of the things demonstrated here! That said, I thought I'd share this in the off-chance it is useful to somebody else.

Preliminaries

Entering the Nix development shell

Sometimes it can be useful to run commands such as dune directly from your shell. To do this you can call nix develop in your shell, which will load a new bash shell with the appropriate tools in your $PATH:

nix develop

Enabling Direnv

Alternatively you can use direnv or nix-direnv to load the development tools automatically in the current shell. Direnv can also be extremely useful for setting up your editor, for example with vscode-direnv.

You can enable direnv to load the development tools onto your $PATH by running:

echo "use flake" > .envrc
direnv allow

If you decide to use direnv you'll probably want to add .direnv and .envrc to your global gitignore or local excludes in order to avoid committing these files to git.

Flake Usage

Running the executable

The hello executable can be run with the nix command with:

nix run

This should print Hello, World! in your shell.

Building the package

The hello package can be built with the nix command with:

nix build

This should create a result symlink in the current directory, which will contain the hello executable and library.

Running the tests

The tests can be run using the nix command with:

nix flake check --print-build-logs

This will also check that flake.nix conforms to the expected flake schema, and will run formatting tests for the Nix files.

Expect test failures can be promoted in a development shell with:

dune promote

This will update the tests to their current expected output.