Working on the Dune Codebase

This section gives guidelines for working on Dune itself. Many of these are general guidelines specific to Dune. However, given that Dune is a large project developed by many different people, it’s important to follow these guidelines in order to keep the project in a good state and pleasant to work on for everybody.


To create a directory-local opam switch with the dependencies necessary to build the tests, run:

$ make dev-switch


Dune uses Dune as its build system, which requires some specific commands to work. Running make dev bootstraps (if necessary) and runs ./dune.exe build @install.

If you want to just run the bootstrapping step itself, build the bootstrap phony target with

$ make bootstrap

You can always rerun this to bootstrap again.

Once you’ve bootstrapped Dune, you should be using it to develop Dune itself. Here are the most common commands you’ll be running:

# to make sure everything compiles:
$ ./dune.exe build @check
# run all the tests
$ ./dune.exe runtest
# run a particular cram foo.t:
$ ./dune.exe build @foo

Note that tests are currently written for version 4.14.1 of the OCaml compiler. Some tests depend on the specific wording of compilation errors which can change between compiler versions, so to reliably run the tests make sure that ocaml.4.14.1 is installed. The TEST_OCAMLVERSION in the Makefile at the root of the Dune repo contains the current compiler version for which tests are written.

Writing Tests

Most of our tests are written as expectation-style tests. While creating such tests, the developer writes some code and then lets the system insert the output produced during the code execution. The system puts it right next to the code in the source file.

Once you write and commit a test, the system checks that the captured output matches the one produced by a fresh code execution. When the two don’t match, the test fails. The system then displays a diff between what was expected and what the code produced.

We write both our unit tests and integration tests in this way. For unit tests, we use the ppx_expect framework, where we introduce tests via let%expect_test, and [%expect ...] nodes capture expectations:

let%expect_test "<test name>" =
   print_string "Hello, world!";
   [%expect {|
     Hello, world!

For integration tests, we use a system similar to Cram tests for testing shell commands and their behavior:

$ echo 'Hello, world!'
Hello, world!

$ false

$ cat <<EOF
> multi
> line

See also

actions_to_sh tests

An example of expect-tests.


An example of Cram test.

When running Dune inside tests, the INSIDE_DUNE environment variable is set. This has the following effects:

  • Change the default root detection behaviour to use the current directory rather than the top most dune-project / dune-workspace file.

  • Be less verbose when Dune outputs a user message.

  • Error reporting is deterministic by default.

  • Prefer not to use a diff program for displaying diffs.

This list is not exhaustive and may change in the future. In order to find the exact behaviour, it is recommended to search for INSIDE_DUNE in the codebase.


As with any long running software project, code written by one person will eventually be maintained by another. Just like normal code, it’s important to document tests, especially since test suites are most often composed of many individual tests that must be understood on their own.

A well-written test case should be easily understood. A reader should be able to quickly understand what property the test is checking, how it’s doing it, and how to convince oneself that the test outcome is the right one. A well-written test makes it easier for future maintainers to understand the test and react when the test breaks. Most often, the code will need to be adapted to preserve the existing behavior; however, in some rare cases, the test expectation will need to be updated.

It’s crucial that each test case makes its purpose and logic crystal clear, so future maintainers know how to deal with it.

When writing a test, we generally have a good idea of what we want to test. Sometimes, we want to ensure a newly developed feature behaves as expected. Other times, we want to add a reproduction case for a bug reported by a user to ensure future changes won’t reintroduce the faulty behaviour. Just like when programming, we turn such an idea into code, which is a formal language that a computer can understand. While another person reading this code might be able to follow and understand what the code does step by step, it isn’t clear that they’ll be able to reconstruct the original developer’s idea. Even worse, they might understand the code in a completely different way, which would lead them to update it incorrectly.

Setting Up Your Development Environment Using Nix

You can use Nix to setup the development environment. This can be done by running nix develop in the root of the Dune repository.

Note that Dune only takes OCaml as a dependency and the rest of the dependencies are used when running the test suite.

Running nix develop can take a while the first time, therefore it is advisable to save the state in a profile.

`sh nix develop --profile nix/profiles/dune `

And to load the profile:

`sh nix develop nix/profiles/dune `

This profile might need to be updated from time to time, since the bootstrapped version of Dune may become stale. This can be done by running the first command.

We have the following shells for specific tasks:

  • nix develop .#slim for a dev environment with fewer dependencies that is faster to build.

  • nix develop .#slim-melange: same as above, but additionally includes the melange and mel packages

  • Building documentation requires nix develop .#doc.

  • For running the Coq tests, you can use nix develop .#coq. NB: Coq native is not currently installed; this will cause some of the tests to fail. It’s currently better to fallback to opam in this case.

Releasing Dune

Dune’s release process relies on dune-release. Make sure you install and understand how this software works before proceeding. Publishing a release consists of two steps:

  • Updating to reflect the version being published.

  • Running $ make opam-release to create the release tarball. Then publish it to GitHub and submit it to opam.

Major & Feature Releases

Given a new version x.y.z, a major release increments x, and a feature release increments y. Such a release must be done from the main branch. Once you publish the release, be sure to publish a release branch named x.y.

Point Releases

Point releases increment the z in x.y.z. Such releases are done from the respective x.y branch of the respective feature release. Once released, be sure to update in the main branch.

Adding Stanzas

Adding new stanzas is the most natural way to extend Dune with new features. Therefore, we try to make this as easy as possible. The minimal amount of steps to add a new stanza is:

  • Extend Stanza.t with a new constructor to represent the new stanza

  • Modify Dune_file to parse the Dune language into this constructor

  • Modify the rules to interpret this stanza into rules, usually done in Gen_rules`


Dune is incredibly strict with versioning of new features, modifications visible to the user, and changes to existing rules. This means that any added stanza must be guarded behind the version of the Dune language in which it was introduced. For example:

; ( "cram"
  , let+ () = Dune_lang.Syntax.since Stanza.syntax (2, 7)
    and+ t = Cram_stanza.decode in
    [ Cram t ] )

Here, Dune 2.7 introduced the Cram stanza, so the user must enable (lang dune 2.7) in their dune project file to use it.

since isn’t the only primitive for making sure that versions are respected. See Dune_lang.Syntax for other commonly used functions.

Experimental & Independent Extensions

Sometimes, Dune’s versioning policy is too strict. For example, it doesn’t work in the following situations:

  • When most Dune independent extensions only exist inside Dune for development convenience, e.g., build rules for Coq. Such extensions would like to impose their own versioning policy.

  • When experimental features cannot guarantee Dune’s strict backwards compatibility. Such features may dropped or modified at any time.

To handle both of these use cases, Dune allows the definition of new languages (with the same syntax). These languages have their own versioning scheme and their own stanzas (or fields). In Dune itself, Syntax.t represents such languages. Here’s an example of how the Coq syntax is defined:

let coq_syntax =
  Dune_lang.Syntax.create ~name:"coq" ~desc:"the coq extension (experimental)"
   [ ((0, 1), `Since (1, 9)); ((0, 2), `Since (2, 5)) ]

The list provides which versions of the syntax are provided and which version of Dune introduced them.

Such languages must be enabled in the dune project file separately:

(lang dune 3.14)
(using coq 0.8)

If such extensions are experimental, it’s recommended that they pass ~experimental:true, and that their versions are below 1.0.

We also recommend that such extensions introduce stanzas or fields of the form ext_name.stanza_name or ext_name.field_name to clarify which extensions provide a certain feature.

Dune Rules

Creating Rules

A Dune rule consists of 3 components:

  • Dependencies that the rule may read when executed (files, aliases, etc.), described by 'a Action_builder.t values.

  • Targets that the rule produces (files and/or directories), described by 'a Action_builder.With_targets.t' values.

  • Action that Dune must execute (external programs, redirects, etc.). Actions are represented by Action.t values.

Combined, one needs to produce an Action.t Action_builder.With_targets.t value to create a rule. The rule may then be added by Super_context.add_rule or a related function.

To make this maximally convenient, there’s a Command module to make it easier to create actions that run external commands and describe their targets and dependencies simultaneously.

Loading Rules

Dune rules are loaded lazily to improve performance. Here’s a sketch of the algorithm that tries to load the rule that generates some target file t.

  • Get the directory that contains t. Call it d.

  • Load all rules in d into a map from targets in that directory to rules that produce it.

  • Look up the rule for t in this map.

To adhere to this loading scheme, we must generate our rules as part of the callback that creates targets in that directory. See the Gen_rules module for how this callback is constructed.


User documentation lives in the ./doc directory.

In order to build the user documentation, you must install python-sphinx, sphinx_rtd_theme and sphinx-copybutton.

Build the documentation with

$ make doc

For automatically updated builds, you can install sphinx-autobuild, and run

$ make livedoc

Nix users may drop into a development shell with the necessary dependencies for building docs nix develop .#doc.


For structure, we use the Diátaxis framework. The core idea is that documents should fit in one of the following categories:

  • Tutorials, focused on learning

  • How-to guides, focused on task solving

  • Reference, focused on information

  • Explanations, focused on understanding

Most features do not need a document in each category, but the important part is that a single document should not try to be in several categories at once.

ReStructured Text

For code blocks containing Dune files, use .. code:: dune and indent with 3 spaces. Use formatting consistent with how Dune formats Dune files (most importantly, do not leave orphan closing parentheses).

In a document that only contains Dune code blocks, it is possible to use the .. highlight:: dune directive to have dune be the default lexer, and then it is possible to use the :: shortcut to end a line with a single : and start a code block. See the source of Lexical Conventions for an example.

For links, prefer references that use :doc: (link to a whole document) or :term: (link to a definition in the glossary) to :ref:.

Use the right lexers: - dune for dune and related files - opam for opam files - console for shell sessions and commands (start with $) - cram for cram tests


Use American spelling.

Use Title Case for titles and headings (every word except “little words” like of, and, or, etc.).

For project names, use the following capitalization:

  • Dune is the project, dune is the command. Files are called dune files.

  • dune-project should always be written in monospace.

  • OCaml

  • OCamlFormat, and ocamlformat is the command.

  • odoc, always in monospace.

  • opam. Can be capitalised as Opam at the beginning of sentences only, as the official name is formatted opam. Even in titles, headers, and subheaders, it should be all lowercase: opam. The command is opam.

  • esy. Can be capitalised as Esy.

  • Nix. The command is nix.

  • Js_of_ocaml can be abbreviated JSOO.

  • MDX, rather than mdx or Mdx

  • PPX, rather than ppx or Ppx; ppxlib

  • UTop, rather than utop or Utop.


Dune vendors some code that it uses internally. This is done to make installing Dune easy as it requires nothing but an OCaml compiler as well as to prevent circular dependencies. Before vendoring, make sure that the license of the code allows it to be included in Dune.

The vendored code lives in the vendor/ subdirectory. To vendor new code, create a shell script update-<library>.sh, that will be launched from the vendor/ folder to download and unpack the source and copy the necessary source files into the vendor/<library> folder. Try to keep the amount of source code imported minimal, e.g., leave out dune-project files. For the most part, it should be enough to copy .ml and .mli files. Make sure to also include the license if there is such a file in the code to be vendored to stay compliant.

As these sources get vendored not as subprojects but parts of Dune, you need to deal with public_name. The preferred way is to remove the public_name and only use the private name. If that is not possible, the library can be renamed into dune-private-libs.<library>.

To deal with the modified dune files in update-<library>.sh scripts, you can commit the modified files to dune and make the update-<library>.sh script to use git checkout to restore the dune file.

For larger modifications, it is better to fork the upstream project in the ocaml-dune organisation and then vendor the forked copy in Dune. This makes the changes better visible and easier to update from upstream in the long run while keeping our custom patches in sync. The changes to the dune files are to be kept in the Dune repository.

It is preferable to cut out as many dependencies as possible, e.g., ones that are only necessary on older OCaml versions or build-time dependencies.

General Guidelines

Dune has grown to be a fairly large project that over time has acquired its own style. Below is an attempt to enumerate some important points of this style. These rules aren’t axioms and we may break them when justified. However, we should have a good reason in mind when breaking them. Finally, the list isn’t exhaustive by any means and is subject to change. Feel free to discuss anything in particular with the team.

  • Parameter signatures should be self descriptive. Use labels when the types alone aren’t sufficient to make the signature readable.


val display_name : string -> string -> _ Pp.t


val display_name : first_name:string -> last_name:string -> _ Pp.t
  • Avoid type aliases when possible. Yes, they might make some type signatures more readable, but they make the code harder to grep and make Merlin’s inferred types more confusing.

  • Every .ml file must have a corresponding .mli. The only exception to this rule is .ml files with only type definitions.

  • Do not write .mli only modules. They offer no advantages to .ml modules with type definitions and one cannot define exceptions in .mli only modules

  • Every module should have toplevel documentation that describes the module briefly. This is a good place to discuss its purpose, invariants, etc.

  • Keep interfaces short & sweet. The less functions, types, etc., there are, the easier it is for users to understand, use, and ultimately modify the interface correctly. Instead of creating elaborate interfaces with the hope of future-proofing every use case, embrace change and make it easier to throw out or replace the interface.

    Ideally the interface should have one obvious way to use it. A particularly annoying violator of this principle is the “logic-less chain of functions” helper. For example:

let foo t = bar t |> baz

If bar and baz are already public, then there’s no need to add yet another helper to save the caller a line of code.

  • Define bindings as close to their use site as possible. When they’re far apart, reading code requires scrolling and IDE tools to understand the code.


let dir = .. in
(* 50 odd lines or so that don't use [dir] *)
f dir


let dir = .. in
f dir
  • A corollary to the previous guideline: keep the scope of bindings as small as possible.


let x1 = f foo in let x2 = f bar in
let y1 = g foo in let y2 = g bar in
let dx = x2 -. x1 in
let dy = y2 -. y1 in
dx^2 +. dy^2


let dx =
  let x1 = f foo in let x2 = f bar in
  x2 -. x1
let dy =
  let y1 = g foo in let y2 = g bar in
  y2 -. y1
dx^2 +. dy^2
  • Prefer Code_error.raise instead of assert false. The reader often has no idea what invariant is broken by the assert false. Kindly describe it to the reader in the error message.

  • Avoid meaningless names like x, a, b, f. Try to find a more descriptive name or just inline it altogether.

  • If a module Foo has a module type Foo.S and you’d like to avoid repeating its definition in the implementation and the signature, introduce an .ml-only module Foo_intf and write the S only once in there.

  • Instead of introducing a type foo, consider introducing a module Foo with a type t. This is often the place to put functions related to foo.

  • Avoid optional arguments. They increase brevity at the expense of readability and are annoying to grep. Furthermore, they encourage callers not to think at all about these optional arguments even if they often should.

  • Avoid qualifying modules when accessing fields of records or constructors. Avoid it altogether if possible, or add a type annotation if necessary.


let result = A.b () in
match result.A.field with
| B.Constructor -> ...


let result : A.t = A.b () in
match (result.field : B.t) with
| Constructor -> ...
  • When constructing records, use the qualified names in in the record. Do not open the record. The local open syntax pulls in all kinds of names from the opened module and might shadow the values that you’re trying to put into the record, leading to difficult debugging.

Bad; if A.value exists, it will pick that over value:

let value = 42 in
let record = A.{ field = value; other } in


let value = 42 in
let record = { A.field = value; other } in
  • Stage functions explicitly with the Staged module.

  • Do not raise Invalid_argument. Instead, raise with Code_error.raise which allows to attach more informative payloads than just strings.

  • When ignoring the value of a let binding let _ = ..., we add type annotations to the ignored value let (_ : t) = .... We do this convention because:

  • We need to make sure we never ignore Fiber.t accidentally. Functions that return Fiber.t are always free of side effects so we need to bind on the result to force the side effect.

  • Whenever a function is changed to return an error via its return value, we want the compiler to notify all the callers that need to be updated.

  • To write a to_dyn function on a record type, use the following pattern. It ensures that the pattern matching will break when a field is added. To ignore a field, add ; d = _, not ; _.

let to_dyn {a; b; c} =
    [ ("a", A.to_dyn a)
    ; ("b", B.to_dyn b)
    ; ("c", C.to_dyn c)
  • To write an equality function, use the following pattern (this applies to other kinds of binary functions). The same remarks about about pattern matching and ignoring fields apply.

let equal {a; b; c} t =
  A.equal a t.a &&
  B.equal b t.b &&
  C.equal c t.c

Subjective Style Points

There’s some stylistic decisions we made that don’t have logical justification and are basically a matter of taste. Nevertheless, it’s useful to follow them to keep the code consistent.

  • Match patterns should be sorted by the length of their RHS when possible. Keep the shorter clauses near the top.

  • If a module Foo defines a type t, all functions that take t in this module should have t as their first argument. This is the “t comes first” rule.

  • Do not mix |> and @@ in the same expression.

  • Introduce bindings that will allow opportunities for record or label punning.

  • Do not write inverted if-else expressions.


(* try reading this out loud without short circuiting your brain *)
if not x then foo else bar


if x then bar else foo
  • We prefer snake_casing identifiers. This includes the names of modules and module types.

  • Avoid qualifying constructors and record fields. Instead, add type annotations to the type being matched on or being constructed, e.g.,


let foo = Command.Args.S []


let (foo : _ Command.Args.t) = S []


Dune Bench

You can benchmark Dune’s performance by running make bench. This will run a subset of the Duniverse. If you are running the bench locally, make sure that you bootstrap since that is the executable that the bench will run.

The bench will build a specially selected portion of the Duniverse once, called a “clean build”. Afterwards, the build will be run 5 more times and are termed the “Null builds”.

In each run of the CI, there will be an ocaml-benchmarks status in the summary. Clicking Details will show a bench report.

The report contains the following information:

  • The build times for Clean and Null builds

  • The size of the dune.exe binary

  • User CPU times for the Clean and Null builds

  • System CPU times for the Clean and Null builds

  • All the garbage collection stats apart from “forced collections” for Clean and Null builds

Pull requests that add new libraries are likely to increase the size of the dune binary.

Performance gains in Dune can be observed in the Clean and Null build times.

Memory usage can be observed in the garbage collection stats.

Inline Benchmarks

Certain performance-critical parts of Dune are benchmarked using the inline_benchmarks library. These benchmarks are run when running the tests. Their outputs are currently not recorded and are only used to detect performance regressions.

Build-Time Benchmarks

We benchmark the build time of Dune in every PR. The times can be found here:

Melange Bench

We also benchmark a demo Melange project’s build time:

Monorepo Benchmark

The file bench/monorepo/bench.Dockerfile sets up a Docker container for benchmarking Dune building a large monorepo constructed with opam-monorepo. The monorepo is constructed according to the files in Build the Docker image from the root directory of this repo.

E.g., run:

$ docker build . -f bench/monorepo/bench.Dockerfile --tag=dune-monorepo-benchmark

The monorepo benchmark Docker image requires duniverse directory to be mounted as a volume. Generate this directory with a script from the ocaml-monorepo-benchmark repository:

$ git clone
$ cd ocaml-monorepo-benchmark
$ ./ /tmp

This will create a directory /tmp/duniverse. Then to run the benchmark, run the Docker image in a container mounting /tmp/duniverse as a volume at /home/opam/bench-dir/current-bench-data/duniverse (that specific path is a requirement of current-bench). From within the container the benchmarks can be started by running make bench. Do all this in a single command with:

$ docker run -it --volume=/tmp/duniverse:/home/opam/bench-dir/current-bench-data/duniverse dune-monorepo-benchmark bash --login -c 'make bench'

The benchmark will print out a JSON string in the format accepted by current-bench.



When changing the formatting configuration, it is possible to add the reformatting commit to the .git-blame-ignore-revs file. The commit will disappear from blame views. It is also possible to configure git to have the same behavior locally.

It is recommended to edit that file in a second PR, to make sure that the referenced commit has not changed.