FAQ¶
Why Do Many Dune Projects Contain a Makefile
?¶
Many Dune projects contain a root Makefile
. It’s often only there for
convenience for the following reasons:
There are many different build systems out there, all with a different CLI. If you have been hacking for a long time, the one true invocation you know is
make && make install
, possibly preceded by./configure
.You often have a few common operations that aren’t part of the build, so
make <blah>
is a good way to provide them.make
is shorter to type thandune build @install
How to Add a Configure Step to a Dune Project¶
The with-configure-step example shows one way to add a configure step that
preserves composability; i.e., it doesn’t require manually running the
./configure
script when working on multiple projects simultaneously.
Can I Use topkg
with Dune?¶
While it’s possible to use the topkg-jbuilder, it’s not recommended.
dune-release subsumes topkg-jbuilder
and is specifically tailored to Dune
projects.
How Do I Publish My Packages with Dune?¶
Dune is just a build system and considers publishing outside of its scope. However, the dune-release project is specifically designed for releasing Dune projects to opam. We recommend using this tool for publishing Dune packages.
Where Can I Find Some Examples of Projects Using Dune?¶
The dune-universe repository contains a snapshot of the latest versions of all opam packages that depend on Dune. Therefore, it’s a useful reference to find different approaches for constructing build rules.
What is Jenga?¶
jenga is a build system developed by Jane Street, mainly for internal use. It was never usable outside of Jane Street, so it’s not recommended for general use. It has no relationship to Dune apart from Dune being the successor to Jenga externally. Eventually, Dune is expected to replace Jenga internally at Jane Street as well.
How to Make Warnings Non-Fatal¶
jbuilder formerly displayed warnings, but most of them wouldn’t stop the build. However, Dune makes all warnings fatal by default. This can be a challenge when porting a codebase to Dune. There are two ways to make warnings non-fatal:
The
jbuilder
compatibility executable works even withdune
files. You can use it while some warnings remain and then switch over to thedune
executable. This is the recommended way to handle the situation.You can pass
--profile release
todune
. It will set up different compilation options that usually make sense for release builds, including making warnings non-fatal. This is done by default when installing packages from opam.You can change the flags used by the
dev
profile by adding the following stanza to adune
file:
(env
(dev
(flags (:standard -warn-error -A))))
How to Turn Specific Errors into Warnings¶
Dune is strict about warnings by default in that all warnings are treated as
fatal errors. To change certain errors into warnings for a project, you can add
the following to dune-workspace
:
(env (dev (flags :standard -warn-error -27-32)))
In this example, the warnings 27 (unused-var-strict) and 32 (unused-value-declaration) are treated as warnings rather than errors.
How to Display the Output of Commands as They Run¶
When Dune runs external commands, it redirects and saves their output, then displays it when complete. This ensures that there’s no interleaving when writing to the console.
But this might not be what the you want. For example, when you debug a hanging build.
In that case, one can pass -j1 --no-buffer
so the commands are directly
printed on the console (and the parallelism is disabled so the output stays
readable).
How Can I Generate an mli
File From an ml
File¶
When a module starts as just an implementation (.ml
file), it can be
tedious to define the corresponding interface (.mli
file).
It is possible to use the ocaml-print-intf
program (available on opam
through $ opam install ocaml-print-intf
) to generate the right mli
file:
$ dune exec -- ocaml-print-intf ocaml_print_intf.ml
val root_from_verbose_output : string list -> string
val target_from_verbose_output : string list -> string
val build_cmi : string -> string
val print_intf : string -> unit
val version : unit -> string
val usage : unit -> unit
The ocaml-print-intf
program has special support for Dune, so it will
automatically understand external dependencies.
How Can I Build a Single Library?¶
You might want to do this when you don’t have all the dependencies installed to
compile an entire project, or parts of the project don’t build for whatever
reason. Maybe you want to check if your changes compile or produce build
artifacts needed by ocaml-lsp-server
.
Suppose you have a library defined in src/foo/dune
:
(library
(public_name my_library)
...)
You can build this library on its own by running the following from the project root directory:
$ dune build %{cmxa:src/foo/my_library}
Note that the path (src/foo
in the example above) is relative to the current
directory - not the project root. If the library defines a name
distinct from
its public_name
then that can be used interchangeably with the public_name
in this command.
Files and Directories Whose Names Begin with “.” (Period) are Ignored by source_tree
¶
Dune’s default behaviour is to ignore files and directories starting with “.”
when copying directories with source_tree
. This is to avoid accidentally
copying the .git
directory into the _build
directory during a build.
This is a common source of confusion when interoperating with other libraries that use hidden directories for configuration, such as Rust. For example consider this rule which builds a Rust library contained in a subdirectory foo-rs:
(rule
(target foo.a)
(deps
(source_tree foo-rs))
(action
(progn
(chdir
foo-rs
(run cargo build --release))
(run mv foo-rs/target/release/%{target} %{target}))))
The build config for the Rust project will be in a directory
foo-rs/.cargo/config.toml
, and by default the .cargo
directory won’t
get copied into the _build
directory and so the Rust project will build
with an incorrect configuration.
To fix this, create a dune
file at the top level of the Rust project (i.e.,
foo-rs/dune
):
(dirs :standard .cargo)
If you’re following the standard advice for embedding Rust projects into Dune
projects then you likely already have a dune
project inside your Rust
project that looks like:
(dirs :standard \ target)
(data_only_dirs vendor)
In this case you can update it to look like this:
(dirs :standard .cargo \ target)
(data_only_dirs vendor)
How Can I Write Inline Tests in a Package Without my Users Needing to Install ppx_inline_test
?¶
If you came to OCaml from Rust and noticed that Dune has a feature for running inline tests you might be wondering how to do the OCaml equivalent of:
// define a private function
fn foo() { ... }
// test the function right next to its definition
#[test]
fn test_of_foo() { ... }
That is, writing tests for private functions right next to the definition of
those functions. The Inline Tests documentation describes how to do this
using the ppx_inline_test
package; however, if you do this in your package,
then your package must unconditionally depend on the ppx_inline_test
package. Opam has a notion of test-only dependencies (its with-test
flag),
but you cannot use this with ppx_inline_test
. The consequence of this is
that anyone depending on your package is also transitively depending on
ppx_inline_test
as well as all of its dependencies.
The reason for this is OCaml code with preprocessor directives (such as those
used for inline tests with ppx_inline_test
) is technically not valid OCaml
code until it has been preprocessed. Unlike the cargo build system used for
Rust, Dune does not have a preprocessor built into it. Instead, it relies on
external tools (such as ppx_inline_test
) to parse the code and replace any
preprocessor directives with valid OCaml. Dune doesn’t know how to parse OCaml
code at all so it can’t even remove inline tests from the code in cases where
ppx_inline_test
is unavailable.
The blessed workaround for folks who want to use ppx_inline_test
in their
packages but don’t want to add it as a dependency is to create a new
(unreleased) package which contains all the tests. In the original package,
expose all the private APIs you intend to test via public modules named
something foreboding such as For_test
so your users know not to rely on
their contents and then have the test package define tests that call your
“private” APIs through the For_test
modules.