How to Generate Opam Files from dune-project

This guide will show you how to configure Dune so that it generates opam files.

Declaring Package Dependencies

The goal of this first step is to add (package) stanzas in your dune-project file. These stanzas declare the metadata that your package uses in the language of opam packages. See Declaring a Package.

The next step depends on whether you are starting from a clean slate (new package) or adapting an existing opam file.

For a New Package (No Existing Opam File)

If your project does not have any opam files, you will have to find your package dependencies. In the simple case, collect all the libraries that appear in the (libraries) fields of your project and put this list in the (depends) field of the corresponding (package). See The OCaml Ecosystem for the difference between libraries and packages.

Example: you have a library that looks like:

 (public_name frobnitz)
 (libraries lwt fmt))

You can declare the package as:

 (name frobnitz)
 (depends lwt fmt))

Also add common metadata using (authors), (maintainers), (license), (source), as well as a (synopsis) and a (description) for

For an Existing Package

If you already have an opam file (or several of them), you can convert it by following the rules in package.

For example, if your opam file looks like:

opam-version: 2.0
authors: ["Anil Madhavapeddy" "Rudi Grinberg"]
maintainer: [""]
name: "cohttp-async"
synopsis: "HTTP client and server for the Async library"
description: "A _really_ long description"
license: "ISC"
bug-reports: ""
homepage: ""
dev-repo: "git+"
build: [
  ["dune" "subst"] {dev}
    "@runtest" {with-test}
    "@doc" {with-doc}
depends: [
  "dune" { >= "3.4" }
  "odoc" { with-doc }
  "cohttp" { >= "1.0.2" }
  "conduit-async" { >= "1.0.3" }
  "async" { >= "v0.10.0" }

You can express this as:

(source (github mirage/ocaml-cohttp))
(license ISC)
(authors "Anil Madhavapeddy" "Rudi Grinberg")
(maintainers "")

 (name cohttp-async)
 (synopsis "HTTP client and server for the Async library")
 (description "A _really_ long description")
  (cohttp (>= 1.0.2))
  (conduit-async (>= 1.0.3))
  (async (>= v0.10.0))))

General Notes and Tips

  • Do not declare a dependency on the dune and odoc packages. Dune will generate them with the right constraints.

  • For fields that are common between packages (like (authors) or (license)), you can use a global one rather than replicate it between packages.

  • If you use a platform such as GitHub you can use (source) as a shorthand instead of specifying (bug_reports), (homepage), etc.

  • (package) stanzas do not support all opam fields or complete syntax for dependency specifications. If the package you are adapting requires this, keep the corresponding opam fields in a pkg.opam.template file. See Packages.

  • It is not necessary to specify (version), this will be added at release time if you use dune-release.

Generating Opam Files

If you have existing *.opam files, make a backup of them because the instructions in this section will overwrite them.

Now that you have declared package metadata in dune-project, you can add (generate_opam_files) in (dune-project).

From now on, commands like dune build and dune runtest are going to regenerate the contents of opam files from the metadata in (package) stanzas. If you only want to generate the opam file, run dune build <project_name>.opam.

Run dune build once and observe that the opam files have been created or updated. Make sure to add these changes to your version control system.