OCaml Build System Migration

When to Use This Skill

Invoke this skill when:

• Converting a project from ocamlbuild to dune
• Discussing _tags, .mllib, or pkg.ml files
• Migrating from topkg to dune
• Understanding ocamlbuild artifacts

Process Overview

1. Analyze the Existing Build

Read these files to understand the project:

• _tags - ocamlbuild compilation flags and package dependencies
• pkg/pkg.ml - topkg package description
• pkg/META - findlib metadata
• *.mllib files - module lists for libraries
• opam - package dependencies

2. Create dune-project

(lang dune 3.20)
(name <package-name>)
(generate_opam_files true)

3. Create Library dune Files

For each library (from .mllib files):

(library
 (name <library_name>)
 (public_name <package.subname>)
 (libraries <dependencies>))

For optional libraries (from pkg/pkg.ml):

(library
 (name <library_name>)
 (public_name <package.subname>)
 (optional)
 (libraries <dependencies>))

4. Handle Toplevel Init Files

Files like *_top_init.ml shouldn't be compiled as modules:

(library
 (name lib_name)
 (modules lib_module)  ; Explicitly list modules
 (libraries deps))

(install
 (package pkg)
 (section lib)
 (files lib_top_init.ml))

5. Handle Warnings

If the original code triggers warnings:

(library
 (name lib)
 (flags (:standard -w -27)))

Common warnings to suppress in vendored code:

• -w -27 - unused variable

6. Create Test dune File

(test
 (name test_name)
 (libraries lib_name alcotest))

For optional tests:

(executable
 (name test_optional)
 (modules test_optional)
 (optional)
 (libraries lib some_optional_lib))

7. Update opam File

1. Rename opam to <package>.opam
2. Remove ocamlbuild/topkg dependencies
3. Add dune:

depends: [
  "ocaml" {>= "4.14.0"}
  "dune" {>= "3.0"}
]

4. Update build commands:

build: [
  ["dune" "subst"] {dev}
  ["dune" "build" "-p" name "-j" jobs]
  ["dune" "runtest" "-p" name "-j" jobs] {with-test}
  ["dune" "build" "@doc" "-p" name "-j" jobs] {with-doc}
]

8. Documentation dune File

If there's a doc/ directory:

(documentation
 (package <package-name>))

9. Remove ocamlbuild Files

Files to delete:

• _tags
• .merlin
• pkg/pkg.ml
• pkg/META
• *.mllib files
• *.itarget files
• The pkg/ directory

Mapping Reference

ocamlbuild	dune
_tags: package(foo)	(libraries foo)
_tags: thread	(libraries threads)
foo.mllib with Foo	(library (name foo) (modules foo))
pkg/pkg.ml: Pkg.mllib ~cond:x	(library ... (optional))
pkg/META	Auto-generated by dune
opam	<package>.opam

Common Issues

"Library not found" for optional deps

Use (optional) on the library stanza.

Unused variable warnings

Add (flags (:standard -w -27)).

Module in wrong library

Use (modules ...) to explicitly list modules.

Toplevel init files

Exclude from library with (modules ...) and use (install ...).

Verification

After migration:

dune build @check    # Verify syntax
dune build           # Build project
dune runtest         # Run tests
dune build @doc      # Build docs