depot/third_party/nixpkgs/doc/languages-frameworks/ocaml.section.md

OCaml

OCaml libraries should be installed in $(out)/lib/ocaml/${ocaml.version}/site-lib/. Such directories are automatically added to the $OCAMLPATH environment variable when building another package that depends on them or when opening a nix-shell.

Given that most of the OCaml ecosystem is now built with dune, nixpkgs includes a convenience build support function called buildDunePackage that will build an OCaml package using dune, OCaml and findlib and any additional dependencies provided as buildInputs or propagatedBuildInputs.

Here is a simple package example.

• It defines an (optional) attribute minimalOCamlVersion that will be used to throw a descriptive evaluation error if building with an older OCaml is attempted.

• It uses the fetchFromGitHub fetcher to get its source.

• useDune2 = true ensures that the latest version of Dune is used for the build (this may become the default value in a future release).

• It sets the optional doCheck attribute such that tests will be run with dune runtest -p angstrom after the build (dune build -p angstrom) is complete, but only if the Ocaml version is at at least "4.05".

• It uses the package ocaml-syntax-shims as a build input, alcotest and ppx_let as check inputs (because they are needed to run the tests), and bigstringaf and result as propagated build inputs (thus they will also be available to libraries depending on this library).

• The library will be installed using the angstrom.install file that dune generates.

{ lib,
  fetchFromGitHub,
  buildDunePackage,
  ocaml,
  ocaml-syntax-shims,
  alcotest,
  result,
  bigstringaf,
  ppx_let }:

buildDunePackage rec {
  pname = "angstrom";
  version = "0.15.0";
  useDune2 = true;

  minimalOCamlVersion = "4.04";

  src = fetchFromGitHub {
    owner  = "inhabitedtype";
    repo   = pname;
    rev    = version;
    sha256 = "1hmrkdcdlkwy7rxhngf3cv3sa61cznnd9p5lmqhx20664gx2ibrh";
  };

  checkInputs = [ alcotest ppx_let ];
  buildInputs = [ ocaml-syntax-shims ];
  propagatedBuildInputs = [ bigstringaf result ];
  doCheck = lib.versionAtLeast ocaml.version "4.05";

  meta = {
    homepage = "https://github.com/inhabitedtype/angstrom";
    description = "OCaml parser combinators built for speed and memory efficiency";
    license = lib.licenses.bsd3;
    maintainers = with lib.maintainers; [ sternenseemann ];
  };

Here is a second example, this time using a source archive generated with dune-release. It is a good idea to use this archive when it is available as it will usually contain substituted variables such as a %%VERSION%% field. This library does not depend on any other OCaml library and no tests are run after building it.

{ lib, fetchurl, buildDunePackage }:

buildDunePackage rec {
  pname = "wtf8";
  version = "1.0.2";

  useDune2 = true;

  minimalOCamlVersion = "4.02";

  src = fetchurl {
    url = "https://github.com/flowtype/ocaml-${pname}/releases/download/v${version}/${pname}-v${version}.tbz";
    sha256 = "09ygcxxd5warkdzz17rgpidrd0pg14cy2svvnvy1hna080lzg7vp";
  };

  meta = with lib; {
    homepage = "https://github.com/flowtype/ocaml-wtf8";
    description = "WTF-8 is a superset of UTF-8 that allows unpaired surrogates.";
    license = licenses.mit;
    maintainers = [ maintainers.eqyiel ];
  };
}