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` (see note below)
  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.

- It also accepts a `duneVersion` parameter (valid values are `"1"`, `"2"`, and
  `"3"`). The recommended practice is to set it only if you don't want the default
  value and/or it depends on something else like package version. You might see
  a not-supported argument `useDune2`. The behavior was `useDune2 = true;` =>
  `duneVersion = "2";` and `useDune2 = false;` => `duneVersion = "1";`. It was
  used at the time when dune3 didn't exist.

- 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.

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

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

  minimalOCamlVersion = "4.04";

  src = fetchFromGitHub {
    owner = "inhabitedtype";
    repo = "angstrom";
    tag = version;
    hash = "sha256-MK8o+iPGANEhrrTc1Kz9LBilx2bDPQt7Pp5P2libucI=";
  };

  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.

```nix
{
  lib,
  fetchurl,
  buildDunePackage,
}:

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

  minimalOCamlVersion = "4.02";

  src = fetchurl {
    url = "https://github.com/flowtype/ocaml-wtf8/releases/download/v${version}/wtf8-v${version}.tbz";
    hash = "sha256-d5/3KUBAWRj8tntr4RkJ74KWW7wvn/B/m1nx0npnzyc=";
  };

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

The build will automatically fail if two distinct versions of the same library
are added to `buildInputs` (which usually happens transitively because of
`propagatedBuildInputs`). Set `dontDetectOcamlConflicts` to true to disable this
behavior.