# 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 `minimumOCamlVersion` 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 sets the `doCheck` (optional) attribute to `true` which means that tests will be run with `dune runtest -p angstrom` after the build (`dune build -p angstrom`) is complete. It uses `alcotest` as a build input (because it is 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.

    { stdenv, fetchFromGitHub, buildDunePackage, alcotest, result, bigstringaf }:

    buildDunePackage rec {
      pname = "angstrom";
      version = "0.10.0";

      minimumOCamlVersion = "4.03";

      src = fetchFromGitHub {
        owner  = "inhabitedtype";
        repo   = pname;
        rev    = version;
        sha256 = "0lh6024yf9ds0nh9i93r9m6p5psi8nvrqxl5x7jwl13zb0r9xfpw";
      };

      buildInputs = [ alcotest ];
      propagatedBuildInputs = [ bigstringaf result ];
      doCheck = true;

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

    { stdenv, fetchurl, buildDunePackage }:

    buildDunePackage rec {
      pname = "wtf8";
      version = "1.0.1";

      minimumOCamlVersion = "4.01";

      src = fetchurl {
        url = "https://github.com/flowtype/ocaml-${pname}/releases/download/v${version}/${pname}-${version}.tbz";
        sha256 = "1msg3vycd3k8qqj61sc23qks541cxpb97vrnrvrhjnqxsqnh6ygq";
      };

      meta = with stdenv.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 ];
      };
    }