OCaml

User guide

OCaml libraries are available in attribute sets of the form ocaml-ng.ocamlPackages_X_XX where X is to be replaced with the desired compiler version. For example, ocamlgraph compiled with OCaml 4.12 can be found in ocaml-ng.ocamlPackages_4_12.ocamlgraph. The compiler itself is also located in this set, under the name ocaml.

If you don't care about the exact compiler version, ocamlPackages is a top-level alias pointing to a recent version of OCaml.

OCaml applications are usually available top-level, and not inside ocamlPackages. Notable exceptions are build tools that must be built with the same compiler version as the compiler you intend to use like dune or ocaml-lsp.

To open a shell able to build a typical OCaml project, put the dependencies in buildInputs and add ocamlPackages.ocaml and ocamlPackages.findlib to nativeBuildInputs at least. For example:

let
 pkgs = import <nixpkgs> {};
 # choose the ocaml version you want to use
 ocamlPackages = pkgs.ocaml-ng.ocamlPackages_4_12;
in
pkgs.mkShell {
  # build tools
  nativeBuildInputs = with ocamlPackages; [ ocaml findlib dune_2 ocaml-lsp ];
  # dependencies
  buildInputs = with ocamlPackages; [ ocamlgraph ];
}

Packaging guide

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 (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 accept duneVersion parameter (valid value are "1", "2", and "3"). The recommended practice it to set 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 existed.
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";

  minimalOCamlVersion = "4.04";

  src = fetchFromGitHub {
    owner  = "inhabitedtype";
    repo   = pname;
    rev    = 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.

{ lib, fetchurl, buildDunePackage }:

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

  minimalOCamlVersion = "4.02";

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

  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 ];
  };
}

Note about minimalOCamlVersion. A deprecated version of this argument was spelled minimumOCamlVersion; setting the old attribute wrongly modifies the derivation hash and is therefore inappropriate. As a technical dept, currently packaged libraries may still use the old spelling: maintainers are invited to fix this when updating packages. Massive renaming is strongly discouraged as it would be challenging to review, difficult to test, and will cause unnecessary rebuild.

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.

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OCaml

User guide

Packaging guide

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