tjh.dev
nixpkgs mirror (for testing)
github.com/NixOS/nixpkgs
nix
1# OCaml {#sec-language-ocaml}
2
3## User guide {#sec-language-ocaml-user-guide}
4
5OCaml 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`.
6
7If you don't care about the exact compiler version, `ocamlPackages` is a top-level alias pointing to a recent version of OCaml.
8
9OCaml 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`.
10
11To 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.
12For example:
13```nix
14let
15 pkgs = import <nixpkgs> {};
16 # choose the ocaml version you want to use
17 ocamlPackages = pkgs.ocaml-ng.ocamlPackages_4_12;
18in
19pkgs.mkShell {
20 # build tools
21 nativeBuildInputs = with ocamlPackages; [ ocaml findlib dune_2 ocaml-lsp ];
22 # dependencies
23 buildInputs = with ocamlPackages; [ ocamlgraph ];
24}
25```
26
27## Packaging guide {#sec-language-ocaml-packaging}
28
29OCaml 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`.
30
31Given 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`.
32
33Here is a simple package example.
34
35- It defines an (optional) attribute `minimalOCamlVersion` (see note below)
36 that will be used to throw a descriptive evaluation error if building with
37 an older OCaml is attempted.
38
39- It uses the `fetchFromGitHub` fetcher to get its source.
40
41- It also accept `duneVersion` parameter (valid value are `"1"`, `"2"`, and
42 `"3"`). The recommended practice it to set only if you don't want the default
43 value and/or it depends on something else like package version. You might see
44 a not-supported argument `useDune2`. The behavior was `useDune2 = true;` =>
45 `duneVersion = "2";` and `useDune2 = false;` => `duneVersion = "1";`. It was
46 used at the time when dune3 didn't existed.
47
48- It sets the optional `doCheck` attribute such that tests will be run with
49 `dune runtest -p angstrom` after the build (`dune build -p angstrom`) is
50 complete, but only if the Ocaml version is at at least `"4.05"`.
51
52- It uses the package `ocaml-syntax-shims` as a build input, `alcotest` and
53 `ppx_let` as check inputs (because they are needed to run the tests), and
54 `bigstringaf` and `result` as propagated build inputs (thus they will also be
55 available to libraries depending on this library).
56
57- The library will be installed using the `angstrom.install` file that dune
58 generates.
59
60```nix
61{ lib,
62 fetchFromGitHub,
63 buildDunePackage,
64 ocaml,
65 ocaml-syntax-shims,
66 alcotest,
67 result,
68 bigstringaf,
69 ppx_let }:
70
71buildDunePackage rec {
72 pname = "angstrom";
73 version = "0.15.0";
74
75 minimalOCamlVersion = "4.04";
76
77 src = fetchFromGitHub {
78 owner = "inhabitedtype";
79 repo = pname;
80 rev = version;
81 hash = "sha256-MK8o+iPGANEhrrTc1Kz9LBilx2bDPQt7Pp5P2libucI=";
82 };
83
84 checkInputs = [ alcotest ppx_let ];
85 buildInputs = [ ocaml-syntax-shims ];
86 propagatedBuildInputs = [ bigstringaf result ];
87 doCheck = lib.versionAtLeast ocaml.version "4.05";
88
89 meta = {
90 homepage = "https://github.com/inhabitedtype/angstrom";
91 description = "OCaml parser combinators built for speed and memory efficiency";
92 license = lib.licenses.bsd3;
93 maintainers = with lib.maintainers; [ sternenseemann ];
94 };
95}
96```
97
98Here 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.
99
100```nix
101{ lib, fetchurl, buildDunePackage }:
102
103buildDunePackage rec {
104 pname = "wtf8";
105 version = "1.0.2";
106
107 minimalOCamlVersion = "4.02";
108
109 src = fetchurl {
110 url = "https://github.com/flowtype/ocaml-${pname}/releases/download/v${version}/${pname}-v${version}.tbz";
111 hash = "sha256-d5/3KUBAWRj8tntr4RkJ74KWW7wvn/B/m1nx0npnzyc=";
112 };
113
114 meta = {
115 homepage = "https://github.com/flowtype/ocaml-wtf8";
116 description = "WTF-8 is a superset of UTF-8 that allows unpaired surrogates";
117 license = lib.licenses.mit;
118 maintainers = [ lib.maintainers.eqyiel ];
119 };
120}
121```
122
123The build will automatically fail if two distinct versions of the same library
124are added to `buildInputs` (which usually happens transitively because of
125`propagatedBuildInputs`). Set `dontDetectOcamlConflicts` to true to disable this
126behavior.