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# OCaml {#sec-language-ocaml}
## User guide {#sec-language-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:
```nix
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 {#sec-language-ocaml-packaging}
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.
- `useDune2 = true` ensures that Dune version 2 is used for the
build (this is the default; set to `false` to use Dune version 1).
- 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";
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.
```nix
{ 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 ];
};
}
```
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.
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