nixpkgs/doc/languages-frameworks/ocaml.section.md

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# OCaml {#sec-language-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.
```nix
{ lib
, 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 = 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 = "1.0.1";
minimumOCamlVersion = "4.01";
src = fetchurl {
url = "https://github.com/flowtype/ocaml-${pname}/releases/download/v${version}/${pname}-${version}.tbz";
sha256 = "1msg3vycd3k8qqj61sc23qks541cxpb97vrnrvrhjnqxsqnh6ygq";
};
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 ];
};
}
```