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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.