133 lines
5.6 KiB
Markdown
133 lines
5.6 KiB
Markdown
# OCaml {#sec-language-ocaml}
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## User guide {#sec-language-ocaml-user-guide}
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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`.
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If you don't care about the exact compiler version, `ocamlPackages` is a top-level alias pointing to a recent version of OCaml.
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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`.
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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.
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For example:
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```nix
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let
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pkgs = import <nixpkgs> {};
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# choose the ocaml version you want to use
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ocamlPackages = pkgs.ocaml-ng.ocamlPackages_4_12;
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in
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pkgs.mkShell {
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# build tools
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nativeBuildInputs = with ocamlPackages; [ ocaml findlib dune_2 ocaml-lsp ];
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# dependencies
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buildInputs = with ocamlPackages; [ ocamlgraph ];
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}
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```
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## Packaging guide {#sec-language-ocaml-packaging}
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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`.
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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`.
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Here is a simple package example.
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- It defines an (optional) attribute `minimalOCamlVersion` (see note below)
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that will be used to throw a descriptive evaluation error if building with
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an older OCaml is attempted.
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- It uses the `fetchFromGitHub` fetcher to get its source.
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- It also accept `duneVersion` parameter (valid value are `"1"`, `"2"`, and
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`"3"`). The recommended practice it to set only if you don't want the default
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value and/or it depends on something else like package version. You might see
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a not-supported argument `useDune2`. The behavior was `useDune2 = true;` =>
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`duneVersion = "2";` and `useDune2 = false;` => `duneVersion = "1";`. It was
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used at the time when dune3 didn't existed.
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- It sets the optional `doCheck` attribute such that tests will be run with
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`dune runtest -p angstrom` after the build (`dune build -p angstrom`) is
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complete, but only if the Ocaml version is at at least `"4.05"`.
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- It uses the package `ocaml-syntax-shims` as a build input, `alcotest` and
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`ppx_let` as check inputs (because they are needed to run the tests), and
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`bigstringaf` and `result` as propagated build inputs (thus they will also be
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available to libraries depending on this library).
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- The library will be installed using the `angstrom.install` file that dune
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generates.
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```nix
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{ lib,
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fetchFromGitHub,
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buildDunePackage,
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ocaml,
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ocaml-syntax-shims,
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alcotest,
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result,
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bigstringaf,
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ppx_let }:
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buildDunePackage rec {
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pname = "angstrom";
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version = "0.15.0";
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minimalOCamlVersion = "4.04";
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src = fetchFromGitHub {
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owner = "inhabitedtype";
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repo = pname;
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rev = version;
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hash = "sha256-MK8o+iPGANEhrrTc1Kz9LBilx2bDPQt7Pp5P2libucI=";
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};
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checkInputs = [ alcotest ppx_let ];
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buildInputs = [ ocaml-syntax-shims ];
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propagatedBuildInputs = [ bigstringaf result ];
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doCheck = lib.versionAtLeast ocaml.version "4.05";
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meta = {
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homepage = "https://github.com/inhabitedtype/angstrom";
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description = "OCaml parser combinators built for speed and memory efficiency";
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license = lib.licenses.bsd3;
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maintainers = with lib.maintainers; [ sternenseemann ];
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};
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```
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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.
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```nix
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{ lib, fetchurl, buildDunePackage }:
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buildDunePackage rec {
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pname = "wtf8";
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version = "1.0.2";
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minimalOCamlVersion = "4.02";
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src = fetchurl {
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url = "https://github.com/flowtype/ocaml-${pname}/releases/download/v${version}/${pname}-v${version}.tbz";
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hash = "sha256-d5/3KUBAWRj8tntr4RkJ74KWW7wvn/B/m1nx0npnzyc=";
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};
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meta = with lib; {
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homepage = "https://github.com/flowtype/ocaml-wtf8";
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description = "WTF-8 is a superset of UTF-8 that allows unpaired surrogates.";
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license = licenses.mit;
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maintainers = [ maintainers.eqyiel ];
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};
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}
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```
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Note about `minimalOCamlVersion`. A deprecated version of this argument was
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spelled `minimumOCamlVersion`; setting the old attribute wrongly modifies the
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derivation hash and is therefore inappropriate. As a technical dept, currently
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packaged libraries may still use the old spelling: maintainers are invited to
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fix this when updating packages. Massive renaming is strongly discouraged as it
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would be challenging to review, difficult to test, and will cause unnecessary
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rebuild.
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The build will automatically fail if two distinct versions of the same library
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are added to `buildInputs` (which usually happens transitively because of
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`propagatedBuildInputs`). Set `dontDetectOcamlConflicts` to true to disable this
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behavior.
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