View source for OCaml

Most packages related to OCaml, in particular the OCaml compiler and many libraries, belong to the '''ocamlPackages''' attribute set of nixpkgs.

== Findlib, ocamlfind ==

OCaml libraries are usually located using [http://projects.camlcity.org/projects/findlib.html findlib] and the associated '''ocamlfind''' tool. These tools are found under the '''ocamlPackages.findlib''' attribute and rely on the '''OCAMLPATH''' environment variable.

A hook in the '''findlib''' package will automatically populate this variable with the paths to the other libraries. For instance, when starting a shell with
<code>nix-shell --packages ocamlPackages.findlib ocamlPackages.batteries</code>
will set the '''OCAMLPATH''' variable so that '''ocamlfind''' can locate the '''batteries''' library.

This hook also sets the '''CAML_LD_LIBRARY_PATH''' environment variable that is used for locating the dynamically loaded shared libraries (aka stublibs).

== Scripting with OCaml ==
OCaml can be used in nix-shell scripts as follows:
<syntaxHighlight lang=ocaml>
#!/usr/bin/env nix-shell
(*
#!nix-shell --pure -i ocaml -p ocaml
*)

print_string "Hello world! 🚀 \n";;
</syntaxHighlight>

== Using Emacs ==

[https://github.com/ocaml/tuareg Tuareg] is an Emacs OCaml mode. It can be installed with
<code>nix-env -iA nixpkgs.emacsPackagesNg.tuareg</code>.

Emacs must be configured to be able to find this package: you may need to add a line like this to your '''.emacs''' file, somewhere between <code>(require 'package)</code> and <code>(package-initialize)</code>:

<syntaxHighlight lang=emacs>
(add-to-list 'package-directory-list "~/.nix-profile/share/emacs/site-lisp/elpa")
</syntaxHighlight>

There is some more documentation in the [https://github.com/NixOS/nixpkgs/blob/master/pkgs/top-level/emacs-packages.nix nixpkgs sources].

[https://github.com/ocaml/merlin Merlin] provides additional functionality to Emacs. It can be installed through <code>nix-env -iA nixpkgs.ocamlPackages.merlin</code>.

Beware that merlin is specific to one particular version of OCaml (it won’t work correctly with a compiler of a different version).

To configure Emacs and enable the merlin mode, add the following to your '''.emacs''':

<syntaxHighlight lang=emacs>
(add-to-list 'load-path "~/.nix-profile/share/emacs/site-lisp")
(require 'merlin)
(add-hook 'tuareg-mode-hook 'merlin-mode t)
</syntaxHighlight>

Also, a specific variable ('''merlin-command''') of the merlin mode must be overridden (its default is '''opam'''):

<syntaxHighlight lang=emacs>
(custom-set-variables
  '(merlin-command "ocamlmerlin")
)
</syntaxHighlight>

A useful complement to merlin is [https://www.typerex.org/ocp-indent.html '''ocp-indent'''], a customizable tool to indent OCaml code. It can be installed using <code>nix-env -iA nixpkgs.ocamlPackages.ocpIndent</code>. To enable it in Emacs, don’t forget to add the following line to your '''.emacs''':

<syntaxHighlight lang=emacs>
(require 'ocp-indent)
</syntaxHighlight>

== Specific version of the OCaml compiler ==

Various versions of the '''ocamlPackage''' attribute set are available, corresponding to various versions of OCaml. For instance, the attribute set '''ocaml-ng.ocamlPackages_4_04''' contains the OCaml compiler at version 4.04 and OCaml libraries compiled with that particular compiler.

=== Custom version ===

So as to get the set of OCaml libraries built with/for a custom version of the OCaml compiler, e.g., to enable '''flambda''' support, you may use the '''ocamlPackages.overrideScope''' function:

<syntaxHighlight lang=nix>
ocamlPackagesFlambda = ocamlPackages.overrideScope (self: super: {
  ocaml = super.ocaml.override { flambdaSupport = true; };
});
</syntaxHighlight>

More details: https://github.com/NixOS/nixpkgs/pull/53357#issuecomment-451727433

[[Category:Languages]]