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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 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 nix-shell --packages ocamlPackages.findlib ocamlPackages.batteries 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:

#!/usr/bin/env nix-shell
#!nix-shell --pure -i ocaml -p ocaml

print_string "Hello world! 🚀 \n";;

Using Emacs

Tuareg is an Emacs OCaml mode. It can be installed with nix-env -iA nixpkgs.emacsPackagesNg.tuareg.

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 (require 'package) and (package-initialize):

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

There is some more documentation in the nixpkgs sources.

Merlin provides additional functionality to Emacs. It can be installed through nix-env -iA nixpkgs.ocamlPackages.merlin.

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:

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

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

  '(merlin-command "ocamlmerlin")

A useful complement to merlin is ocp-indent, a customizable tool to indent OCaml code. It can be installed using nix-env -iA nixpkgs.ocamlPackages.ocpIndent. To enable it in Emacs, don’t forget to add the following line to your .emacs:

(require 'ocp-indent)

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:

ocamlPackagesFlambda = ocamlPackages.overrideScope' (self: super: {
  ocaml = super.ocaml.override { flambdaSupport = true; };

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