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nix-opam-switch

Create and maintain Opam switches where the compiler and tools are built by Nix.

This brings several advantages:

  • Several switches can share the same compiler and tools. Reduces disk usage and makes switch creation potentially instant.

  • The compiler and the tools might be fetched from Nix's online cache. If the compiler have not been built before, there's a chance that it could be fetched in seconds.

  • Tools' dependencies are not installed in the switch. They cannot interfere with the projects' dependencies.

  • Switches created by this tool do not expire on NixOS. The compiler and other binaries contain absolute paths to C libraries that are not recorded as GC roots and are eventually collected.

The tools installed alongside the compiler are:

  • Merlin
  • ocp-indent
  • OCamlformat (with version specified in ./.ocamlformat)

Install

This tool relies on Nix and Opam to be also installed, they are not part of the closure to avoid messing up with their mutable states.

Save this to a file and run nix-env -if the_file.nix:

{ pkgs ? import <nixpkgs> { } }:

# Or use the following in your NixOS configuration, nixpkgs overlay, etc..
pkgs.callPackage (pkgs.fetchgit {
  url = "https://github.com/Julow/nix-opam-switch";
  rev = "d9035b22b3e362210e190bab36f58fd18743992e";
  sha256 = "sha256-svGlLtYysrgNA3/E1kWsgDvGU51lAXdH8z7qIn/2tD0=";
}) { }

Or installation from source (useful to hack on the code):

git clone https://github.com/Julow/nix-opam-switch
cd nix-opam-switch
nix-env -if .

nix-env can be repeated to update the package.

Usage

Local switch

To create a local switch with a recent version of OCaml, Merlin, ocp-indent and OCamlformat installed:

$ nix-opam-switch create .

To create a local switch with a specific version of OCaml:

$ nix-opam-switch list-available
...
4.14
5.0
$ nix-opam-switch create . 5.0

Global switch

To create a global switch:

$ nix-opam-switch create 4.14
$ nix-opam-switch create 4.14-foo 4.14
$ nix-opam-switch create 4.14-foo # Shortcut for the previous command

Re-installing OCamlformat

To install the version of OCamlformat specified by the .ocamlformat in the current directory:

$ nix-opam-switch ocamlformat

To install a specified version of OCamlformat:

$ nix-opam-switch ocamlformat 0.24.1