So far, I'm very happy with how things work and can do most of my tasks as on any other Linux distribution. Some tasks, however, require some more reading and thinking.

Today I wanted to jot down a quick how-to on testing an addition to the Perl packages in the Nix Packages collection (Nixpkgs).

#coding #perl #nixos

This how-to is based on:

• https://nixos.wiki/wiki/Perl#Adding_something_from_CPAN_to_nixpkgs
• https://dataswamp.org/~solene/2021-09-18-nix-cpan-pip.html#_Perl
• https://nixos.org/manual/nixpkgs/stable/#how-to-develop
• https://nixos.org/manual/nixpkgs/stable/#submitting-changes-making-patches

First, download nix-generate-from-cpan by adding it to /etc/nixos/configuration.nix and running sudo nixos-rebuild switch.

I won't go into the details on the first step here, because this is a task you should already be familiar with after having spent the first few hours configuring your NixOS installation.

The next steps involve patching the <nixpkgs> “collection” (which is part of the default nixpkgs “channel”) .

git clone --depth=1 https://github.com/NixOS/nixpkgs nixpkgs.git
cd nixpkgs.git

Then you create the patch for the pkgs/top-level/perl-packages.nix file as follows:

nix-generate-from-cpan Cal::DAV
attribute name: CalDAV
module: Cal::DAV
version: 0.6
package: Cal-DAV-0.6.tar.gz (Cal-DAV-0.6, CalDAV)
path: authors/id/S/SI/SIMONW
downloaded to: /home/andi/.cpanplus/authors/id/S/SI/SIMONW/Cal-DAV-0.6.tar.gz
sha-256: e436ecb61db9a2fa3ba4b2111149dfd31bdf7aa6acbc12cc49584e50db63e79b
unpacked to: /home/andi/.cpanplus/5.36.0/build/CM_4_0IbZK/Cal-DAV-0.6
runtime deps: DataICal HTTPDAV LWP
build deps: 
description: A CalDAV client
license: perl_5
RSS feed: https://metacpan.org/feed/distribution/Cal-DAV
===
  CalDAV = buildPerlPackage {
    pname = "Cal-DAV";
    version = "0.6";
    src = fetchurl {
      url = "mirror://cpan/authors/id/S/SI/SIMONW/Cal-DAV-0.6.tar.gz";
      sha256 = "e436ecb61db9a2fa3ba4b2111149dfd31bdf7aa6acbc12cc49584e50db63e79b";
    };
    propagatedBuildInputs = [ DataICal HTTPDAV LWP ];
    meta = {
      description = "A CalDAV client";
      license = with lib.licenses; [ artistic1 gpl1Plus ];
    };
  };

Add the expression in alphabetical order to pkgs/top-level/perl-packages.nix. Only copy/paste the “Nix expression” after the === characters into the file.

To test the patch, spin-up a nix-shell which includes the new Perl module. Create the default.nix expression file in any working directory:

cat <<EOF > default.nix 
with (import <nixpkgs> {});
let
  CalDAV = perlPackages.buildPerlPackage {
    pname = "Cal-DAV";
    version = "0.6";
    src = fetchurl {
      url = "mirror://cpan/authors/id/S/SI/SIMONW/Cal-DAV-0.6.tar.gz";
      sha256 = "e436ecb61db9a2fa3ba4b2111149dfd31bdf7aa6acbc12cc49584e50db63e79b";
    };
    propagatedBuildInputs = [ perlPackages.DataICal perlPackages.HTTPDAV perlPackages.LWP ];
    meta = {
      description = "A CalDAV client";
      license = with lib.licenses; [ artistic1 gpl1Plus ];
    };
  };

in
mkShell {
  buildInputs = [ CalDAV perl ];
  # putting perl here is only required when not using NixOS, this tell you want Nix perl binary
}
EOF

The expression includes the result of the previous nix-generate-from-cpan Cal::DAV with slight modifications.

Notice that propagatedBuildInputs needs to refer to already installed packages prefixed by perlPackages.

More information on how to develop with nix-shell.

Solènes article nicely shows how the let expression can also include an entirely new block of code to define a variable. In this example, the let expression uses the buildPerlPackage function, which is a “builder” for building Perl packages (as the name indicates).

With that said, you can launch the development shell to test any Perl script with the newly added Perl library as follows:

nix-shell

If you don't want to create a “descriptive” shell expression, you can also launch the shell more in an ad-hoc fashion using the following command:

nix-shell -p perlPackages.CalDAV perl -I nixpkgs=~/Downloads/nixpkgs.git

See also the example for Perl in the nix-shell Manual.

If you are happy with how the module works in the nix-shell, you can add it to your system/profile (using nix-env or nixos-rebuild) and even contribute it to the upstream <nixpkgs> repository.

This post mostly reminds myself of all the new terminology I learned the past few days.