Title: NixOS optional features in packages

	Title: NixOS optional features in packages
	Author: Solène
	Date: 14 October 2020
	Tags: nixos linux nix
	Description:

	As a claws-mail user, I like to have calendar support in the mail
	client to be able to "accept" invitations. In the default NixOS
	claws-mail package, the vcalendar module isn't installed with the
	package. Still, it is possible to add support for the vcalendar
	module without ugly hack.

	It turns out, by default, the claws-mail package in Nixpkg has an
	optional build option for the vcalendar module, we need to tell
	nixpkg we want this module and claws-mail will be compiled.

	As stated in the [NixOS
	manual](https://nixos.org/manual/nixos/stable/#sec-customising-packages
	),
	the optionals features can't be searched yet. So what's possible
	is to search for your package in the [NixOS packages
	search](https://search.nixos.org/packages), click on the package
	name to get to the details and click on the link named "Nix expression"
	that will open a link to the package definition on GitHUB, [claws-mail
	nix
	expression](https://github.com/NixOS/nixpkgs/blob/nixos-unstable/pkgs/a
	pplications/networking/mailreaders/claws-mail/default.nix#L111)

	As you can see on the claws-mail nix expression code, there are lot
	of lines with optional, those are features we can enable. Here
	is a sample:

	[..]
	++ optional (!enablePluginArchive) "--disable-archive-plugin"
	++ optional (!enablePluginLitehtmlViewer)
	"--disable-litehtml_viewer-plugin"
	++ optional (!enablePluginPdf) "--disable-pdf_viewer-plugin"
	++ optional (!enablePluginPython) "--disable-python-plugin"
	[..]

	In your `configuration.nix` file, where you define the package list
	you want, you can tell you want to enable the plugin vcalendar,
	this is done as in the following example:


	environment.systemPackages = with pkgs; [
	kakoune git firefox irssi minetest
	(pkgs.claws-mail.override { enablePluginVcalendar = true;})
	];

	When you rebuild your system to match the configuration definition,
	claws-mail will be compiled with the extras options you defined.

	Now, I have claws-mail with vCalendar support.