NixOS optional features in packages

Author: Solène

Date: 14 October 2020

Tags: nixos linux nix

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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/applications/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.