Add: Overlays concept / tutorial - introduction to overlays

[hsjobeki]

Learning goals

• Understand overlays concept

• What is the problem

• How do overlays solve it

• How to use them -> override pkgs within the fixed point

Prerequisites

• Understand a package set
• Understand a fixed point

Outline

Prior art

• https://discourse.nixos.org/t/what-are-overlays/14680
• https://wiki.nixos.org/wiki/Overlays
• https://nixcademy.com/posts/mastering-nixpkgs-overlays-techniques-and-best-practice/

Willing to help?

Yes

Priorities

Add 👍 to issues you find important.

[66-firebat]

Did the overlays section ever get added to nix.dev?

Could we include an updated version of this explanation: https://blog.layus.be/posts/2020-06-12-nix-overlays.html

[hsjobeki]

Yes, I'd be happy to review any contributions.

If you want to do it:

Structure for explaining overlays could be:

• Introduce terminology (prerequisites, keep it simple, write last)
• Start with a simple example
• Explain what it does
• Give a more complex example
• Explain what it does

Avoid:

Upfront explanations, without examples: (order: example -> explanation)

• Bad: Overlays provide a method to extend and change nixpkgs. They replace constructs like packageOverride and overridePackages ... (copied from discourse)
• Better (show examples, maybe not the best one from the wiki):

Consider this simple example
final: prev: {
   google-chrome = prev.google-chrome.override {
     commandLineArgs =
       "--proxy-server='https=127.0.0.1:3128;http=127.0.0.1:3128'";
   };
};
It sets the default proxy in Google Chrome

It seems counter-intuitive, but show the code first. Understanding builds gradually from that examples later.

[66-firebat]

Thanks @hsjobeki , will try to get to it this weekend! I also noticed that devshells were not included in the wiki, so maybe I can have a look at that as well. I really like this style of example-accompanied descriptions, and I look forward to contributing!