her.esy.fun/src/drafts/XXXX-how-i-use-nix/index.org

#+Title: How I use nix
#+Author: Yann Esposito
#+Email: yann@esposito.host
#+Date: [2020-06-14 Sun]
#+KEYWORDS: nix, programming
#+DESCRIPTION: In this article I explain how I use nix.
#+DESCRIPTION: As a brew replacement, as home environment manager,
#+DESCRIPTION: to have reproductible dev environment.
#+LANGUAGE: en
#+LANG: en
#+OPTIONS: H:5 auto-id:t toc:nil
#+STARTUP: showeverything

Have you ever written a small script and you update your system and this
stop working?
Have you copied your tool/script to another machine it doesn't work because
some dependency is missing?
Have you tried to sync your dotfiles to another env and there are a few
details not working? Some missing dependency?
If the answer is yes, then [[https://nixos.org/nix][nix]] can help.

** Scripts
:PROPERTIES:
:CUSTOM_ID: scripts
:END:

Suppose you want to write a portable script.
For example, the script I use to minify my CSS.
Here it is:

#+begin_src shell
#!/usr/bin/env nix-shell
#!nix-shell --pure
#!nix-shell -i bash
#!nix-shell -I nixpkgs="https://github.com/NixOS/nixpkgs/archive/19.09.tar.gz"
#!nix-shell -p bash minify

minify "$1" > "$2"
#+end_src

So let's analyze each line of the header block:

- ~#!/usr/bin/env nix-shell~ :: basic, use ~nix-shell~ to run the script.
- ~#!nix-shell --pure~ :: only use dependencies installed in this nix shell
  environment. A bit as if the PATH environment variable was emptied.
- ~#!nix-shell -i bash~ :: tell ~nix-shell~ to run ~bash~
- ~#!nix-shell -I nixpkgs="https://github.com/NixOS/nixpkgs/archive/19.09.tar.gz"~ ::
  pin the nixpkgs using this archive.
- ~#!nix-shell -p bash minify~ :: install ~bash~ and ~minify~ in the nix shell.

Now if the script is run on a machine with ~nix~ installed you can be
pretty sure it will work as expected.
Even if I update my OS and I forget about this script for a few years.
As long as I can install nix on the new system and I could download the tar
file the script will be run the same way as the day I wrote it.

** Temporary working env
:PROPERTIES:
:CUSTOM_ID: temporary-working-env
:END:

Quite often, I need to do something, and run a specific command that need
me to install a very specific command.
And I'm pretty sure I will not use this tool ever again.

For those cases, what I do, is generally run my command directly with a
fresh ~nix-shell~.

#+begin_src shell
> nix-shell -p httpie
[nix-shell:~]$ ... here I can use httpie ...
#+end_src

** Home Manager
:PROPERTIES:
:CUSTOM_ID: home-manager
:END:

A few years ago I used =brew= to install the tools I need.
With =nix= you can install a new tool with ~nix-env -i~ instead of ~brew install~.
Still recently I prefer to use [[https://github.com/rycee/home-manager][home-manager]].

The main advantage is that it is even more reproductible and can easily be
shared accross different machines.

Mainly when I need a new binary I add it in a description list in the file
=~/.config/nixpkgs/home.nix=.
It looks like this:

#+begin_src nix
home.packages = with pkgs; [
  # emacs
  emacsMacport
  imagemagick
  gnupg
  # shell
  direnv
  ...
];
#+end_src

then I simply run ~home-manager switch~ and I've got all those tools in my env.

*** Pinning the packages
:PROPERTIES:
:CUSTOM_ID: pinning-the-packages
:END:

#+begin_src nix
{ config, pkgs, ... }:
let
  # ...
  pkgs = import (fetchGit {
    name = "nixpkgs20";
    url = "https://github.com/NixOS/nixpkgs";
    # obtained via
    # git ls-remote https://github.com/NixOS/nixpkgs nixpkgs-20.03-darwin
    ref = "refs/heads/nixpkgs-20.03-darwin";
    rev = "58f884cd3d89f47672e649c6edfb2382d4afff6a";
  }) {};
  # ...
in {
  # ...
}
#+end_src

*** Specific tools
:PROPERTIES:
:CUSTOM_ID: specific-tools
:END:

There are a few noticiable artifact here:

The first one is ~weechat~ is a very specify build of weechat with the
plugin I need. So here is the block I use:

#+begin_src nix
let
  ...
  rel20weechat = rel20.weechat.override {
      configure = { availablePlugins, ... }: {
        # plugins = with availablePlugins; [ python perl guile ];
        scripts = with pkgs.weechatScripts; [ wee-slack ];
      };
    };
  ...
in
{
  ...
  home.packages = with pkgs; [... rel20weechat ...];
  ...
}
#+end_src

First ~rel20~ pin a nixpkgs some specific commit of nix packages.
Second, I overriden the default packages of the =weechat= package.

*** Another nice tool is =sws=
:PROPERTIES:
:CUSTOM_ID: another-nice-tool-is--sws-
:END:

I use macOS so even though I'm using a darwin focused nixpkgs
sometimes a few package can be broken and can't be installed.


That occurred with [[https://hackage.haskell.org/package/sws][=sws=]] during the upgrade to 20.03 on darwin.
This is a simple tool that need haskell to be compiled locally and
installed.
Here is how I could install it:

#+begin_src nix
let
  ...
  rel19 = import (fetchGit {
    name = "nixpkgs19";
    url = "https://github.com/NixOS/nixpkgs";
    ref = "refs/heads/nixpkgs-19.09-darwin";
    rev = "2f9bafaca90acd010cccd0e79e5f27aa7537957e";
  }) {};
  ...
in
  home.packages = with pkgs; [
    ...
    ghc
    rel19.haskellPackages.sws
    ...
  ]
#+end_src

So I used the older version from 19.09.

[fn:desktop]: I'm using macOS, and I have multiple macs.
A laptop and two desktop machines.
Using the 27″ iMac is my ultimate work station.
It really enhance my productivity.
Still the laptop is a superior work environment for more casual tasks.
I guess I might write someday about my full work environment.

** Install
:PROPERTIES:
:CUSTOM_ID: install
:END:

First, let's start by the bad news.
Recent macOS security policy made nix a bit harder to install on a mac.
See [[https://hydra.nixos.org/build/119559243/download/1/manual/#sect-macos-installation][macOS Installation instructions]].

Once you have nix installed you should update the nix-channel.
Mainly a nix-channels is where are the definitions of all the packages.
See [[https://hydra.nixos.org/build/119559243/download/1/manual/#sec-channels][nixOS documentation]].