https://github.com/NixOS/nixpkgs/blob/master/lib/modules.nix
    mkIf, mkMerge
https://nixos.org/manual/nix/stable/#ssec-builtins
    
currentSystem
    "x86_64-linux"

isInt
isAttrs
    is attribute set
attrNames <attr-set>
    get attribute names
    (a list of keys of a given set)

<derivation>.drvAttrs

intersectAttrs

toString

reflection
    functionArgs
        nix-repl> add = { a ? 3, b }: a+b
        nix-repl> builtins.functionArgs add
            { a = true; b = false; }

map
filter

trace

toPath
    deprecated, use toString with path-param instead
pathExists
    https://github.com/NixOS/nix/blob/master/tests/lang/eval-okay-pathexists.nix
readFile
    zsh = { ... initExtraBeforeCompInit = builtins.readFile ../../configs/zsh/fedora_zshrc.zsh; }
    home.file.".p10k.zsh".text = builtins.readFile "/etc/nixos/dot.p10k.zsh";
    home.file.".aws/config".source = ../configs/aws/aws_config;
    home.file.".dircolors".source = sources.LS_COLORS.outPath + "/LS_COLORS";

getEnv
    getEnv "HOME"; # home directory