lib: add usage docs

SUMMARY.md

@@ -8,6 +8,7 @@
   - [Cachix](./cachix/README.md)
   - [Extern](./extern/README.md)
   - [Hosts](./hosts/README.md)
+  - [Lib](./lib/README.md)
   - [Modules](./modules/README.md)
   - [Overlays](./overlays/README.md)
   - [Overrides](./overrides/README.md)

lib/README.md

@@ -0,0 +1,88 @@
+# Lib
+The lib directory mirrors the upstream concepts of [`nixpkgs:./lib`][nixpkgs-lib], 
+[`nixpkgs:./nixos/lib`][nixpkgs-nixos-lib] and [`nixpkgs:./pkgs/pkgs-lib`][nixpkgs-pkgs-lib],
+but also occasionally [`nixpkgs:./pkgs/build-support`][nixpkgs-pkgs-build-support].
+
+It comes with functions necesary to declutter `devos` itself, but you are
+welcome to extend it to your needs.
+
+For example:
+
+- you want to add a library function that depends on some packages
+and use it throughout your devos environment: place it into `./lib`
+as if you would place it into [`nixpkgs:./pkgs/pkgs-lib`][nixpkgs-pkgs-lib].
+
+- you want to add library functions that don't depend on `pkgs`: place
+them into `./lib` as if you would place them into [`nixpkgs:./lib`][nixpkgs-lib].
+
+- need to try out a newish custom build support: place it here before
+upstreaming into [`nixpkgs:./pkgs/build-support`][nixpkgs-pkgs-build-support].
+
+- you want to reutilize certain module configuration functions or helpers:
+place them into `./lib` as if you would place them into [`nixpkgs:./nixos/lib`][nixpkgs-nixos-lib].
+
+Once your library grows, we recoomend you start organizing them into subfolders
+analogous `nixpkgs`:
+
+| `nixpkgs`              | `devos`            |
+| ---------------------- | ------------------ |
+| `./lib`                | `./lib`            |
+| `./pkgs/pkgs-lib`      | `./lib/pkgs-lib`   |
+| `./nixos/lib`          | `./lib/nixos-lib`  |
+| `./pkgs/build-support` | `./lib/pkgs-build` |
+
+
+## Example
+lib/nixos-lib/mkCustomI3BindSym/default.nix:
+```nix
+{ pkgs, writers, ... }:
+{ name, cmd, workspace, baseKey }:
+let
+  isWorkspaceEmpty = writers.writePython3 "is-workspace-empty" {
+    libraries = [ pkgs.python3Packages.i3ipc ];
+  } (builtins.readFile ./is-workspace-empty.py);
+
+  ws = builtins.toString workspace;
+in
+''
+
+  # ${name}
+  #bindsym ${baseKey}+${ws} workspace ${ws}; exec ${cmd}
+  bindsym ${baseKey}+${ws} workspace ${ws}; exec bash -c "${isWorkspaceEmpty} && ${cmd}"
+''
+```
+
+lib/nixos-lib/mkCustomI3BindSym/is-workspace-empty.py:
+```python
+# returns 0/1 if current workspace is empty/non-empty
+
+import i3ipc
+
+i3 = i3ipc.Connection()
+tree = i3.get_tree()
+
+
+def current_workspace():
+    return tree.find_focused().workspace()
+
+
+if current_workspace().leaves():
+    print("Error current workspace is not empty")
+    exit(1)
+exit(0)
+```
+
+lib/default.nix:
+```nix
+{ nixos, pkgs, ... }:
+# ...
+{
+  # ...
+  mkCustomI3BindSym = pkgs.callPackage ./nixos-lib/mkCustomI3BindSym { };
+}
+```
+
+[nixpkgs-lib]: https://github.com/NixOS/nixpkgs/tree/master/lib
+[nixpkgs-pkgs-lib]: https://github.com/NixOS/nixpkgs/tree/master/pkgs/pkgs-lib
+[nixpkgs-pkgs-build-support]: https://github.com/NixOS/nixpkgs/tree/master/pkgs/build-support
+[nixpkgs-nixos-lib]: https://github.com/NixOS/nixpkgs/tree/master/nixos/lib