Stumpwm is a window manager written entirely in Common Lisp. It attempts to be highly customizable while relying entirely on the keyboard for input. You will not find buttons, icons, title bars, tool bars, or any of the other conventional GUI widgets.
These design decisions reflect the growing popularity of productive, customizable lisp based systems.
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Pros
Pro Integration with Common Lisp Packages
StumpWM is really just a Common Lisp core that also happens to run your WM. You can execute any Common Lisp code you want through the REPL, related to or unrelated to your actual WM. You can tie together your workflow with any of the complex Lisp Packages or utilities. It's really just all the power of Lisp in a WM.
Pro You can tweak it without restarting
As StumpWM is a Common Lisp app, you can have it run arbitrary lisp code at runtime. This means you can tweak your configuration and see the effects immediately.