Ratpoison, a tiling window manager for Linux, did this. The devs hit the point where they realized they were basically implementing parts of Lisp to allow more

Ratpoison, a tiling window manager for Linux, did this. The devs hit the point where they realized they were basically implementing parts of Lisp to allow more configuration. They ended up creating StumpWM in Common Lisp to be like Ratpoison if it allowed more configuration.