Sounds scary but it's giving you some compile-time guarantees about correctness.

StumpWM and XMonad do the same and they are quite easy to use, especially the former.

They also lead to very space-efficient setups. Windows can be tightly packed.

StumpWM is in a special position here as "compile-time" on Common Lisp is roughly the same thing as "runtime".

Sure, my statement referred to dwm, and it also applies to XMonad.

The only compile time guarantee you'd have by making changes is that it would run, not it being correct nor functional.

Static types do provide some guarantees as they rule out an entire class of runtime errors. In case of XMonad, since Haskell's type system is more expressive, the class of runtime errors ruled out at compile-time is broader.