xmonad is a tiling window manager for the X Window System, written in the functional programming language Haskell. Its developers write:

"xmonad is a tiling window manager for the X Window system, implemented, configured and dynamically extensible in Haskell. This demonstration presents the case that software dominated by side effects can be developed with the precision and efficiency we expect from Haskell by utilising purely functional data structures, an expressive type system, extended static checking and property-based testing. In addition, we describe the use of Haskell as an application configuration and extension language."

It makes use of Haskell features and tools such as: QuickCheck, GHC extensions like pattern guards, monads, monad transformers, zippers, and the Cabal packaging system, in addition to Haskell bindings to Xlib and xft fonts (with rebinding to XCB planned when the Haskell bindings are released).