Xmonad is written according to this pattern Most of the code (Layout.hs and StackSet.hs) is pure and has extensive unit (property-based) tests for all the pure

Xmonad is written according to this pattern Most of the code (Layout.hs and StackSet.hs) is pure and has extensive unit (property-based) tests for all the pure functional core. https://github.com/xmonad/xmonad/tree/master/tests/Properties https://github.com/xmonad/xmonad/tree/master/tests/Propertie... Then there is a layer that talks to X11 and interfaces with the functional core. (Core.hs and Main.hs) https://github.com/xmonad/xmonad https://github.com/xmonad/xmonad