Xmonad is a tiling window manager for X. Windows are arranged automatically to tile the screen without gaps or overlap, maximising screen use. All features of the window manager are accessible from the keyboard: a mouse is strictly optional. Xmonad is written and extensible in Haskell. Custom layout algorithms, and other extensions, may be written by the user in config files. Layouts are applied dynamically, and different layouts may be used on each workspace. A guiding principle of the user interface is predictability: users should know in advance precisely the window arrangement that will result from any action, leading to an intuitive user interface.
Xmonad provides three tiling algorithms by default: tall, wide and fullscreen. In tall or wide mode, all windows are visible and tiled to fill the plane without gaps. In fullscreen mode only the focused window is visible, filling the screen. Alternative tiling algorithms (and much more) are provided as extensions. Sets of windows are grouped together on virtual workspaces and each workspace retains its own layout. Multiple physical monitors are supported via Xinerama, allowing simultaneous display of several workspaces.
Adhering to a minimalist philosophy of doing one job, and doing it well, the entire code base remains tiny, and is written to be simple to understand and modify. By using Haskell as a configuration language arbitrarily complex extensions may be implemented by the user using a powerful `scripting’ language, without needing to modify the window manager directly. For example, users may write their own tiling algorithms, in Haskell, in their config files.
From the outset correctness and robustness have been a goal of the project. Using a combination of static and dynamic techniques, a wide range of errors are prevented and checkable, leading to better, cleaner code, and a more stable application. In particular, the expressive Haskell type system is used to prevent certain classes of bugs, while type-based automatic testing and verification tools, are used to ensure the user interface policies are correct (for example, that a layout algorithm never produces gaps or overlapping windows, or that window manager actions never produce an inconsistent internal state).