XMonad is a tiling window manager that can be configured entirely in Haskell.
Over at FP Zulip, when I floated the idea of writing Haskell DSL for i3 configuration, someone suggested to just use XMonad instead. XMonad is a window manager where your configuration file is just Haskell code.
The second step of the #Make Tech Simple Again project was to switch from i3 to XMonad. Why? Because this opens me up to write complex workflows in Haskell with its type-safety benefits.