mindfuck | Includes | Interpreter library

I have known about these monads for well over a decade now, and have only just recently seen a realistic application of them. It's in a bit of an unusual setting. A coworker and I are using functional reactive programming via the 'reflex' library, and are working on a library to help with building terminal-graphics applications. If you're familiar with 'reflex-dom', it's similar in nature, except that our basic monad, rather than putting subsequent widgets one after the other in the DOM, instead just stacks terminal character-cell-based "images" on top of each other, and it's up to the user to carve up the screen sensibly. We wanted to provide something a little nicer than this, which would keep track of remaining screen real-estate to some extent, and let the user place some "tiles" in rows and columns, such that a do-block basically corresponds to either a column or row of tiles on the screen.

In addition to handling the problem of layout, we also want the tiles to be able to manage keyboard focus, allowing the user to press tab to cycle through them, or shift-tab to go in reverse. It was here that the forwards-and-backwards-in-time state monad transformer became quite handy: we can have the current state in either direction be an Event (of an empty tuple). Each tile can send an event to the previous and next widgets (and receive an event from them), notifying widgets when they are receiving keyboard focus and so should stop blocking key presses from reaching their child widgets. So schematically, the tile widget looks something like:

use Number class