I once saw a Clojure implementation of red-black trees that had an inline, visual diagram of each tree pattern to match on, and how to mutate the graph (at one

I once saw a Clojure implementation of red-black trees that had an inline, visual diagram of each tree pattern to match on, and how to mutate the graph (at one of the Strange Loop conferences.) extremely slick, since code was generated from the diagrams (or by, can't recall which.) that's an extremely niche case, certainly not enough to justify including visualizer widgets in an IDE, but here's another: form builders. they've fallen out of favor recently, but UI layouts are extremely visual and lend themselves well to visual programming. DrRacket is a neat little playground for experimenting with this kind of thing.