Debugging red-black trees
Here’s a nice example which shows that GUIs can be made quickly enough that they can be used for “one off” or “throw away” programs. Christophe Poucet wrote this little GUI to help him debug a C++ implementation of a red-black tree:
After countless hours of hacking an RB tree in C++, and getting frustrated, I finally coded up a haskell program to check the consistency. With the help of this I was easily able to find the final bug which I doubt I would’ve ever found without the visual view of the tree, allowing me to detect exactly what case was failing.