[Haskell-cafe] "Prompt" Monad

I just came across last year's thread about Ryan Ingram's "Prompt" monad ( http://www.mail-archive.com/haskell-cafe@haskell.org/msg33040.html ) and wondered if it might be useful for debugging and program analysis purposes. In particular, I thought about enforcing program decisions interactively. Consider for example modifying the heuristic function of a search algorithm at specific breakpoints. Is this possible with the "Prompt" monad? Furthermore, could somebody post the full code of it (and maybe a short example how to use it if at hand)? I was not able to run the code from the previous posts and the referring links are dead. Thanks a lot, Martin