"Printf-style debugging" is possible with Haskell thanks to Debug.Trace, as long as the value you're interested in has a Show instance. Using tracing, we'll start to explore a method of collecting dependency/prerequisite information from .do scripts through the use of a redo-ifchange program. Along the way we'll also look at filter (a List function) and Maps (provided by the collections package). Follow along on GitHub at https://github.com/jekor/redo Notices and updates on Twitter at @jekor https://twitter.com/jekor All Haskell from Scratch videos:    • Haskell from Scratch