What's Your Go-To Haskell Trick for Beginners?

Recursion is usually a pain point—not quite how to run recursive code, but rather how to think up recursive code in the first place. I had no trouble because I found myself thinking in terms of induction proofs. Later I found that Graham Hutton, Ralf Hinze, et al. advocate the same. You don’t have the code yet, but you start the proof [template] anyway and discover a solution along the way. I now teach it at the beginning: my lecture notes.