I'm currently doing a Functional Programming course and I'm quite amused by the concept of higher-order functions and functions as first class citizens. However, I can't yet think of many practically useful, conceptually amazing, or just plain interesting higher-order functions. (Besides the typical and rather dull map , filter , etc functions).

Do you know examples of such interesting functions?

Maybe functions that return functions, functions that return lists of functions (?), etc.

I'd appreciate examples in Haskell, which is the language I'm currently learning :)