Recursion Review

Recursion is so dang useful with tree-like structures because we don't always know how deeply they're nested. Stop and think about how you would write nested loops to traverse a tree of arbitrary depth... it's not easy, is it?

for item in tree:
    for nested_item in item:
        for nested_nested_item in nested_item:
            for nested_nested_nested_item in nested_nested_item:
                # ... WHEN DOES IT END???

I most often use recursion on tree-like problems (file systems, nested dictionaries, etc.). If I'm just iterating over a one-dimensional list, then a loop (gasp...) is typically simpler, even if it's not as "pure" in the academic sense.

Remember: the rules of functional programming are just philosophies to help you write better code, but it's not always the right tool for the job. The same goes for any programming paradigm.