Comment by eru

What's a plain old data structure?

Linked lists are recursive, but you can use loops just fine to deal with them.

Similarly, it depends on what you do with your trees. If you want to go down a single path (eg to find an element in a search tree), loops aren't too bad.

And arrays don't necessarily ask for loops. Eg implementing quicksort or heap sort with loops is possible (and depending on your language, compiler and target machine might have performance benefits), but I'd hardly call it ergonomical.

Despite all my criticism you are hinting at a good point though: it's often useful to define your data structures together with combinators that work on them. So eg you want to define various tree traversals together with your tree. (Most people are familiar with the combinators 'map' and 'filter' for lists but that's a very small pool.)

Whether those combinators are implemented with recursion or loops is an implementation detail that the user of the combinator doesn't need to worry about too much.