Comment by mumblemumble

It can be, though there are some interesting philosophical issues there.

The example that I always keep coming back to is Smalltalk, which is the only language I know of that represents pure object-oriented programming. Similar to how, for the longest time, Haskell was more-or-less the only purely functional language. Anyway, in Smalltalk you generally would not do that. You'd tell the object to iterate its own children, and give it some block (Smalltalk equivalent of an anonymous function) that tells it what to do with them.

Looping over a data structure from the outside is, if you want to get really zealous about it, is more of a procedural and/or functional way of doing things.