[dbaupp]

You are getting the join from itertools. There's no join on iterators in the standard library, only on &[T] (which is why the collect is needed).

A quick search of itertools docs indicates there's no join method in it that takes two parameters, only a method that takes a &str like the slice one in std, and a freestanding version of it that takes the iterator as the first argument (instead of as the method's self) and the &str as the second.

Based on reading that documentation, I think steve's original comment was meant to say "second collect", not "second map".