I know that to be Traversable, you need only have a foreach method. Iterable requires an iterator method.

Both the Scala 2.8 collections SID and the "Fighting Bitrot with Types" paper are basically silent on the subject of why Traversable was added. The SID only says "David McIver... proposed Traversable as a generalization of Iterable."

I have vaguely gathered from discussions on IRC that it has to do with reclaiming resources when traversal of a collection terminates?

The following is probably related to my question. There are some odd-looking function definitions in TraversableLike.scala, for example:

def isEmpty: Boolean = {
  var result = true
  breakable {
    for (x <- this) {
      result = false
      break
    }
  }
  result
}

I assume there's a good reason that wasn't just written as:

def isEmpty: Boolean = {
  for (x <- this)
    return false
  true
}