title: Why do Scala's index methods return -1 instead of None if the element is not found?
body: <p>I've always been wondering why in Scala the various index methods for determining the position of an element in a collection (e.g. <code>List.indexOf</code>, <code>List.indexWhere</code>) return <code>-1</code> to indicate the absence of the given element in the collection, instead of a more idiomatic <code>Option[Int]</code>. Is there some particular advantage to returning <code>-1</code> instead of <code>None</code>, or is this just for historical reasons?</p>