Difference between fold and foldLeft or foldRight?

Question

NOTE: I am on Scala 2.8—can that be a problem?

Why can\'t I use the fold function the same way as foldLeft or foldRight?

Answer 1

fold, contrary to foldRight and foldLeft, does not offer any guarantee about the order in which the elements of the collection will be processed. You'll probably want to use fold, with its more constrained signature, with parallel collections, where the lack of guaranteed processing order helps the parallel collection implements folding in a parallel way. The reason for changing the signature is similar: with the additional constraints, it's easier to make a parallel fold.