foldLeft v. foldRight - does it matter?

Question

Previously, Nicolas Rinaudo answered my question on Scala\'s List foldRight Always Using foldLeft?

Studying Haskell currently, my understanding is that foldRig

Answer 1

It depends, consider the following:

scala> val l = List(1, 2, 3)
l: List[Int] = List(1, 2, 3)

scala> l.foldLeft(List.empty[Int]) { (acc, ele) => ele :: acc }
res0: List[Int] = List(3, 2, 1)

scala> l.foldRight(List.empty[Int]) { (ele, acc) => ele :: acc }
res1: List[Int] = List(1, 2, 3)

As you can see, foldLeft traverses the list from head to the last element. foldRight on the other hand traverses it from the last element to head.

If you use folding for agregation, there should be no difference:

scala> l.foldLeft(0) { (acc, ele) => ele + acc }
res2: Int = 6

scala> l.foldRight(0) { (ele, acc) => ele + acc }
res3: Int = 6