gistfile1.txt

scala> foo
res7: Array[String] = Array(one, two, three)

scala> foo.foldLeft(List[String]()) { (z, f) => z :+ f.toUpperCase }
res8: List[String] = List(ONE, TWO, THREE)

scala> foo.foldRight(List[String]()) { (z, f) => z :+ f.toUpperCase }
<console>:9: error: value toUpperCase is not a member of List[String]
              foo.foldRight(List[String]()) { (z, f) => z :+ f.toUpperCase }
                                                               ^

scala> foo.foldRight(List[String]()) { (f, z) => z :+ f.toUpperCase }
res10: List[String] = List(THREE, TWO, ONE)