reduceLeft

trait Collection[A] {
  def reduceLeft[B :> A](op: (B, A) => B): B
}

reduceLeft applies the binary operator op to each element, going from left to right, and the previous op result. The first time op is applied it’s fed with the two first elements.

On single-element collections that only element is returned and the binary operator op is not applied.

On empty collections this function throws an UnsupportedOperationException exception.