「fold書けないプログラマー」と聞いてドキっとしたので書いて確かめて気を落ち着かせる

fold.ml

let foldl f z xs =
  let rec go acc xs =
    match xs with
    | [] -> acc
    | x :: xs -> go (f acc x) xs
  in go z xs;;

let foldr f z xs =
  let rec go xs =
    match xs with
    | [] -> z
    | (x :: rest) -> f x (go rest)
  in go xs;;