List operations

list_ops.ml

let rec fold ~init ~f = function
    [] -> init
  | x :: xs -> fold ~init:(f init x) ~f xs

let length xs =
  xs
  |> fold ~init:0 ~f:(fun acc _ -> acc + 1)

let reverse xs =
  xs
  |> fold ~init:[] ~f:(fun acc x -> x :: acc)

let append xs ys =
  xs
  |> reverse
  |> fold ~init:ys ~f:(fun acc x -> x :: acc)

let map ~f xs =
  xs
  |> reverse
  |> fold ~init:[] ~f:(fun acc x -> f x :: acc)

let filter ~f xs =
  xs
  |> reverse
  |> fold ~init:[] ~f:(fun acc x -> if f x then x :: acc else acc)

let concat xs =
  xs
  |> reverse
  |> fold ~init:[] ~f:(fun acc x -> append x acc)
  

list_ops.re

let rec fold ::init ::f =>
  fun
  | [] => init
  | [x, ...xs] => fold init::(f init x) ::f xs;

let length xs => 
  xs
  |> fold init::0 f::(fun acc _ => acc + 1);

let reverse xs => 
  xs 
  |> fold init::[] f::(fun acc x => [x, ...acc]);

let append xs ys =>
  xs
  |> reverse
  |> fold init::ys f::(fun acc x => [x, ...acc]);

let map ::f xs =>
  xs 
  |> reverse
  |> fold init::[] f::(fun acc x => [f x, ...acc]);

let filter ::f xs =>
  xs 
  |> reverse 
  |> fold
      init::[]
      f::(fun acc x => if (f x) { [x, ...acc] } else { acc });

let concat xs =>
  xs
  |> reverse
  |> fold init::[] f::(fun acc x => append x acc);