gistfile1.ml

module type S = sig
  type 'a t
  val fold : 'a t -> init:'acc -> f:('acc -> 'a -> 'acc) -> 'acc
end

module MyFolder = struct
  type 'a t = (int * 'a) list
      
  let fold (a : 'a t) ~init ~f =
    let rec aux prev = function
      | [] -> prev
      | hd :: tl -> aux (f prev hd) tl in
    aux init a
end