File (based on lists)

list_file.ml

(* Empty the list l into m, reversing it *)
let rec clear l m = match l with
    | [] -> m
    | h :: t -> clear t (h :: m)

type 'a file = {top: 'a list; back: 'a list}

let empty () = {top= []; back = []}

let add a f = match f with
    | {top = x; back = y} -> { top = x; back = a :: y }

let rec remove = function
    | { top = []; back = [] } -> empty ()
    | { top = []; back = l } -> remove { top = (clear l []); back = [] }
    | { top = h :: t; back = l } -> { top = t; back = l }

let rec head = function
    | { top = []; back = [] } -> empty ()
    | { top = []; back = h :: [] } -> h
    | { top = h :: []; back = [] } -> h
    | { top = _; back = h :: t } -> head { top = []; back = t }
    | { top = h :: t; back = [] } -> head { top = t; back = [] }