So, how about reversing a list using our leftee function from our previous post?

Let’s work it out now. Here’s the original tail recursive function for your reference:

let tr_lst_rev lst =
  let rec aux_lst_rev lst acc =
    match lst with
    | [] -> acc
    | h::t -> aux_lst_rev t (h::acc)
  in
  aux_lst_rev lst [];;

All you need to observe is how the accumulator acc changes - here we take a list element we are currently processing and stick it before (at the head) of the current accumulator acc.

let tr_lst_rev lst = leftee (fun a x -> x::a) [] lst;;

In most functional programming languages our leftee is actually called fold_left and is tail recursive. There’s also a fold_right which is not tail recursive, because of the way it applies the function to the list elements. More on this in a later post!