Tail-recursive merge sort in OCaml

Question

I’m trying to implement a tail-recursive list-sorting function in OCaml, and I’ve come up with the following code:

let tailrec_merge_sort l =
  let split l = 
           


        

Answer 1

The expression

merge (sort left) (sort right)

is not tail-recursive; it calls both (sort left) and (sort right) recursively while there is remaining work in the call (merge).

I think you can fix it with an extra continuation parameter:

  let rec sort l k =
    match l with
    | [] -> k [] 
    | [a] -> k [a] 
    | list -> let left, right = split list in sort left (fun leftR -> sort right (fun rightR -> k (merge leftR rightR)))
  in sort l (fun x -> x)