Tail-recursive merge sort in OCaml

Posted by CFP on Stack Overflow See other posts from Stack Overflow or by CFP
Published on 2010-03-27T14:17:13Z Indexed on 2010/03/27 14:23 UTC
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Hello world!

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 = 
    let rec _split source left right =
      match source with
        | [] -> (left, right)
        | head :: tail -> _split tail right (head :: left) 
    in _split l [] []

  let merge l1 l2 = 
    let rec _merge l1 l2 result =
      match l1, l2 with
        | [], [] -> result
        | [], h :: t | h :: t, [] -> _merge [] t (h :: result)
        | h1 :: t1, h2 :: t2 ->
            if h1 < h2 then _merge t1 l2 (h1 :: result)
            else            _merge l1 t2 (h2 :: result)
    in List.rev (_merge l1 l2 [])

  let rec sort = function
    | [] -> []
    | [a] -> [a]
    | list -> let left, right = split list in merge (sort left) (sort right)
  in sort l

Yet it seems that it is not actually tail-recursive, since I encounter a "Stack overflow during evaluation (looping recursion?)" error.

Could you please help me spot the non tail-recursive call in this code? I've searched quite a lot, without finding it. Cout it be the let binding in the sort function?

Thanks a lot, CFP.

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