# Re: Map is not tail recursive

From: Marc Rouaix (rouaix@my-dejanews.com)
Date: Wed Jan 13 1999 - 18:40:39 MET

To: caml-list@pauillac.inria.fr
Date: Wed, 13 Jan 1999 17:40:39 -0000
From: "Marc Rouaix" <rouaix@my-dejanews.com>
Subject: Re: Map is not tail recursive

It looks like this didn't get sent the first time. I made the suggestion of using a map function that maps chunks of the list at a time. Also, in a separate mail, I suggested using a map function that tested the length of the list and then chose what function to invoke, but I suppose that's a questionable strategy since finding the length of a list requires traversing it.

Anyway...

(* returns nth cons of a list, or [] if there is no such cons *)
let rec nth_tail ls n =
if n = 0 then ls else
match ls with
[] -> []
| x::xs -> nth_tail xs (n-1)

(* jump_map n should compute the same function as List.map for any n>0. But
jump_map n will use min(list_size, n+list_size/n) stack space whereas
List.map uses list_size stack space. jump_map costs an extra list
traversal compared to List.map. *)
let rec jump_map jump fn lst =
let rec do_a_chunk last stub chunk =
if chunk == last then stub else
(fn (List.hd chunk))::(do_a_chunk last stub (List.tl chunk))
in
if lst == [] then [] else
let last = nth_tail lst jump
in do_a_chunk last (jump_map jump fn last) lst

(* It may be a bad idea to use this because of the cost of List.length, but
you get the idea. *)
let general_map fn lst =
let n = List.length lst in
if n < 1000 then List.map fn lst
else jump_map (truncate (sqrt (float n))) fn lst

```---
Marc
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