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Re: Map is not tail recursive
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Date: -- (:)
From: Marc Rouaix <rouaix@m...>
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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