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From: Luc Maranget <luc.maranget@inria.fr>
Message-Id: <200306121443.QAA0000013156@beaune.inria.fr>
Subject: Re: [Caml-list] How to find out if a function is tail recursive?
To: steck@stecksoft.com
Date: Thu, 12 Jun 2003 16:43:12 +0200 (MET DST)
Cc: rich@annexia.org ('Richard Jones'), caml-list@inria.fr
In-Reply-To: <000201c330ef$473ea270$0264a8c0@blimpy> from "Paul Steckler" at jun 12, 2003 10:31:14 
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X-Spam: no; 0.00; caml-list:01 printf:01 tail-rec:01 --luc:01 rec:01 maranget:02 recursive:03 tail:03 argument:03 cons:03 let:04 luc:05 function:09 isn't:11 operator:12 
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> 
> > let rec range a b =
> >   if a > b then []
> >   else a :: range (a+1) b
> >   ;;
> > 
> > let list = range 1 1000000;;
> > 
> > Printf.printf "length = %d\n" (List.length list);;
> > 
> > Can you tell me why this function isn't tail recursive, and 
> > share any useful tips on how to tell whether a function is or 
> > is not tail recursive?
> 
> The recursive call to range is not the result for its 
> caller.  Instead, the result of recursive call is 
> an argument to the cons (the :: operator).
> 
> -- Paul

Hence, here is a tail-recursive range function.

let rec do_range a b r =
  if a > b then r
  else do_range a (b-1) (b::r) (* tail-rec call *)
  ;;

let range a b = do_range a b []
;;

--Luc

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