From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Original-To: caml-list@yquem.inria.fr Delivered-To: caml-list@yquem.inria.fr Received: from nez-perce.inria.fr (nez-perce.inria.fr [192.93.2.78]) by yquem.inria.fr (Postfix) with ESMTP id 34048BB81 for ; Fri, 11 Nov 2005 21:18:11 +0100 (CET) Received: from pauillac.inria.fr (pauillac.inria.fr [128.93.11.35]) by nez-perce.inria.fr (8.13.0/8.13.0) with ESMTP id jABKIA0h008403 for ; Fri, 11 Nov 2005 21:18:10 +0100 Received: from nez-perce.inria.fr (nez-perce.inria.fr [192.93.2.78]) by pauillac.inria.fr (8.7.6/8.7.3) with ESMTP id VAA31900 for ; Fri, 11 Nov 2005 21:18:10 +0100 (MET) Received: from mail.physik.uni-muenchen.de (mail.physik.uni-muenchen.de [192.54.42.129]) by nez-perce.inria.fr (8.13.0/8.13.0) with ESMTP id jABKI9wE008400 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=FAIL) for ; Fri, 11 Nov 2005 21:18:10 +0100 Received: from localhost (unknown [127.0.0.1]) by mail.physik.uni-muenchen.de (Postfix) with ESMTP id 5EB102001B for ; Fri, 11 Nov 2005 21:18:09 +0100 (CET) Received: from mail.physik.uni-muenchen.de ([127.0.0.1]) by localhost (mail.physik.uni-muenchen.de [127.0.0.1]) (amavisd-new, port 10024) with LMTP id 17336-01-18 for ; Fri, 11 Nov 2005 21:18:09 +0100 (CET) Received: from mailhost.cip.physik.uni-muenchen.de (kaiser.cip.physik.uni-muenchen.de [141.84.136.1]) by mail.physik.uni-muenchen.de (Postfix) with ESMTP id 468D620007 for ; Fri, 11 Nov 2005 21:18:09 +0100 (CET) Received: from eiger.cip.physik.uni-muenchen.de (eiger.cip.physik.uni-muenchen.de [141.84.136.54]) by mailhost.cip.physik.uni-muenchen.de (Postfix) with ESMTP id 9CF7626E89 for ; Fri, 11 Nov 2005 21:18:08 +0100 (CET) Received: by eiger.cip.physik.uni-muenchen.de (Postfix, from userid 3092) id 3D7A334FFF; Fri, 11 Nov 2005 21:18:08 +0100 (CET) Received: from localhost (localhost [127.0.0.1]) by eiger.cip.physik.uni-muenchen.de (Postfix) with ESMTP id 349F72258E for ; Fri, 11 Nov 2005 21:18:08 +0100 (CET) Date: Fri, 11 Nov 2005 21:18:08 +0100 (CET) From: Thomas Fischbacher To: caml-list@inria.fr Subject: What on earth is this? Message-ID: X-BOFH: Daemons did it MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Virus-Scanned: amavisd-new at physik.uni-muenchen.de X-Miltered: at nez-perce with ID 4374FC82.000 by Joe's j-chkmail (http://j-chkmail.ensmp.fr)! X-Miltered: at nez-perce with ID 4374FC81.001 by Joe's j-chkmail (http://j-chkmail.ensmp.fr)! X-Spam: no; 0.00; beginner's:01 memoizing:01 stdout:01 gettimeofday:01 gettimeofday:01 printf:01 fprintf:01 rec:01 memoized:01 hashtbl:01 hashtbl:01 rec:01 memoized:01 right-hand:01 rec':01 X-Spam-Checker-Version: SpamAssassin 3.0.3 (2005-04-27) on yquem.inria.fr X-Spam-Level: X-Spam-Status: No, score=0.0 required=5.0 tests=none autolearn=disabled version=3.0.3 Maybe I just need a good night of sleep, maybe I overlooked something really dumb and rather should have asked on the beginner's list. Anyway, I find the behaviour demonstrated below quite confusing. Why don't the definitions ###_v1 and ###_v2 work? Any idea, anyone? ===> (* (C) T. Fischbacher, 2005 - Demonstrating a simple form of memoizing... *) #load "unix.cma";; let timing ?(tagfun= fun _ -> "Time passed: ") ?(channel=stdout) f x = let tag = tagfun x in let t0 = Unix.gettimeofday () in let result = f x in let t1 = Unix.gettimeofday () in let () = Printf.fprintf channel "%s%f sec\n%!" tag (t1-.t0) in result ;; let rec fibonacci n = if n < 2 then 1 else fibonacci (n-1) + fibonacci (n-2) ;; (* # timing fibonacci 35;; Time passed: 4.254157 sec - : int = 14930352 *) let memoized ht f arg = try Hashtbl.find ht arg with | Not_found -> let result = f arg in let () = Hashtbl.add ht arg result in result ;; let rec mem_fibonacci_v1 = let mem = Hashtbl.create 10 in memoized mem (fun n -> if n < 2 then 1 else mem_fibonacci_v1 (n-1) + mem_fibonacci_v1 (n-2)) ;; (* Error: "This kind of expression is not allowed as right-hand side of `let rec'" *) let mem_fibonacci_v2 = let mem = Hashtbl.create 10 in let rec mf = memoized mem (fun n -> if n < 2 then 1 else (mf (n-1)) + (mf (n-2))) in mf ;; (* Error: "This kind of expression is not allowed as right-hand side of `let rec'" *) let mem_fibonacci_v3 = let mem = Hashtbl.create 10 in let mf yfp = memoized mem (fun n -> if n < 2 then 1 else yfp () (n-1) + yfp () (n-2)) in let rec yc x = x (fun () -> (yc x)) in yc mf ;; (* This definition worked. And indeed: # timing mem_fibonacci_v3 35;; Time passed: 0.000149 sec - : int = 14930352 # timing mem_fibonacci_v3 35;; Time passed: 0.000005 sec - : int = 14930352 # *) <=== -- regards, tf@cip.physik.uni-muenchen.de (o_ Thomas Fischbacher - http://www.cip.physik.uni-muenchen.de/~tf //\ (lambda (n) ((lambda (p q r) (p p q r)) (lambda (g x y) V_/_ (if (= x 0) y (g g (- x 1) (* x y)))) n 1)) (Debian GNU)