Path: csiph.com!news.mixmin.net!aioe.org!.POSTED!not-for-mail
From: Rod Pemberton <NoHaveNotOne@bcczxcfre.cmm>
Newsgroups: comp.lang.forth
Subject: Re: BEGIN WHILE UNTIL THEN
Date: Thu, 7 Jan 2016 05:13:10 -0500
Organization: Aioe.org NNTP Server
Lines: 71
Message-ID: <20160107051310.67828226@_>
References: <08c4616c-21ad-4bdf-9fe4-a9e5ee2e5dad@googlegroups.com>
NNTP-Posting-Host: n4wpt9zq8xR26Ttf9mo2BA.user.gioia.aioe.org
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
X-Complaints-To: abuse@aioe.org
X-Notice: Filtered by postfilter v. 0.8.2
X-Newsreader: Claws Mail 3.13.1 (GTK+ 2.24.10; x86_64-unknown-linux-gnu)
Xref: csiph.com comp.lang.forth:44658

On Mon, 4 Jan 2016 14:50:25 -0800 (PST)
Julian Fondren <julian.fondren@gmail.com> wrote:

> The following code is from SwoopPortable.f
> 
>   : UPDATE-LINKS ( super link -- )   >R
>      BEGIN
>         DUP @ DUP WHILE                \ prev cur
>         R@ BEGIN                          \ prev cur sup
>            @ DUP WHILE                 \ prev cur sup'
>            2DUP = UNTIL
>            2DROP  R> @ SWAP !  EXIT
>         THEN DROP NIP
>      REPEAT R> DROP  2DROP ;
> 
> Can someone rewrite this with normal control-flow?
> 

Reformatting, ...

: UPDATE-LINKS ( super link -- )
  >R
  BEGIN
    DUP @ DUP
  WHILE                \ prev cur
    R@
    BEGIN              \ prev cur sup
      @ DUP
    WHILE              \ prev cur sup'
      2DUP =
    UNTIL
      2DROP R> @ SWAP !
      EXIT
    THEN
      DROP NIP
  REPEAT
  R> DROP 2DROP
;

I'm not sure I understand fully how this
BEGIN-WHILE-UNTIL-THEN thing works ...

My guess is that a false condition (NULL pointer
terminating a linked-list) at the WHILE in the inner
loop continues at the THEN's "DROP NIP" for end-of-list.
Otherwise, if not end-of-list, then BEGIN-WHILE-UNTIL
cycles until a pointer match is found.  When found,
the code after the UNTIL is executed for the true match.

If so, then the inner BEGIN-WHILE-UNTIL-THEN loop
is really a BEGIN-UNTIL followed by an IF-ELSE-THEN,
where the BEGIN-UNTIL is checking for two conditions
to exit.  Since the BEGIN-UNTIL now exits for two
conditions, you must determine which condition caused
it to exit, afterwards.  E.g., the BEGIN-WHILE-UNTIL-THEN
is something to the effect of this BEGIN-UNTIL with
two exit conditions, which is not tested, and is
possibly incorrect for the sample code above ...

BEGIN
  @ 2DUP = OVER 0= OR  \ OR two exit conditions
UNTIL
IF
  DROP ... EXIT   \ DROP one since IF took one ...
ELSE
  DROP NIP
THEN


Rod Pemberton