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From: Helmar Wodtke <helmwo@gmail.com>
Newsgroups: comp.lang.forth
Subject: Re: DO... LOOP question
Date: Tue, 27 Mar 2012 09:19:21 -0700 (PDT)
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Am Dienstag, 27. M=E4rz 2012 17:33:43 UTC+2 schrieb Coos Haak:
> Op Tue, 27 Mar 2012 04:34:07 -0700 (PDT) schreef Helmar Wodtke:
>=20
> > Am Dienstag, 27. M=E4rz 2012 13:21:43 UTC+2 schrieb Albert van der Hors=
t:
> >=20
> >> ... something about WHILE ...
> >=20
> > In this case the Forth has to support multiple WHILEs for one loop.
> > Difference at the moment between using WHILE and LEAVE is that WHILE ca=
n be there one time and LEAVE can be there multiple times.
> >=20
> Nope, you can use WHILE multiple times. But you have to add UNLOOP in eve=
ry
> ELSE part:
>=20
> .. DO .. WHILE .. WHILE .. LOOP .. ELSE UNLOOP THEN ELSE UNLOOP THEN ..
>=20
> But this is much simpler, and according to the standard.
> .. DO .. IF LEAVE THEN .. IF LEAVE THEN .. LOOP
>=20
> Or with an other word, ?LEAVE
> : ?LEAVE POSTPONE IF POSTPONE LEAVE POSTPONE THEN ; IMMEDIATE
> .. DO .. ?LEAVE .. ?LEAVE .. LOOP
>=20
> --=20
> Coos
>=20
> CHForth, 16 bit DOS applications
> http://home.hccnet.nl/j.j.haak/forth.html

Oh, what a messy thing.
My non-ANS-part works in a way where WHILE is possible multiple times.

It all looks simply like this:

  purpose: implement loop constructs
   status: mature

: do`      (loop-push)` begin` ;
: (?do)    2dup` (loop-push)` <>` ;
: ?do`     (?do) if` begin` (while) ;
: loop`    (loop)` repeat` unloop` ;
: +loop`   (+loop)` repeat` unloop` ;
: leave`   ahead` (while) ;
: until`   0=3D` while` repeat` ;
: while:`  begin` ?dup` while` ;
: for`     while:` 1-` >r` ;
: next`    r>` repeat` ;
: recurse` body compile ;

You see some (xxx)-words, that implement some "primitves" - "while" is basi=
cally a primitive too.
The ANS layer implements it inside this block:

 {~ Fix
    256 buffer pad
    : begin`   seal there ;
    : while`   if` swap ;
    : while:`  begin` ?dup` while` ;
    : until`   0=3D` while`
      : repeat` back then` ;
    : ans-:    smudge ::` current-xt ! is-immediate off
               parse-name current-name place ;
    : ans-;`   seal ;` smudge ;
    : ans-immediate current-xt @ if is-immediate on ;then immediate ;
    : exit`    seal ;;` ;
    : ans-recurse` current-xt @ ?dup if compile ;then recurse` ;
    : key      (key) if c@ ;then EOF ;
    : execute  321 <private-state exec private-state> ;
    /pad buffer pad
  ~}

Main problem with ANS was, that it does not allow the WHILE to be multiple =
times - it's not very good, what it takes as implementation predictions the=
re... Otherwise WHILE could be doubled without problem.
You see that WHILE is implemented in ANS layer basically as:

: WHILE POSTPONE IF SWAP ; IMMEDIATE

(well, "POSTPONE" is not the right word in all cases...)
About that "SWAP" you can think of a long time, but it's on "compilation st=
ack"... All that CS-things.

Regards,
-Helmar