Re: Global Warming - Forth as answer

From	Bernd Paysan <bernd.paysan@gmx.de>
Newsgroups	comp.lang.forth
Subject	Re: Global Warming - Forth as answer
Date	2013-05-15 23:38 +0200
Organization	1&1 Internet AG
Message-ID	<kn0v7u$tfd$1@online.de> (permalink)
References	(3 earlier) <kn07f7$o62$1@dont-email.me> <2013May15.170815@mips.complang.tuwien.ac.at> <kn08r4$nc$1@dont-email.me> <2013May15.172242@mips.complang.tuwien.ac.at> <kn0o96$u8u$1@dont-email.me>

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rickman wrote:
> How is OVER_+ reversible exactly?  You can recover the data you started
> with using OVER -, but is that really the same as saying it is
> reversible?

Yes.  The idea of a reversible program is that you can run backwards (with 
now doing the reverse operations), and get back to the original state.

> If OVER is not reversible, that means any side effects have
> to be factored in.  OVER can be reversed with a DROP otherwise.  But
> OVER clobbers a memory location.  I suppose OVER_+ would have to be
> atomic such as ADD A,B can be reversed by SUB A,B, but I don't think
> this is really what the concept is about.

The problem here is that this sort of "concept" doesn't transform into real 
hardware.  And the real hardware needs to do reversible operations, not just 
conceptually reversible operations.  Which is why clobbering some memory or 
registers is not an option.

Some languages make fun of "if" and end it with a "fi".  In a reversible 
language, this makes perfectly sense: "fi" is a conditional branch when 
running backwards.  Whether to branch depends on what happened when running 
fowards.

-- 
Bernd Paysan
"If you want it done right, you have to do it yourself"
http://bernd-paysan.de/

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Global Warming - Forth as answer visualforth@rocketmail.com - 2013-05-14 15:21 -0700
  Re: Global Warming - Forth as answer hughaguilar96@yahoo.com - 2013-05-14 18:40 -0700
    Re: Global Warming - Forth as answer Steve <nospam275@gmail.com> - 2013-05-14 21:00 -0700
      Re: Global Warming - Forth as answer visualforth@rocketmail.com - 2013-05-15 22:24 -0700
        Re: Global Warming - Forth as answer Steve <nospam275@gmail.com> - 2013-05-19 19:25 -0700
  Re: Global Warming - Forth as answer albert@spenarnc.xs4all.nl (Albert van der Horst) - 2013-05-15 10:05 +0000
    Re: Global Warming - Forth as answer anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2013-05-15 12:42 +0000
      Re: Global Warming - Forth as answer rickman <gnuarm@gmail.com> - 2013-05-15 10:56 -0400
        Re: Global Warming - Forth as answer anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2013-05-15 15:08 +0000
          Re: Global Warming - Forth as answer rickman <gnuarm@gmail.com> - 2013-05-15 11:19 -0400
            Re: Global Warming - Forth as answer anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2013-05-15 15:22 +0000
              Re: Global Warming - Forth as answer rickman <gnuarm@gmail.com> - 2013-05-15 15:42 -0400
                Re: Global Warming - Forth as answer visualforth@rocketmail.com - 2013-05-15 13:45 -0700
                Re: Global Warming - Forth as answer Bernd Paysan <bernd.paysan@gmx.de> - 2013-05-15 23:38 +0200
                Re: Global Warming - Forth as answer anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2013-05-16 12:53 +0000
                Re: Global Warming - Forth as answer rickman <gnuarm@gmail.com> - 2013-05-16 14:03 -0400
                Re: Global Warming - Forth as answer anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2013-05-17 12:41 +0000
        Re: Global Warming - Forth as answer albert@spenarnc.xs4all.nl (Albert van der Horst) - 2013-05-17 13:17 +0000
          Re: Global Warming - Forth as answer dirk.bruehl@usa.net - 2013-05-17 08:36 -0700
            Re: Global Warming - Forth as answer Steve <nospam275@gmail.com> - 2013-05-19 19:12 -0700
            Re: Global Warming - Forth as answer Steve <nospam275@gmail.com> - 2013-05-20 00:06 -0700
    Re: Global Warming - Forth as answer rickman <gnuarm@gmail.com> - 2013-05-15 10:47 -0400
      Re: Global Warming - Forth as answer albert@spenarnc.xs4all.nl (Albert van der Horst) - 2013-05-17 13:23 +0000
        Re: Global Warming - Forth as answer rickman <gnuarm@gmail.com> - 2013-05-17 09:59 -0400
    Re: Global Warming - Forth as answer Steve <nospam275@gmail.com> - 2013-05-19 18:56 -0700
  Re: Global Warming - Forth as answer oh2aun@gmail.com - 2013-05-22 00:50 -0700

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