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From: aglew@ccvaxa.UUCP
Newsgroups: net.lang
Subject: Re: compound statements ( side note
Message-ID: <800018@ccvaxa>
Date: Fri, 18-Jul-86 17:10:00 EDT
Article-I.D.: ccvaxa.800018
Posted: Fri Jul 18 17:10:00 1986
Date-Received: Mon, 21-Jul-86 04:09:46 EDT
References: <221@bsdpkh.UUCP>
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Nf-From: ccvaxa.UUCP!aglew    Jul 18 16:10:00 1986


>> Dijkstra argues that making the null statement explicit, `skip', with
>> defined semantics, may be as important for computer science as the
>> invention of zero was for arithmetic. 
>
>Mmph.  That's either a misattribution or one of the craziest analogies I'VE
>ever heard.

Reference: _A_Discipline_Of_Programming_, Edsger W. Dijkstra, Prentice-Hall,
    1976. (I am very attached to this book, as it was one of the first books
    on Computer Science, as opposed to programming, that I ever read. I keep
    it on my bookshelf at work, and every now and again try to understand 
    it ;-)

p. 25: The semantics of the statement named `skip' are therefore given by
	    wp(skip,R) = R
    ...
    Note: Those who think it a waste of characters to introduce an explicit
    name such as `skip' for the empty statement while "nothing" expresses its
    semantics so eloquently, should realize that the decimal number system
    was only possible thanks to the introduction of the character "0" for
    the concept zero. (End of note.)

The semantics should be intuitively obvious (`wp' means weakest precondition,
so wp(skip,R)=R means that for R to be true after a skip it must have been 
true before (unless you're time based)). Are they uninteresting because they
are obvious? Maybe so: do you find zero uninteresting? Maybe you do (I
don't), but even so, is interest a prerequisite for a notation?

The point is, programming is a positional notation. S1;S2 is not the same
as S2;S1, hence it is position sensitive, and in any position sensitive
system you need a placeholder.

---

Yep. I guess with a signature like mine any statement I make is open to
doubt. But with a name like mine you take the first decent nickname that
comes along.

Andy "Krazy" Glew. Gould CSD-Urbana.    USEnet:  ihnp4!uiucdcs!ccvaxa!aglew
1101 E. University, Urbana, IL 61801    ARPAnet: aglew@gswd-vms