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From: aglew@ccvaxa.UUCP
Newsgroups: net.lang
Subject: Re: and if you put this in your languag
Message-ID: <800009@ccvaxa>
Date: Thu, 13-Mar-86 21:54:00 EST
Article-I.D.: ccvaxa.800009
Posted: Thu Mar 13 21:54:00 1986
Date-Received: Sat, 15-Mar-86 19:41:41 EST
References: <1187@mmintl.UUCP>
Lines: 68
Nf-ID: #R:mmintl.UUCP:1187:ccvaxa:800009:000:1852
Nf-From: ccvaxa.UUCP!aglew    Mar 13 20:54:00 1986


Why add `and-if'? Use <shudder> gotos:

if A then 
	X;
	if B then Y
	     else goto AB
     else
	goto AB

     AB: begin
		Z
	 end.

Have I shocked you all? Will I have enough flames to break up the Ottawa
River early? (I'm going home this weekend, hurrah!) Are you all going to tell
be the above code is wrong? Hold on: this is Donald E. Knuth's example.

In the uproar that followed Dijkstra's "Goto considered dangerous" letter,
Knuth came out with a bit of reason. He suggested that gotos only be used
where a bit of work (X above) has made one case into another. He also 
suggested that it be illegal to fall through to a label - that the only 
way to reach a label is via a goto. Finally, something I'm not sure whether
it's my idea, or Knuth's, or somebody else's, there is the concept of an
ASIDE. An ASIDE is a labelled block of code that can only be entered via
a goto to the label at the top of the aside (and can only be exited in one of
the normal ways to exit a block). Code that would seemm to fall through to
an aside branches around it. So above you really have
	goto END
	AB: begin Z; goto END end
	END: ...
The aside has saved you a goto and a label - it makes it seem just a bit less
like spaghetti.

Anyway, you can obviously do

if A then 
	X
	if B then Y
	     else Z
     else Z

Although it reads more easily if you do

if !A then Z
      else if B then Y
		else Z

but if Z is not simply a function call then you have to make sure that two
copies of code get updated. That's what macros are for

if ( !A ) {
	#define Z zzzzzzzzzzzzzzz
	Z;
}
else {
	if ( B ) 
		Y;
	else
		Z;
}

A big #define in line looks ugly, and is really just like an aside, except
that it hides a bit of the concept of doing work to get simpler cases.

Andy "Krazy" Glew. Gould CSD-Urbana. 
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