[452] in Coldmud discussion meeting

Re: assignment, while (etc)

daemon@ATHENA.MIT.EDU (Sat Sep 24 15:45:17 1994 )

To: stewarta@netcom.com (Alex Stewart)
Cc: BRANDON@cc.usu.edu, coldstuff@MIT.EDU
In-Reply-To: Your message of "Sat, 24 Sep 1994 00:13:32 PDT."
             <199409240713.AAA07790@netcom12.netcom.com> 
Date: Sat, 24 Sep 1994 11:21:57 -0400
From: Greg Hudson <ghudson@MIT.EDU>


I'd just like to throw a few more ideas into the mix:

> However, the argument that you should use a while (1) ... if (...)
> break; ...  is in my opinion one of the stupidest things anybody
> could deliberately design into a language.

I agree that it's not the best construct for the task at hand.
Perhaps the problem here is that while() isn't really appropriate for
the job.  Consider the meaning of:

	while ((a := .test()) >= 0)
		.do_something(a);

This reads as, "while [go off and set the value of a to .test()] a
isn't negative, do something with a".  This is a rather disjointed
sentence; there are ways of reading this that make grammatical sense
(e.g. "while the value of .test(), which I'll call a, is nonegative,
do something with a"), but they don't come directly from reading the
code (when := is your generic assignment operator) and gloss over the
side-effect, which is exactly how you see people writing C expressions
a lot.

I proposed "while (1) { a := .test(); if (a < 0) break;
.do_something(a); }", which reads as "forever, set a to test, then
stop if a is less than zero, then do something with a", but most
people don't seem to like that.

If while() isn't the appropriate construct, let's try C's for() (which
I realize doesn't exist in Coldmud):

	for (a = .test(); a >= 0; a = .test())
		.do_something(a);

This reads, "start with a equal to .test().  At the end of each loop
iteration, set a equal to .test(), and loop until a is nonnegative,
doing something with a."  This makes much more sense, but you've
duplicated code in the first and third clauses of the for, and
therefore you wind up with a longer and more complicated descriptive
sentence which you would like.

So perhaps the answer is to define a third loop construct (really,
that'd not so many; we don't even have do-while yet) which is
basically a for loop with the intialization and iteration actions
equal.  We would like the descriptive sentence for this loop to be,
"At the beginning of each loop iteration, set a equal to .test(), and
loop until a is nonnegative, doing something with a."  Consider:

	where (a = .test(); a >= 0)
		.do_something(a);

The name "where" is the best I could come up with on short notice, and
of course any name for a new looping construct is going to sound a bit
funny ("for (a;b;c)" is not particularly intuitive; while (a)" is
easily made intuitive mainly because it's so simple).  This construct
takes care of all of the situations for which people want assignment
expressions in while loops.  I argue that a when loop is more
intuitive than a while loop with a side-effect in the condition.

There is onlu one other situation I can think of where assignments in
expressions makes code significantly more compact:

	if ((a := .test1()) >= 0)
		.do_something1(a);
	else if ((a := .test2()) >= 0)
		.do_something2(a);
	else
		.do_something3();

These are really a series of parallel constructions (fallbacks, if you
will), but in the most general case, you are forced to rewrite your
code as a series of nested compound statements inside if-else
statements (often, you can use return statements to get around this).
Again, this is a situation where assignment expressions lead to
compact code, but a rather disjoint English description of the code.

If we think of where() as a while() with a preparatory statement
clause, then we might try to apply the same solution here, but this
requires making up yet another name and complicating your control flow
constructs again.  Assignment expressions are another solution (with
associated defects), and ignoring the problem is also acceptable
(really, I don't use assignment expressions in C, and these things
don't bother me too much).

However, let me throw out just one more crazy idea: leave looping
alone for now and extend the expression syntax:

	(where IDENT := expr) expr

For example:

	while ((where a := .test()) a >= 0)
		.do_something(a);

Okay, I'm done for now.  A final note: "with handler" is there for
readability.  You can reclaim one of the keywords if you like, but I
think it will make the catch construct less readable for new
programmers (although "handler" alone is pretty reasonable).