A new shell. Any takers? [now "if not"]

[Byron Rakitzis]

In article <1991Jan15.213533.9992 at maytag.waterloo.edu> himacdon at maytag.uwaterloo.ca (Hamish Macdonald) writes:
>>>>>> In article <andy.663963786 at xwkg>, andy at xwkg.Icom.Com (Andrew H.
>>>>>> Marrinson) writes:
>
>Andrew> 	if (command) {
>Andrew> 		if (command)
>Andrew> 			command
>Andrew> 	}
>Andrew> 	if not
>Andrew> 		command
>
>Andrew> I suspect the if not applies to the second if, not the first.
>Andrew> Naming it else would cause people to expect it to apply to the
>Andrew> first if, as an else would.  Calling it if not makes it clear
>Andrew> it is different from else: it applies to the last if command
>Andrew> run, and knows nothing about the lexical structure of the
>Andrew> script.
>
>Nope.  The "if not" applies to the first if (by experimentation).
>"rc" must stack the results of the "if"s.  I suspect it uses "if not"
>because the:
>	if not
>		command
>is a separate statement.
>

The reason is that "if not" looks at the preceeding COMMAND, at that
level of scoping. The braced "argument" of the first if never comes
into consideration. The place the if not burns you most, if you think
of it as an else, is here:

	if (command) {
		foo
	} else {
		bar
	}

This causes a syntax error in the parser, because the syntax of
the if statement is "if (cmd) cmd" and an rc command must either
end in newline, semicolon or &. Thus the "correct" way to write
the above command would be:

	if (cmd) {
		foo
	}; else {
		bar
	}

and hence:

	if (cmd) {
		foo
	}
	if not {
		bar
	}

I agree that this is all hopelessly ugly, and I'm still waiting
for an appropriate solution. The best suggestion I've heard to
date is to make an else clause mandatory in an if statement (this
is not without precedent---ML does this, though for different
reasons) and then to use && for a simple-if. Thus:

	if (foo) bar ==> foo && bar

This has its problems also, but at least makes the syntax for
if-statements acceptable.

---

	A human being should be able to change a diaper, plan an
	invasion, butcher a hog, conn a ship, design a building,
	write a sonnet, balance accounts, build a wall, set a bone,
	comfort the dying, take orders, give orders, cooperate,
	act alone, solve equations, analyze a new problem, pitch
	manure, program a computer, cook a tasty meal, fight
	efficiently, die galantly. Specialization is for insects.

		--- Robert A. Heinlein (1916-1988)

Byron Rakitzis, System Administrator
Visualization Lab, College of Architecture
Texas A&M University
College Station, TX 77843

Work: 409-845-5691
Home: 409-693-7273