VBScript Quiz Answers, Parts Nine and Ten

Eric Lippert

30 Mar 2005 10:25 AM

Comments 7

9) Which of these statements is syntactically legal? Why?

(a) Const Z = -+-+-+-10
(b) Const Y = 2 + 2
(c) Const X = .1e310
(d) Dim W(+10)

(a) is legal. The rest are illegal.

(a) is legal because you can put as many unary positive or negative operators on top of a constant as you want, so long as it's a numeric constant. (b) is illegal; as I've discussed before, we do not support "constant folding" in VBScript. (c) is illegal because it's outside the range of a float. (d) is illegal because, bizarrely enough, dimension lists must be just straight integers, no operators whatsoever.

Clearly VBScript is inconsistent insofar as how unary operators and constants work together. These inconsistencies are one those unfortunate corner cases that got missed when the parser was designed.

10) Which of these statements is syntactically legal? Why?

(a) If Blah Then Foo Else Bar
(b) If Blah Then If Foo Then Bar
(c) If Blah Then Foo Bar Else Bar Foo
(d) If Blah Then Foo Bar End If

(a) is illegal, the rest are legal.

The VBScript single-line If-statement parser is all screwed up. Again, some of these were bugs in the first version that we couldn't fix without breaking compatibility, and some are just plain bugs that never got fixed for no particular reason.

(b) is perfectly legal, nothing odd here.

(a) is legal in VB6 but not in VBScript because of a mistake. The parser sees IF ID THEN ID ELSE... and tries to parse "ID ELSE" as a statement. The statement processor (correctly) thinks that the id is a procedure call and it tries to parse the argument list. The argument list processor should detect the Else and figure oh, I'm in an If and the statement is done, but it doesn't figure that out, it just creates a syntax error. (c) is legal because in this case the argument list parser finds an argument list before the Else and doesn't freak out.

A number of people pointed out that having both Foo Bar and Bar Foo in the same program is a little weird. Yes, it is! But the question was what is syntactically legal, not what makes sense. And in fact, it is possible for this to work:

Class ABC
Public Default Sub DEF(X)
Msgbox X.Name
End Sub
Public Name
End Class
Set Foo = New ABC
Foo.Name = "Foo"
Set Bar = New ABC
Bar.Name = "Bar"
Blah = True
If Blah Then Foo Bar Else Bar Foo

(d) is another mistake. The VB6 parser does not allow this, but VBScript does. As with #8, we couldn't fix it when we found it because it would have broken existing ASP page and web pages. The End If is ignored.

Funny story: I actually fixed this one for a beta release of the engines and a certain influential news site was broken by the fix. Bizarrely enough they had a web page with this syntax in it. They said they would give the new version of IE a bad review if upgrading broke even a single one of their hundreds of thousands of pages! Backwards compatibility is very, very important in the scripting world. Of course we rolled the change back, and there's a comment in the code now cautioning future maintenance programmers to never change it.

VBScript, Scripting, Puzzles

Comments

Rob

30 Mar 2005 12:24 PM

Here's one I've struggled with several times:
How can I include a line break character in a constant?

Const blah = "hello" & vbCRLF
-Nope.
Const blah = "hello
"
-Nope.

I've also tried various hex/ascii code angles, but without luck. JScript just uses escaped switches in the string, but I know no such trick for VBScript.

Is it possible?
Eric Lippert

30 Mar 2005 12:26 PM

Sorry, you are correct -- there's no way to do that. Sub-optimal, I know.
Nicholas Allen

30 Mar 2005 12:37 PM

Of all of the quirks that were demonstrated in this quiz, the ones with conditional expressions caused me the most grief in figuring out what was going on and thinking about the implications for real-world programs.

I ended up writing a lot of test programs and often had _no idea_ how the parser was going to react. It really seemed to defy intuition about what a program should do in many relatively simple cases.
mike

31 Mar 2005 7:20 AM

>But the question was what is syntactically
>legal, not what makes sense.

Said like a true Chomsky (linguistic) acolyte, witness the (in)famously syntactically correct sentence "Colorless green ideas sleep furiously."

:-)
Robert

31 Mar 2005 7:22 AM

"They said they would give the new version of IE a bad review"

Who would care? How many people who don't just settle for the browser they are given would be dissuaded from upgrading by a poor newspaper review?

I know the issue of backwards compatibility is wider than this but still.......
Eric Lippert

31 Mar 2005 8:36 AM

> Who would care?

You are correct -- the "home user" market segment is irrelevant with respect to this issue.

However, IT managers of huge corporations would care, particularly because VBScript has much larger usage on intranets than the Internet. Rolling out new software across a large enterprise can be VERY expensive if it is not fully backwards compatible.

And that's one of the functions of the industry press. They did their job -- they found a problem that would be a barrier to upgrading. If we hadn't fixed it, they _should_ have told the IT managers of the world that it was not a 100% backwards compatible release. I don't mean to imply that the news site in question was wrong -- rather, the script team was wrong, twice. We wrote the code wrong in the first place, and then I tried to make a breaking change. That was definately a big learning experience for me.
Rob Eberhardt

27 May 2005 12:51 AM

Fabulous Adventures In Coding

VBScript Quiz Answers, Parts Nine and Ten

VBScript Quiz Answers, Parts Nine and Ten