Mike Schaeffer's Blog

The other day, I had the following (abbreviated) dialog with my little Scheme interpreter:

scheme> (intern! 'xyzzy2 (find-package "keyword"))
; Fatal Error: Assertation Failed: STRINGP(pname) @ (oblist.cpp:451)
c:\vcalc>vcsh.exe

scheme> (intern! 12)
; Fatal Error: Assertation Failed: STRINGP(sym_name) @ (oblist.cpp:269)
c:\vcalc>

Needless to say, 'Fatal errors' aren't good things, and fatal errors in intern!, a core function, are even worse. Without going into too many details, the first call should be returning successfully, and the second should be throwing a runtime type check error. However, the implementation of intern! wasn't checking argument types and passing invalid arguments into lower layers of the interpreter's oblist (symbol table) code, which died with an assertation failure.

To put this in perspective, my implentation of intern! is about five years old, and something that I thought to be a fairly reliable piece of code. At the very least, I didn't think it was susceptable to something as simple as a type checking error that would crash the entire interpreter. Of course, when I looked at my test suite, there wasn't a set of tests for intern!. That might have something to do with it, don't you think?

Here are the morals I'm taking from this little story:

• Do not assume something works, unless you have a complete test suite for it. (Even then be wary, because your test suite is probably not complete.)
• Shoot for more than 60% code coverage on your test cases.
• Don't write your own interpreter, because there are probably hundreds of other bugs just like this one. :-)

I don't usually write posts for the sole purpose of linking to other posts, but this is an exception. This is brilliant. What it is is the USDA's Food Pyramid but adapted to how programmers should spend their time. My one complaint is that it's way too focused on coding. My experience has been that it really pays to spend time on design work and learning to how to better interact with others, be they clients or team-mates. If you can design your way out of a rewrite, or work with your client to recast requirements to save complexity, it can be far more cost effective than even the best raw code.

Last June, I wrote a bit on my experiences with the Cingular 2125 Windows Smartphone. After more than a year, the phone has been a good choice, but there have been several suprises, for both the good and the bad.

• This is the first phone I've used with a web browser that's usable for general web surfing. Most sites render reasonably correctly, and the display is large enough to contain a useful amount of content. It's still not perfect, the browser crashes too often and it is difficult to log into reddit, but this is a vast improvement over conventional phones.
• I installed a 1GB SD Card that is borderline useless. This might be different if I'd been more aggressively installing software or music, but as it is, the primary benefit of having a card like this is that I can now take 40,000 pictures before I run out of space.
• Outlook integration is still incredibly useful, but it's been harder to keep the calendar in sync than I thought. This is probably due to the fact I get lots of meeting invites that change, but it's made it difficult to rely on the phone as the 'authoritative' source for my scheduling information I hoped it would be.
• J2ME is a non-starter on this phone. There is a JVM, but it's buried under a submenu and the applications running on it look more like 'steerage class' than 'first class' citizens of the phone. They aren't integrated with the main application launcher, and their interfaces look like something out of 1988. I really get the impression that the phone has J2ME solely for the purpose of selling into corporate clients with a requirement to run custom J2ME code.
• Given the power of the underlying hardware and the quality of the display, I was hoping to find more games for the phone. My previous two phones both had small collections of J2ME games purchased through my service provider's web site. AT&T has finally started adding games for this phone to their site, but the selection is limited, expensive, and not that great. I did at least find a few games elsewhere that are pretty fun, Atomic Cannon and Nethack. These were both pretty easy to install. Atomic Cannon, in particular, demonstrates the graphics of the phone quite well.
• I don't use the 'Phone as Modem' capability at all. I don't have as many places where I need to use it as I thought. That said, it does work, and would be a nice way to check mail in a pinch.

In my career, I've done a bit of switching back and forth between Emacs and various IDE's. One of the IDE features I've come to depend on is quick access to the compiler. Typically, IDE's make it possible to compile your project with a keystroke, and then navigate from error to error at the press of a key. It's easy to recreate this in Emacs. The following two expressions make Emacs work a lot like Visual Studio in this regard.

(global-set-key [(shift f5)] 'compile)
(global-set-key [f12] 'next-error)

After these forms are evaluated, pressing Shift-F5 invokes the compile command, which asks for a command to be run in an inferior shell, typically make, ant, or some other build utility. The catch is that it runs the command in the directory of the current buffer, which implies that the build script can be found in the same directory as the current source file. For a Java project with a per-package directory hierarchy, this is often not true. There are probably a bunch of ways to fix this, but I've solved it with a Windows NT batch file, ant-up.bat, that repeatedly searches up the directory hierarchy for build.xml. I just compile with ant-up, rather than a direct invocation of ant. This is not the most elegant solution, I'm sure, but it took about five minutes to implement and works well.

@echo off

setlocal

:retry

set last_path=%CD%

echo Searching %CD% ...

if exist build.xml goto compile-now

cd ..

if "%last_path%"=="%CD%" goto abort

goto retry

:compile-now

call ant -k %1 %2 %3 %4 %5

if errorlevel 1 goto failure

goto success

:abort

echo build.xml not found... compile failed

:failure

exit /b 1

:success

exit /b 0