Andrew Binstock: You are one of the fathers of the open source revolution, even if you aren’t widely heralded as such. You previously have stated that you released T_eX as open source because of the problem of proprietary implementations at the time, and to invite corrections to the code—both of which are key drivers for open-source projects today. Have you been surprised by the success of open source since that time?
Donald Knuth: The success of open source code is perhaps the only thing in the computer field that hasn’t surprised me during the past several decades. But it still hasn’t reached its full potential; I believe that open-source programs will begin to be completely dominant as the economy moves more and more from products towards services, and as more and more volunteers arise to improve the code.

For example, open-source code can produce thousands of binaries, tuned perfectly to the configurations of individual users [eg. Gentoo], whereas commercial software usually will exist in only a few versions. A generic binary executable file must include things like inefficient “sync” instructions that are totally inappropriate for many installations; such wastage goes away when the source code is highly configurable. This should be a huge win for open source.

Yet I think that a few programs, such as Adobe Photoshop, will always be superior to competitors like the GIMP—for some reason, I really don’t know why! I’m quite willing to pay good money for really good software, if I believe that it has been produced by the best programmers.

Remember, though, that my opinion on economic questions is highly suspect, since I’m just an educator and scientist. I understand almost nothing about the marketplace.

A. Binstock: …A story states that you once entered a programming contest at Stanford (I believe) and you submitted the winning entry, which worked correctly after a single compilation. Is this story true? In that vein, today’s developers frequently build programs writing small code increments followed by immediate compilation and the creation and running of unit tests. What are your thoughts on this approach to software development?
D. Knuth: The story you heard is typical of legends that are based on only a small kernel of truth. Here’s what actually happened: John McCarthy decided in 1971 to have a Memorial Day Programming Race. All of the contestants except me worked at his AI Lab up in the hills above Stanford, using the WAITS time-sharing system; I was down on the main campus, where the only computer available to me was a mainframe for which I had to punch cards and submit them for processing in batch mode. I used Wirth’s ALGOL W system (the predecessor of Pascal). My program didn’t work the first time, but fortunately I could use Edward Satterthwaite’s excellent offline debugging system for ALGOL W, so I needed only two runs. Meanwhile, the folks using WAITS couldn’t get enough machine cycles because their machine was so overloaded [elsewhere, Knuth sardonically thanks the WAITS developers for all the downtime & inefficiency as they ‘improved’ it]. (I think that the second-place finisher, using that “modern” approach, came in about an hour after I had submitted the winning entry with old-fangled methods.) It wasn’t a fair contest.

As to your real question, the idea of immediate compilation [interpretation?] and “unit tests” appeals to me only rarely, when I’m feeling my way in a totally unknown environment and need feedback about what works and what doesn’t. Otherwise, lots of time is wasted on activities that I simply never need to perform or even think about. Nothing needs to be “mocked up”.

Binstock: …One of the few projects of yours that hasn’t been embraced by a widespread community is literate programming. What are your thoughts about why literate programming didn’t catch on? And is there anything you’d have done differently in retrospect regarding literate programming?
Knuth: Literate programming is a very personal thing. I think it’s terrific, but that might well be because I’m a very strange person. It has tens of thousands of fans, but not millions.

In my experience, software created with literate programming has turned out to be substantially better than software developed in more traditional ways. Yet ordinary software is usually okay—I’d give it a grade of C (or maybe C++), but not F; hence, the traditional methods stay with us. Since they’re understood by a vast community of programmers, most people have no big incentive to change, just as I’m not motivated to learn Esperanto even though it might be preferable to English and German and French and Russian (if everybody switched).

Jon Bentley probably hit the nail on the head when he once was asked why literate programming hasn’t taken the whole world by storm. He observed that a small percentage of the world’s population is good at programming, and a small percentage is good at writing; apparently I am asking everybody to be in both subsets.

Yet to me, literate programming is certainly the most important thing that came out of the T_eX project. Not only has it enabled me to write and maintain programs faster and more reliably than ever before, and been one of my greatest sources of joy since the 1980s—it has actually been indispensable at times. Some of my major programs, such as the MMIX meta-simulator, could not have been written with any other methodology that I’ve ever heard of. The complexity was simply too daunting for my limited brain to handle; without literate programming, the whole enterprise would have flopped miserably.

If people do discover nice ways to use the newfangled multithreaded machines, I would expect the discovery to come from people who routinely use literate programming. Literate programming is what you need to rise above the ordinary level of achievement. But I don’t believe in forcing ideas on anybody. If literate programming isn’t your style, please forget it and do what you like. If nobody likes it but me, let it die.

On a positive note, I’ve been pleased to discover that the conventions of CWEB are already standard equipment within preinstalled software such as Makefiles, when I get off-the-shelf Linux these days.

A B: …What set of tools do you use today for writing TAOCP? Do you use T_eX? LaTeX? CWEB? Word processor? And what do you use for the coding?
D K: My general working style is to write everything first with pencil and paper, sitting beside a big wastebasket. Then I use Emacs to enter the text into my machine, using the conventions of T_eX. I use tex, dvips, and gv to see the results, which appear on my screen almost instantaneously these days. I check my math with Mathematica.

I program every algorithm that’s discussed (so that I can thoroughly understand it) using CWEB, which works splendidly with the GDB debugger. I make the illustrations with MetaPost (or, in rare cases, on a Mac with Adobe Photoshop or Adobe Illustrator). I have some homemade tools, like my own spell-checker for T_eX and CWEB within Emacs. I designed my own bitmap font for use with Emacs, because I hate the way the ASCII apostrophe and the left open quote have morphed into independent symbols that no longer match each other visually. I have special Emacs modes to help me classify all the tens of thousands of papers and notes in my files, and special Emacs keyboard shortcuts that make bookwriting a little bit like playing an organ [Knuth is famously an amateur organ player & owns a custom one]. I prefer rxvt to xterm for terminal input. Since last December, I’ve been using a file backup system called backupfs, which meets my need beautifully to archive the daily state of every file.

According to the current directories on my machine, I’ve written 68 different CWEB programs so far this year. There were about 100 in 2007, 90 in 2006, 100 in 2005, 90 in 2004, etc. Furthermore, CWEB has an extremely convenient “change file” mechanism, with which I can rapidly create multiple versions and variations on a theme; so far in 2008 I’ve made 73 variations on those 68 themes. (Some of the variations are quite short, only a few bytes; others are 5KB or more. Some of the CWEB programs are quite substantial, like the 55-page BDD package that I completed in January.) Thus, you can see how important literate programming is in my life.