Interview with Donald E. Knuth
   
Posted 25 Jan 2000 by advogato

This week, Advogato had the pleasure and honor of interviewing
Prof. Donald E. Knuth. He is the author of the TeX typesetting
system as well as The Art of Computer Programming and a number of
deep, insightful papers and books. The interview took place by
phone on a rainy California winter day. The topics covered the
freeness of TeX and its fonts, how TeX's innovations have slowly
diffused into commercial systems, some history of math typesetting,
the design of TeX from the beginning as an archival system,
literate programming in the age of the Web, MMIX and the Transmeta
chip, how to avoid generating inscrutable error messages, and
taking the TeX ideas to a broader community. Read below to find
more about a remarkable person.

Advogato: The first questions that I have are about free software.
TeX was one of the first big projects that was released as free
software and had a major impact. These days, of course, it's a big
deal. But I think when TeX came out it was just something you did,
right? 

Prof. Knuth: I saw that the whole business of typesetting was being
held back by proprietary interests, and I didn't need any claim to
fame. I had already been successful with my books and so I didn't
have to stake it all on anything. So it didn't matter to me whether
or not whether I got anything financial out of it.

I see. 

There were people who saw that there was a need for such software,
but each one thought that they were going to lock everyone into
their system. And pretty much there would be no progress. They
wouldn't explain to people what they were doing. They would have
people using their thing; they couldn't switch to another, and they
couldn't get another person to do the typesetting for them. The
fonts would be only available for one, and so on.

But I was thinking about FORTRAN actually, the situation in
programming in the '50s, when IBM didn't make FORTRAN an IBM-only
thing. So it became a lingua franca. It was implemented on all
different machines. And I figured this was such a new subject that
whatever I came up with probably wouldn't be the best possible
solution. It would be more like FORTRAN, which was the first fairly
good solution [chuckle]. But it would be better if it was available
to everybody than if there were all kinds of things that people
were keeping only on one machine.

So that was part of the thinking. But partly that if I hadn't
already been successful with my books, and this was my big thing, I
probably would not have said, "well, let's give it away." But since
I was doing it really for the love it and I didn't have a stake in
it where I needed it, I was much more concerned with the idea that
it should be usable by everybody. It's partly also that I come out
of traditional mathematics where we prove things, but we don't
charge people for using what we prove.

So this idea of getting paid for something over and over again,
well, in books that seems to happen. You write a book and then the
more copies you sell the more you get, even though you only have to
write the book once. And software was a little bit like that.

I think that's the model that software publishing generally comes
from. There was a quote that you had in the "Mathematical
Typography" essay reprinted in "Digital Typography" where you said,
"Mathematics belongs to God." 

Yes. When you have something expressed mathematically, I don't see
how you can claim... In the context, that was about fonts. That was
when I had defined the shape of the letter in terms of numbers. And
once I've done that, I don't know how you're going to keep those
numbers a secret...

Proprietary. 

I can conceieve of a number that would be a million digits long and
would be extremely expensive to compute, and once somebody knew
that number, it would solve all kinds of problems. And I suppose
that would make it a little bit harder to say that God already had
given us this number, when it's a number that you can only discover
by a tremendous amount of sweat.

When I made that quote, I didn't think of such things.

Fonts seem like a really interesting edge case for that argument,
because a font is in some ways a mathematical formula, especially a
TeX font, much more so than what came before, but it's also an
artwork. 

Absolutely. It absolutely requires great artistry. So the other
part of this is that artists are traditionally not paid like
scientists. Scientists are supported by the National Science
Foundation to discover science, which benefits the human race.
Artists, or font designers, are not supported by the National Font
Foundation to develop fonts that are going to be beneficial to the
human race. Fonts are beneficial to the human race, they just don't
traditionally get supported that way. I don't know why. They're
both important aspects of our life. It's just that one part has
traditionally gotten funded by a royalty type mechanism and the
other by public welfare grants for the whole country.

Perhaps that has something to do with the absolute necessity in
science to have open access to the results of others, that if you
did science in a closed, proprietary framework that the
disadvantages would be so clear. 

With fonts, it was pretty clear to me.

Ok! That's a question that Federico Mena Quintero suggested. You've
gotten a number of free fonts contributed by artists, in some cases
very beautiful fonts, to TeX and to the Metafont project. In
general, this has been a real struggle for open source development
these days, to get free fonts. Do have any thoughts? 

I think it's still part of this idea of how are the font designers
going to get compensated for what they do. If they were like a
scientist, then they've got their salary for doing their science.
But as font designers, where do they get their salary? And
musicians. It's just a matter of tradition as to how these people
are getting paid.

But how did you address those problems with the fonts that got
contributed to TeX? 

In my case, I hired research associates and they put their fonts
out into the open. Or else, other people learned it and they did it
for the love of it. Some of the excellent fonts came about because
they were for Armenian and Ethiopian and so on, where there wasn't
that much money. It was either them taking time and making the
fonts or else their favorite language would be forever backwards,
so I made tools by which they could do this. But in every case, the
people who did it weren't relying on this for their income.

If we had somebody who would commission fonts and pay the font
designer, the font designer wouldn't be upset at all about having
it open, as long as the font designer gets some support.

And you did some of that. 

Yeah. In fact, I worked with some of the absolute best type
designers, and they were thrilled by the idea that they could tell
what they knew to students and have it published and everything.
They weren't interested in closed stuff. They're interested in
controlling the quality, that somebody isn't going to spoil it, but
we could assure them of that.

Right. Working with the creator of the software. 

Yeah, if they didn't like the software, I could fix it for them.

One of the things that struck me when I was reading "Digital
Typography" is the intensive study that you did, especially in the
area of math typesetting. When I was writing papers, using math
formulas in TeX, I just typed in the commands and out came the math
and it looked pretty good to me. It shouldn't have been surprising,
but it definitely struck me how much attention you paid to the best
mathematics typesetting of past centuries. 

I do strongly think that people, when they start throwing computers
at something, they think that it's a whole new ballgame, so why
should they study the past. I think that is a terrible mistake. But
also, I love to read historical source materials, so I couldn't
resist. I had a good excuse to study these things, and the more I
looked at it, the more interesting it was. But I don't think
responsible computer scientists should be unaware of hundreds of
years of history that went before us. So that was just a natural
thing to approach it that way, for me.

I noticed, for example, that in the proprietary software market for
publishing, that systems are only today acquiring features that
have existed in TeX for a long time, for example whole-paragraph
optimization. There's a big to-do about Adobe InDesign, which
finally... 

They finally implemented the TeX algorithm.

Did they implement the TeX algorithm? 

Yeah, that's what they said.

Did you talk to the people? 

I met three of four of them at the ATYPI meeting in Boston in
October, but that was after I had heard about it, that some friends
had found this in the documentation.

Another similar issue is TrueType fonts. TrueType fonts have this
property of including instructions, computer programs effectively,
in the font, to do hinting. 

Well, I never met Elias or whatever.

Sampo Kaasila? 

I don't know. I know enough about TrueType to know that it's a very
intelligent design, that is similar to Metafont except that it
strips out everything that's slow. So the way the hinting is done
is by program, certainly. Of course, it came out maybe ten years
after Metafont, so probably something got through somehow.

There was the F3 font that Folio was making, if I can remember the
name, what the people in industry called it. Some of the people
that I had worked with on Metafont went into making font designs
that were similar to TrueType, but have not been successful.

There's a fairly major controversy with TrueType right now, that
there a number of patents that are owned now by Apple. It's kind of
interesting to me that that is the case even though it's for the
most part derivative work of what was in Metafont. 

I've been very unhappy with the way patents are handled. But the
more I look at it, the more I decide that it's a waste of time. I
mean, my life is too short to fight with that, so I've just been
staying away. But I know that the ideas for rendering... The main
thing is that TrueType uses only quadratic splines, and that Type1
fonts use cubic splines, which allow you to get by with a lot fewer
points where you have to specify things.

The quadratic has the great advantage that there's a real cheap way
to render them. You can make hardware to draw a quadratic spline
lickety-split. It's all Greek mathematics, the conic sections. You
can describe a quadratic spline by a quadratic equation (x, y) so
that the value of f(x, y) is positive on one side of the curve and
negative on the other side. And then you can just follow along
pixel by pixel, and when x changes by one and y changes by one, you
can see which way to move to draw the curve in the optimal way. And
the mathematics is really simple for a quadratic. The corresponding
thing for a cubic is six times as complicated, and it has extra
very strange effects in it because cubic curves can have cusps in
them that are hidden. They can have places where the function will
be plus on both sides of the cubic, instead of plus on one side and
minus on the other.

The algorithm that's like the quadratic one, but for cubics, turns
out that you can be in something that looks like a very innocuous
curve, but mathematically you're passing a singular point. That's
sort of like a dividing by zero even though it doesn't look like
there's any reason to do so. The bottom line is that the quadratic
curves that TrueType uses allow extremely fast hardware
implementations, in parallel.

The question is whether that matters of course, now that CPU's are
a zillion times faster. 

But for rendering, Metafont was very very slow by comparison,
although I'm amazed at how fast it goes now. Still, it has to be an
order of magnitude better, and certainly that was a factor in
getting TrueType adopted at the time that it was, because machines
weren't that fast then. So TrueType was an intelligently chosen
subset, but certainly all the ideas I've ever heard of about
TrueType were, I believe, well known in the early '60s.

Back to this issue of preserving the past. I was reading some
papers of Edsger Dijkstra. For a while, he used handwritten
manuscripts and then a typewriter to actually distribute the work.
And, his notation became much more typewriter-like, that he would
use an underlined A or a boldfaced A instead of the traditional
\forall symbol. 

I've gotten some of his handwritten notes, but I don't remember the
typewritten ones.

I was looking at the proceedings of the Marktoberdorf summer school
in '90, where there were a couple of papers by him and his group.
In any case, it occurred to me that TeX has made the traditional
mathematical notations so accessible to practicing computer
scientists, students, researchers, etc. It's very likely that if
there hadn't been something like TeX, in other words if
mathematical typesetting had remained strictly in the domain of
book publishers, and people who did publishing as their profession,
it's likely that the standard notations in computer science would
have become much more typewriter-like, kind of ASCII-ized. 

That's interesting.

As it is, if you look at Principles of Programming Languages,
there's all this type theory in there with beautiful Greek letters,
inference rules and so on. The visual aesthetic seems to have been
inspired by the easy availability that TeX provides. 

Certainly, TeX was partly influenced by the needs of computer
science, that pushed beyond what mathematicians had needed.
Computer scientists needed to see mathematical structure, but they
also needed to see the kind of structure you have in programs. So
we had to push the envelope in the ACM Journal in the '60s. To
publish computer science papers, the typesetters using hand methods
in the '60s were being forced to work harder by the computer
scientists than by the mathematicians at that time. It's part of
the need for a way to see the structure that's in the stuff you're
working with. Since I'm writing books about computers, naturally I
made TeX so that it could do that.

It's clear that TeX was inspired by the visual aesthetic of
mathematics. 

Oh yeah.

I'm just saying: to what extent mathematics and computer science
today is being influenced by the visual aesthetic of TeX? 

Well, I don't know. I think the fact that TeX was open-ended means
that people can choose their notation themselves. They don't have
to adopt somebody else's macros particularly. That was the bind
that we were in before. We would have to first write our paper,
then explain it to the printer, and the printer would maybe get it
right. But now we're less hesitant to use a notation that's not
traditional, but that we think is appropriate, because we know that
we don't have to go through a noisy channel in the middle.

One of the other accomplishments of TeX that I continue to be
impressed with is the consistent rendering, the idea that you have
a source file and that on virtually any implementation of TeX,
you'll get the same results. 

That's what I insisted on the most. I didn't want to get paid, but
I didn't want it to change.

If you look at, for exmaple, a lot of the new systems that are
being designed today like Cascading Style Sheets for the Web, they
allow a tremendous amount of latitude for the implementor, so you
don't get anything approaching consistent rendering. 

With the Web, it's a different tradeoff, between appearance and the
ability for a search engine to find it.

Presentation vs semantic markup, right, in general? 

Right.

But it seems to me that TeX's model is actually a way to be better
in both of those regards than the way that the Web has been
evolving, because you do have at least the potential for semantic
markup, and then you get the fact that you know what the
presentation will be. So, one example from the Digital Typography
book is that you described changing the wording of some of the
exercises so that the symbols would line up better, that the
parentheses wouldn't collide with descenders. 

This wouldn't go too well to the web, because people can certainly
set the width of their browser window differently, and I'm not
expecting that I would optimize for all of those things. But if I'm
optimizing for a particular page size, then I would say, "why not."
Mostly, the rewriting is that I can get a figure to be on the same
page where you want to see it, without turning the pages over, if
you care about things like that. There were some parts of The Art
of Computer Programming where I said, well, I gotta think of five
lines of things to say on this page, and then I would think of ten
lines. It was a motivation for research. I knew I needed five more
lines of stuff to say on this page, because otherwise the
illustrations aren't going to come out right.

If you are going to print, and of course a lot of people print from
the Web, and usually on page sizes that are well defined in
advance, with TeX you have at least the possibility of doing that.
If you do reformat the width, then the semantic markup is still
there so maybe it isn't as optimized, but you still get a
reasonable rendering. 

Well, what I was mostly concerned about was that the rendering
would be the same in ten years. What you say is probably important,
but I didn't have that so much in mind as the fact that on
everybody's machine it would come out the same, not only now but in
the future. Because there was so much software rot going on where
stuff wouldn't work any more. And I didn't have time to keep
updating... It's going to take many years to finish TAoCP, so I
wanted to make sure that I could make Vol. 7 match Vol. 1.

But that seemed to be at odds with something else you said in
"Digital Typography" about being surprised that people didn't make
more custom versions of TeX for specialized applications. And if
that had happened, and if some of those had become popular,
wouldn't that have created similar problems? 

I wasn't expecting them to become popular. I was expecting that
they were going to be one-off things. If I were a publisher, that's
what I would have done.

But if you create a one-off, that still creates the potential for
software rot, that ten years from now... 

Okay. I have to save my program. Yeah.

And that's hard. Usually. 

The program is there in a bunch of change files, so as long as the
C language isn't changing, then I'm ok.

Well, it isn't changing much. 

See, the way I would make these custom version is by using the
change file facility that we built in when we started distributing
TeX. It means that you have a master file, and then you have
another file that says, these are the things I changed. And that's
the way people ported TeX to lots of different operating systems,
they would have it in these change files. I could change the master
file, and their change file would probably still work.

I see. I didn't actually know about that aspect of the TeX
distribution. 

There's a change-file mechanism that's been quite successfully
received in all the TeX distributions.

That sounds like it could be useful in other projects as well. 

Isn't hasn't been picked up yet, but I don't know why [laugh].

Speaking of literate programming, the question that I have for you
is: now that people are moving everything to the Web, including
programming, do you think that's another chance for literate
programming to become popular? 

There's a lot of ferment in this direction. I still don't have the
dvipdf program that I got to get installed on my Linux. I guess I
gotta try it. But a guy has worked out now that he can convert to
Acrobat format, a literate program automatically come out in
Acrobat format, so that you can use all the features of the Acrobat
reader to click on stuff to move around in the documentation.

Cross-referencing and so on? 

Yeah, find a variable where it's declared, and all other uses of
the variable. It's certainly a natural thing for hypertext, and
this guy in Brazil has worked out a nice system. Still, the thing
that's holding it back is probably that some programmers just don't
like to document their stuff.

That's certainly a problem we've had in free software, that if
there's a documentation file and then a code file, that the two
often get out of sync. 

These are slowly changing, but most of the horror stories that you
hear about that stuff is because of the illiterate nature of the
program.

So you think that literate programming is a way to make that
better? 

It's so much better than the alternative, but I think Jon Bentley
explained it to me best, not much percentage of the world's
population is really good at programming, and not much percentage
of the world's population is really good at documenting, and here
you need to be doing both. [laughs]

So, I think that in my experiments with Stanford students I found
that more than half of the students that I worked with really hit
it off well with literate programming. Maybe Stanford students
aren't average. [laugh]

Right. 

So you need somebody who's not afraid to write an essay, as well as
not afraid to write a computer program.

They work together perfectly, but you have to be able to
communicate to the computer, and you have to be able to communicate
to the human being, and if you don't do both, then you can't expect
your program to be as successful. Literate programming is just the
best way I know to do both at the same time.

My new book on MMIX, where another example of what I hope people
will consider is just a state-of-the-art, normal way to document
programs. In this case, the program that I wrote for this MMIX
simulator last year, I wouldn't have been able to finish the
program without literate programming. It was just too mind
boggling. Not only was literate programming a nice add-on, without
that tool, I wouldn't have gotten the program done.

I saw the MMIX book just came out. I haven't gotten my copy yet,
but I'm really looking forward to it. That is the new instruction
set that you're using in TAoCP, right? 

Yeah. Well, I'm going to use it eventually. It's there, but it's
going to be a while before I replace it in volumes 1, 2 and 3. I
gotta write volume 4 first.

MMIX is another digression for you, then. 

It'll be used a small amount in Vol. 4. If I talk about machine
code at all in Vol. 4, there are a few places, then it will be in
there.

I was reading up on MMIX at the same time that the Transmeta
processor came out, the Crusoe. I don't know if you're familiar
with that, but it's a VLIW that does on-the-fly translation of a
pretty much arbitrary instruction set into its own native format.
Do you think that MMIX is really going to be very similar to the
architectures that people will need to know, or is the increasing
complexity of CISC chips and new things like VLIW going to make... 

I don't know. These VLIW chips like the IA64 are getting very mixed
reviews. I guess the Pentium translates into a RISC-like language
internally.

Yeah, sort of. 

I think in any case, this is a model that is going to be as close
to efficient and clean at the same time as we're going to get in
the forseeable future. I wanted it to be realistic, but I also
wanted it to be clean and easy to learn.

One of the things that intrigued me about this Crusoe chip that
Transmeta is doing is that they have firmware that does dynamic
translation of the source architecture. Obviously they're pushing
x86 as the most commercially important source language, but I was
just wondering if maybe MMIX would be a good candidate for... 

If they want to play with it. But that will be just an academic
exercise right now rather than a commercial one, because there
isn't that much code written for it.

Well, the x86 has some serious problems, that the memory bandwidth
is a lot higher than it needs to be because it doesn't have enough
registers. 

Oh, absolutely.

So, for this particular chip, it might make more sense for gcc to
generate MMIX code. 

Oh, I see what you mean. Well, that's interesting.

I don't know. I was just wondering if you had some thoughts on
that. 

Well, okay. So we need a library, all that kind of code. It's for
the MMIXmasters to develop, because I haven't got time to mix with
it.

The other thread I was interested about was this whole development
of commercial TeX packages in the '80s, for PC's and so on. Those
didn't catch on, and PostScript did, and the world is quite
different because of that. Do you have have some stories about why
that would have happened, or what you might have done differently? 

Those programs were kind of heroic. At that time, PC's were
limited. If you wanted to have an array with more than than
65kbytes in it, you had to go in to a special extended addressing
mode. So it wasn't easy to port TeX to a PC in those days, with the
186 and the zero-86.

Except in the mathematics and physics community, there weren't that
many people who were interested in the archival quality of stuff,
or making something that looks best instead of good. I never
expected TeX to be the universal thing that people would turn to
for the quick-and-dirty stuff. I always thought of it as something
that you turned to if you cared enough to send the very best.

It made me almost scared when I heard that college students in
Japan were being forced to learn TeX, because I never wanted
anybody to be compelled to dot their i's and cross their t's. I
always thought of it for somebody who wanted something to pride in
putting a little spit and polish on documenting. I didn't think of
it as the world standard or something like that at all.

The things that, in my experience, are the most difficult for
people who are learning TeX or using TeX, are for one thing, the
difficulty of using PostScript fonts. And from what I understand,
that's accidental. 

Oh, there's no problem whatsoever, except since on every machine
you needed to interface a different way, and everybody had a
different subset of the fonts. There were six different PostScript
Times Roman fonts, and they all had slightly different spacing, so
there was no way to make that easy. Then Adobe was going to put out
another Times Roman, because they could improve it. There was no
idea of archiving these fonts or keeping them in a way that wasn't
going to change from year to year the way other things in TeX were
doing, so that was one of the hangups. Also, we didn't have that
many drivers until about 1990 for PostScript. I had been using them
for different projects, but since I wanted to make sure that I was
technology independent for my own books, I did everything in
bitmaps that I generated in a form that I knew would work 50 years
from now. But when I would do a special project that was not for my
books, but for something where I was supposed to make camera ready
copy for some journal, then I would use commercial fonts. We
finally made the virtual font mechanism popular, and people started
understanding it and it took another ten years. Now, that's being
used a lot.

It's better, but it's still difficult to just take a PostScript
font and drop it into TeX and have TeX use it. That might be more
of a packaging issue. 

Yeah, You've got to change a couple of lines in some mysterious
files that are not well documented.

And I've had problems with encodings too. 

You have to be sure that you get the right font metrics.

It just seems like a hassle when I've tried to do it. 

Well, it's a hassle to install new fonts right now, because the TeX
world and the Microsoft world are separate.

Yeah. 

So you've got to install the fonts in a few different places.

The other issue that is definitely intimidating for beginning users
are the error messages and error reporting, especially when you're
using a macro package like LaTeX. It seems like the error messages
that you get have virtually no relationship whatsoever to what you
did wrong. I'm wondering, is that because of the macro nature of
that process, an inherent limitation of systems that do what TeX
does by evaluating huge piles of macros? 

There is something inherent. If you have a language that has a
certain global character to it so that if you make a mistake on
line 100, there's no way for the computer to find out about it
until line 200, because even though you made a mistake, it still
made sense. Then you get into the situation that you're describing,
where error messages are pretty inscrutable.

So if I had put more care into making a language that was more
restrictive, that it would check a lot more stuff, then you'd catch
an error much quicker.

Right. 

Let me explain. TeX does have some things like this. It'll say that
a macro can be defined to be \outer, which means that you're not
supposed to have this macro in the middle of a parameter to another
macro. So if somebody forgot a right brace, then it'll catch it
before you've gone too far in the program. But suppose that I had
made something that you have to end something on the same line it
begins. For example, take a programming language. If you have a
string starting with a double quote mark, you've got to finish the
string on that same line. So if you forget a double quote mark, you
know about it right away. But if you didn't have that rule, then
you could leave out one double quote mark, and everthing that was
outside of a string would be treated as inside of a string, and
vice versa.

And you generally get an error message the next time a quote mark
appears in the program. 

Yeah, you're bound to get an inscrutable error message. So, there's
part of it inherent that TeX is a language that allows you to go on
and on and still be syntactly correct.

And the other problem that I see with macros is that it makes it
much more difficult to do an incremental update to a display. Of
course, that's something that seems very interesting now, with big
high-res displays commonly available, and probably less important
when TeX was originally being created. If you make even a small
change to a TeX document, you pretty much are forced to go
through... 

TeX doesn't know that your small change hasn't changed everything.

Right. So you can't really have a version of TeX that lets you edit
in one window and have those... 

But on this Pentium III, TeX just generates 50 pages by the time
you hit carriage return.

Yeah, but there's still a difference between that and incremental
editing where you're actually typing and seeing your changes
updated in real time. 

It's so close to real time now, even if I'm going through dvips and
Ghostview, and going up to page 20. Right now, my Linux machine at
school is ten times faster than my SparcStation at home, so it's
worthwhile for me to go to school, just to get this effect, when
I'm doing a lot of editing of spacing. So I'm sitting here with a
50 page document, and I say, "maybe I'll add a thin space here,"
and I'll re-TeX the entire document, and I'll re-convert to
PostScript, and it's just a fraction of a second. And I can see
right away that the thin space has gone in there. I can't believe
it. And this is going to be so much more common in the future.

It's hard for me to believe because I know what TeX is doing, I
know what dvips is doing, and yet it's fast, it's done.

Well, you've heard of Gates' law, which is that the speed of
software halves every 18 months, right? [laugh] And I guess that
TeX is an exemption to that. 

Well, it hasn't changed.

Exactly. Well, one of the things that I'm personally interested in
is making the kind of stuff that's in TeX more accessible, to users
in general, so I'm working on interactive editing in the Gnome
project, which is a Linux thing. I'm very interested in doing
things like the whole paragraph optimization and the nice
hyphenation with all the different languages. Also, in the area of
graphics, I've been doing work with splines and using some of the
work that John Hobby did for the smooth fitting of splines to just
the control points, so you don't have to fiddle with the Bezier
control points. I've got some prototype software around that does
that interactively. I'm very excited by it, because I feel like
somebody who is not a mathematician but who is a font designer
might be able to sit down with that, and really get the benefit of
a lot of the ideas that were in Metafont, but not have to deal with
it as mathematics. You just deal with it directly on the screen.
Now, to you, you're a mathematician. 

No, no. That's right. It never became natural. I tried to explain
how, but even the idea of multiple masters was a pretty hard sell.

Yeah. But in any case, I'm very grateful to have the enormous body
of work that's been done by you and your graduate students on TeX,
Metafont, and related packages. My work is so much easier. All I
have to do is adapt this stuff and steal all your best ideas. 

Well, it's not... The hard stuff is still getting it so that it
responds right to the people who need it. For that, you have to
really be in that community. Also, to see how the users...

That's what I'm trying to do. 

Like this error message business that you brought up. You have the
same kind of thing in a C language program, where if you do
something wrong, you might get a hundred error messages. But the
amazing thing about the literate programming tools, CWEB, is that
if I make five CWEB errors, I will tend to get five error messages.

Interesting. 

Not only the first one will make sense, but I'll be able to look at
the whole thing and fix them all and recompile. And it's always
been that way. There's something about that design that localizes
the errors. I'm not sure what the key is.

Well, that would be interesting to look into. 

That's one of the tradeoffs that you have between how much
redundancy is there so that the computer can check as you're going.
If you're going to catch an error right away, that means that
people have to write things a little bit redundant. That means that
you put a little bit of burden, but maybe it's still better to have
that little bit of burden than to have your errors become
inscrutable.

Aha. Unless you're one of those people that always types things
right the first time. 

Oh, no no. I have never met such a person.

All right, that's all the questions that I had. 

Good luck to you.

It's really been a pleasure.