Peter Seibel - Coders at Work - Reflections on the craft of programming

Then when I went to university at Cambridge, microprocessors were just beginning to take off. So there was a university computing club. There was a big university computing mainframe kind of machine, called Phoenix, with an extremely elaborate accounting system.

The time at which you used it was very significant. You were given a certain amount of units of the machine’s currency and the more memory your program took, the more units you consumed; the longer your program took, the more units you consumed. But the lower the load was, the fewer units your program consumed. So as a result, us undergraduates, who didn’t get a very large allocation, simply spent our nights there because from about 9:00 at night it became rather cheap to run your program.

So we would be there 9:00 to 3:00 a.m., writing our programs. And what did we write in mainly? BCPL, I think. So this again was completely hobbyist stuff. I was doing a maths degree at the time. So zero formal teaching in computer science.

There wasn’t a whole undergraduate degree at that time either. That was 1976 to ’79. There was a final year course so you could graduate in computer science. But you couldn’t do three years of computer science—you had to do something else like maths or natural sciences beforehand. In fact I did maths and finished up with a year of electrical sciences. Mainly because I thought computing was my hobby—it’d be a bit like cheating to do it as your degree as well; degrees should be hard.

But maths turned out to be a bit too hard because Cambridge is stuffed with extremely brainy mathematicians so I switched to electrical sciences.

Seibel:And electrical sciences—that’s what we’d call electrical engineering in the U.S.?

Peyton Jones:That’s right. At that stage my same friend that I was at school with, Thomas Clarke, was also here at Cambridge. So Thomas and I built various computers. You would buy yourself a microprocessor and lots of 7400 series TTL and wire it up. Our biggest problem, I remember, was printers. Printers and screens. Those were the hard bits.

Seibel:Because they’re expensive.

Peyton Jones:They’re so expensive—yes. You could get the electrical parts for the kind of money students could afford but printers were typically big, fridge-sized line printer things. They had a lot of mechanics in them that made them completely out of our price bracket. That and storage devices—any kind of permanent storage device tended to be tricky. So we tended to have computers with a keyboard, a screen, and not much else. And some kind of primitive tape mechanism.

Seibel:You guys were building these computers from scratch in ’76 to ’79. Isn’t that about the same time the Altair was coming out?

Peyton Jones:That’s right. Hobbyist computers were definitely starting to come out. But we considered those to be rather cheating.

The thing about this machine that we built ourselves was that software was the problem. I think my most advanced program for this machine was Conway’s Game of Life. That worked very nicely. But writing any kind of serious program, like a programming language, was just too much work because it had very limited permanent storage medium. And it was all typing in hexadecimal stuff—no assembler.

Seibel:So more raw machine code.

Peyton Jones:Of course the Cambridge mainframe understood BCPL so we were writing lots of BCPL programs. We were actually writing a compiler then for a programming language that we’d invented. We never completed this compiler—it was very elaborate. There were these two completely divorced worlds. Writing compilers in a high-level language for a mainframe and diddling with hardware on the other end.

Seibel:What was the first interesting program you remember writing?

Peyton Jones:A program to extract 24-digit square roots on this schools computer and fit it into 99 memory locations.

Seibel:So you had one spare!

Peyton Jones:That’s right. It was some kind of Newton-Raphson approximation to do square roots. I was terribly proud of it. What after that? I suppose the next scale up must have been this compiler that we never completed. It was written in BCPL and it was extremely elaborate. We were extremely ambitious with it. There was no type system so we just had enormous sheets of printout with pictures, diagrams of structures and arrows between them.

Seibel:You mean in BCPL there was no type system.

Peyton Jones:Yeah, that’s right. So essentially we wrote out our types by drawing them on large sheets of papers with arrows. That was our type system. That was a pretty large program—in fact it was over ambitious; we never completed it.

Seibel:Do you think you learned any lessons from that failure?

Peyton Jones:That was probably when I first became aware that writing a really big program you could end up with problems of scale—you couldn’t keep enough of it in your head at the same time. Previously all the things I had written, you could keep the whole thing in your head without any trouble. So it was probably the first time I’d done any serious attempt at long-standing documentation.

Seibel:But even that wasn’t enough, in this case?

Peyton Jones:Well, we had a lot of other things to do, like get degrees. This was all between 9:00 p.m. and 3:00 a.m.

Seibel:And is there anything you wish you had done differently about learning to program?

Peyton Jones:Well, nobody every taught me to program. I’m not sure I ever really missed that. Today I feel as if my main programming blank spot is that I don’t have a deep, visceral feel for object-oriented programming. Of course I know how to write object-oriented programs and all that. But something different happens when you do something at scale. When you build big programs that last for a long time and you use class hierarchies in a complex way and you build frameworks—that’s what I mean by a deep, visceral understanding. Not the kind of stuff that you’d learn immediately from a book.

I feel that as a lack because I don’t feel I can really be authoritative about what you can and can’t do with object-oriented programming. I’m always very careful in what I say, particularly not to be negative about imperative programming because it’s an incredibly sophisticated and rich programming paradigm. But somehow because of the way my life developed, I never really spent several years writing big C++ programs. That’s how you get some kind of deep, visceral feel and I never have.

Seibel:I think that feeling is usually revulsion.

Peyton Jones:That’s right—but it’s a well-informed revulsion rather than a superficial, “Oh, that sucks” kind of revulsion.

Seibel:So you finished your three years at Cambridge, then what?

Peyton Jones:Then I thought, “Alright, better do a little bit of work on computing.” So I spent one year doing a postgraduate diploma in computer science—my sole formal education in computer science.

Seibel:Is that kind of like a master’s degree?

Peyton Jones:Kind of like a master’s degree. I had a great year. I suspect it was very similar to the computer science tripos, the undergraduate degree. But it was intended for students who hadn’t done any other computer science.

Seibel:Then you spent a couple years in industry before getting back into research. What were you doing then?