Seibel: Speaking of practical work, in the middle of working on The Art of Computer Programming you took what turned into a ten-year break to write your typesetting system TeX. I understand you wrote the first version of TeX completely away from the computer.
Knuth: When I wrote TeX originally in 1977 and ’78, of course I didn’t have literate programming but I did have structured programming. I wrote it in a big notebook in longhand, in pencil. Six months later, after I had gone through the whole project, I started typing into the computer. And did the debugging in March of ’78 while I had started writing the program in October of ’77. The code for that is in the Stanford archives—it’s all in pencil—and of course I would come back and change a subroutine as I learned what it should be. This was a first-generation system, so lots of different architectures were possible and had to be discarded until I’d lived with it for a while and knew what was there. And it was a chicken-and-egg problem—you couldn’t typeset until you had fonts but then you couldn’t have fonts until you could typeset. But structured programming gave me the idea of invariants and knowing how to make black boxes that I could understand. So I had the confidence that the code would work when I finally would debug it. I felt that I would be saving a lot of time if I waited six months before testing anything. I had enough confidence that the code was approximately right.
Seibel: And the time savings would be because you wouldn’t spend time building scaffolding and stubs to test incomplete code?
Knuth: Right.