Seibel: Do you think you were a dramatically better programmer when you finished TeX than when you started?

Knuth: Well, yes, because of literate programming.

Seibel: So you had better tools, but had you actually improved your skills?

Knuth: I learned a terrific amount while I was doing it. One of the things I learned was how much software occupies the brain. It was a much more difficult task than I expected. I couldn’t teach classes full-time and write software full-time. I could teach classes full-time and write a book full-time
but software required so much attention to detail. It filled that much of my brain to the exclusion of other stuff. So it gave me a special admiration for people who do large software projects—I would never have guessed it without having been faced with that myself.

Seibel: So programming is harder than writing books, and somewhere I read something where you said that it’s impossible to estimate how long it will take to write books. Does that then mean that it’s even harder to estimate how long programming will take?

Knuth: Yeah, right. That’s a very good corollary.

This year I’ve written probably three major programs which are pushing one hundred pages of code—literate code, with 8.5✕11 pages. Two of them are related to each other, so it’s more like two and a half major programs. And about 150 small programs. Probably more than I did the previous year. So I
programmed galore this year on small programs but also, a couple of them were things that took a month or more to do.

Seibel: And did you expect them to take a month?

Knuth: Well, I expected one of them to take a month. I knew that it wasn’t going to be easy but I didn’t know how much richness there was going to be, so I added more features as I got to using them. I think it is always going to be true that a person who manages programmers should not expect it to
be predictable.

Coders at Work – Page 572 – Donald Knuth