settled that, he wrote down a program that met the proof’s requirement. You lose a lot when you restrict the role of mathematics to program verification as opposed to program construction or derivation.

Another experience in 1959 was attending the “zeroth” IFIP Congress in Paris. My international contacts had started in December 1958, with the meetings for the design of ALGOL 60. My boss, Aad van Wijingaarden, had had a serious car accident, and Jaap Zonneveld and I, as his immediate underlings, had to replace him. Zonneveld was a numerical analyst, while I did the programming work. The ALGOL 60 meetings were about the first time that I had to carry out discussions spontaneously in English. It was tough.

you’ve remarked that learning many
different languages is useful to pro-
gramming.

Oh yes, it’s useful. There is an enormous difference between one who is monolingual and someone who at least knows a second language well, because it makes you much more conscious about language structure in general. You will discover that certain constructions in one language you just can’t translate. I was once asked what were the most vital assets of a competent programmer. I said “ mathematical inclination” because at the time it was not clear how mathematics could contribute to a programming challenge. And I said “exceptional mastery” of his native tongue because you have to think in terms of words and sentences using a language you are familiar with.

how was ALgoL 60 a turning point?

Computing science started with AL-
GOL 60. Now the reason that ALGOL 60
was such a miracle was that it was not
a university project but a project cre-
ated by an international committee.
It also introduced about a half-dozen
profound novelties. First of all, it in-
troduced the absence of such arbitrary
constraints as, say, ruling out the sub-
scripted subscript, the example I men-
tioned. A second novelty was that at
least for the context-free syntax, a for-
mal definition was given. That made a
tremendous difference! It turned pars-
ing into a rigorous discipline, no longer
a lot of handwaving. But perhaps more
important, it made compiler writing
and language definition topics worthy
of academic attention. It played a ma-
jor role in making computing science
academically respectable. The third
novelty was the introduction of the
type “Boolean” as a first-class citizen.
It turns the Boolean expression from a
statement of fact that may be wrong or
right into an expression that has a val-
ue, say, “true” or “false.” How great that
step was I learned from my mother’s
reaction. She was a gifted mathemati-
cian, but she could not make that step.
For her, “three plus five is ten” was not
a complicated way of saying “false”; it
was just wrong.

all the programming
i did was on paper.
So i was quite
used to developing
programs without
testing them.

who had to agree with the report, none saw that sentence. That’s how recursion was explicitly included.

Was this called recursion at that time?

Oh yes. The concept was quite well known. It was included in LISP, which was beginning to emerge at that time. We made it overlookable. And F.L. Bauer would never have admitted it in the final version of the ALGOL 60 Report, had he known it. He immediately founded the ALCOR Group. It was a group that together would implement a subset of ALGOL 60, with recursion emphatically ruled out.

What were other novelties in ALgoL 60?

A fifth novelty that should be mentioned was the block structure. It was a tool for structuring the program, with the same use of the word “structure” as I used nine years later in the term “structured programming.” The concept of lexical scope was beautifully blended with nested lifetimes during execution, and I have never been able to figure out who was responsible for that synthesis, but I was deeply impressed when I saw it.

Finally, the definition of the semantics was much less operational than it was for existing programming languages. FORTRAN was essentially defined by its implementation, whereas with ALGOL 60 the idea emerged that the programming language should be defined independent of computers, compilers, stores, etc.; the definition should define what the implementation should look like. Now these are five or six issues that for many years the United States has missed, and I think that is a tragedy. It was the obsession with speed, the power of IBM, the general feeling at the time that programming was something that should be doable by uneducated morons picked from the street, it should not require any sophistication. Yes… false dreams paralyzed a lot of American computing science.

When did you understand that pro-
gramming was a deep subject?

I had published a paper called “ Recursive Programming,” again in Numerische Mathematik. 8 In 1961, I was beginning to realize that programming really was an intellectual challenge.