# Happy birthday, Dana!

Today Dana Scott is celebrating the 90th birthday today. Happy birthday, Dana! I am forever grateful for your kindness and the knowledge that I received from you. I hope to pass at least a part of it onto my students.

On the occasion Steve Awodey assembled selected works by Dana Scott at CMU-HoTT/scott repository. It is an amazing collection of papers that had deep impact on logic, set theory, computation, and programming languages. I hope in the future we can extend it and possibly present it in better format.

As a special treat, I recount here the story the invention of the famous $D_\infty$ model of the untyped $\lambda$-calculus. I heard it first when I was Dana's student. In 2008 I asked Dana to recount it in the form of a short interview.

These days domain theory is a mature branch of mathematics. It has had profound influence on the theory and practice of programming languages. When did you start working on it and why?

Dana Scott: I was in Amsterdam in 1968/69 with my family. I met Strachey at IFIP WG2.2 in summer of 1969. I arranged leave from Princeton to work with him in the fall of 1969 in Oxford. I was trying to convince Strachey to use a type theory based on domains.

One of your famous results is the construction of a domain $D_\infty$ which is isomorphic to its own continuous function space $D_\infty \to D_\infty$. How did you invent it?

D. S.: $D_\infty$ did not come until later. I remember it was a quiet Saturday in November 1969 at home. I had proved that if domains $D$ and $E$ have a countable basis of finite elements, then so does the continuous function space $D \to E$. In understanding how often the basis for $D \to E$ was more complicated than the bases for $D$ and $E$, I then thought, “Oh, no, there must exist a bad $D$ with a basis so 'dense' that the basis for $D \to D$ is just as complicated – in fact, isomorphic.” But I never proved the existence of models exactly that way because I soon saw that the iteration of $X \mapsto (X \to X)$ constructed a suitable basis in the limit. That was the actual $D_\infty$ construction.

Why do you say “oh no”? It was an important discovery!

D. S.: Since, I had claimed for years that the type-free $\lambda$-calculus has no “mathematical” models (as distinguished from term models), I said to myself, “Oh, no, now I will have to eat my own words!”

The existence of term models is guaranteed by the Church-Rosser theorem from 1936 which implies that the untyped lambda calculus is consistent?

D. S.: Yes.

The domain $D_\infty$ is an involved construction which gives a model for the calculus with both $\beta$- and $\eta$-rules. Is it easier to give a model which satisfies the $\beta$-rule only?

D. S.: Since the powerset of natural numbers $P\omega$ (with suitable topology) is universal for countably-based $T_0$-spaces, and since a continuous lattice is a retract of every superspace, it follows that $P\omega \to P\omega$ is a retract of $P\omega$. This gives a non-$\eta$ model without any infinity-limit constructions. But continuous lattices had not yet been invented in 1969 – that I knew of.

D.S.: I would recommend these two:

Thank you very much!

Dana Scott: You are welcome. 