CUE: An open-source data validation language

There are two core aspects of CUE that make it different from the usual programming or configuration languages:

- Types are values
- Values (and thus types) are ordered into a lattice

These properties are relevant almost to everything that makes CUE what it is. They simplify the language, as many concepts that are distinct in other languages fold together. The resulting order independence simplifies reasoning about values for both humans and machines.

It also forces formal rigor on the language, such as defining exactly what it means to be optional, a default value, or null. Making sure all values fit in a value lattice leaves no wiggle room.

The application of set-theoretic types is getting more and more mainstream as they turn out to be very effective in (partially) typing dynamic languages (i.e. Typescript). Other popular examples are Scala(3) and Kotlin, together with other (less mainstream) examples such as Ceylon, Typed Racket and the Avail programming language.

I've dabbled in Avail a few years ago, and was very impressed by Avail's type driven compiler, but not so much by its (extreme) DSL support. In contrast to Avail, I believe CUE is much more pragmatic while not allowing any 'tactical' compromises.

CUE has its origins at Google: the ability to compose CUE code/specifications is really useful at the scale that Google operates. The creator of CUE - Marcel van Lohuizen - has recently quit his job at Google to become a full-time developer of CUE. I think Marcel has created something really interesting, so I understand his decision!