Matt Brubeck

12 Mar 2010

Discovering Urbit: Functional programming from scratch

C. Guy Yarvin recently published a project in language and systems design. His own writing about his work is entertaining but verbose, so I will attempt to explain it in my own words here.

Nock, Urbit, Watt

First there is Nock, “a tool for defining higher-level languages – comparable to the lambda calculus, but meant as foundational system software rather than foundational meta­mathe­matics.” Its primitives include positive integers with equality and increment operators, cons cells with car/cdr/cadr/etc., and a macro for convenient branching. Nock uses trees of integers to represent both code and data. Nock is aggressively tiny; the spec linked above is just 33 terse lines.

Next, Yarvin provides the rationale for Nock. In short, he asks how a planet-wide computing infrastructure (OS, networking, and languages) would look if designed from first priniciples for robustness and interoperability. The answer he proposes is Urbit: a URI-like name­space distributed globally via content-centric networking, with a feudal structure for top-level names and cryptographic identities. Urbit is a static functional name­space: it is both referentially transparent and monotonic (a name, once bound to a value, cannot be un- or re-bound).

Why does this require a new formal logic and a new programming language? In Urbit, all data and code are distributed via the global namespace. For interoperability, the code must have a standard format. Nock's minimal spec is meant to be an un­ambiguous, unchanging, totally standardized basis for computation in Urbit. Above it will be Watt, a self-hosting language that compiles to Nock. Urbit itself will be implemented in Watt, so Nock and Watt are designed to treat data as code using metacircular evaluation.

The code

A prototype implementation of Watt is on GitHub. It is not yet self-hosting; the current compiler is written in C. Watt is a functional language with static types called “molds” and a mechanism for explicit lazy evaluation. (I was suprised to find I had accidentally created an in­com­patible lazy dialect of Nock – despite its goal of unambiguous semantics – just by implementing it in Haskell.)

The code is not fully documented, but the repository contains draft specs for both Watt and Urbit. Beware: the syntax and terminology are a bit unconventional. Yarvin has offered a few exercises to help get started with Nock and Watt:

The Nock challenge:
Write a decrement operator in Nock, and an interpreter that can evaluate it.
Basic Watt:
Write an integer square root function in Watt.
Advanced Watt:
How would you write a function that tests whether molds A and B are orthogonal (no noun is in both A and B)? Or compatible (any noun in A is also in B)? Are these functions NP-complete? If so, how might one work around this in practice?

If you want to learn more, start with these problems. You can email your solutions to Yarvin.

Will it work?

I find Urbit intellectually appealing; it is a simple and clean architecture that could potentially replace a lot of complex system software. But can we get there from here?

Yarvin imagines Urbit as the product of an ages-old Martian civilization:

Since Earth code is fifty years old, and Martian code is fifty million years old, Martian code has been evolving into a big ball of mud for a million times longer than Earth software. (And two million times longer than Windows.) …

Therefore, at some point in Martian history, some abject fsck of a Martian code-monkey must have said: fsck this entire fscking ball of mud. For lo, its defects cannot be summarized; for they exceed the global supply of bullet points; for numerous as the fishes in the sea, like the fishes in the sea they fsck, making more little fscking fishes. For lo, it is fscked, and a big ball of mud. And there is only one thing to do with it: obliterate the trunk, fire the developers, and hire a whole new fscking army of Martian code-monkeys to rewrite the entire fscking thing.

… This is the crucial inference we can draw about Mars: since the Martians had 50 million years to try, in the end they must have succeeded. The result: Martian code, as we know it today. Not enormous and horrible – tiny and diamond-perfect. Moreover, because it is tiny and diamond-perfect, it is perfectly stable and never changes or decays. It neither is a big ball of mud, nor tends to become one. It has achieved its final, permanent and excellent state.

Do Earthlings have the will to throw out the whole ball of mud and start from scratch? I doubt it. We can build Urbit but no one will come, unless it solves some problem radically better than current software. Moldbug thinks feudalism will produce better online reputation, but feudal reputation does not require feudal identity; it is not that much harder to build Moldbug's reputation system on Earth than on Mars. I still have not figured out the killer app that will get early adopters to switch to Urbit.