XIX

Plan 9 programs, rewritten in OCaml

What's New · About · The Books · Source Code

XIX (a recursive acronym for "XIX is XIX") is a port of the C source code of major Plan 9 programs to OCaml, an industrial-strength functional programming language. The ports include the build system mk, the shell rc, the ARM and MIPS toolchains (5a, 5c, 5l), the windowing system rio, and more.

The project also includes programs beyond Plan 9: ocaml-light, a simplified OCaml compiler used to run OCaml programs on Plan 9; efuns, an Emacs clone; ogit, a git implementation; and mmm, a web browser.

XIX is a companion project to Principia Softwarica, a series of literate programming books explaining Plan 9 source code. Each OCaml port has its own book (see the full list).

Publications

IWP9 paper — a short paper submitted to the International Workshop on Plan 9 describing the XIX project

What's New

March 2026 — We're live! The XIX website is now online.
Q1 2026 — OCaml editor (oed) v1; resumed Plan 9 port (orio runs on Plan 9 via ocaml-light).
Q4 2025 — ELF/Linux support in assembler and linker; multi-architecture toolchain; new utilities (ocat, opwd, owc, oed); Dockerfile.
2019–2024 — Hiatus (working on Semgrep).
2018 — Git algorithms (diff3, diff_myers); syncweb code indexing.
2017 — Git port (ogit); rio and libdraw port; OCaml kernel on Raspberry Pi.
2016 — Ports of mk, rc, 5a, 5l, 5c, cpp, lex, yacc to OCaml.
2015 — Project started: port of 5a (assembler), import of lex/yacc.

About

Why port Plan 9 to OCaml?

The original motivation comes from the Principia Softwarica project, a series of books explaining Plan 9 source code. One strategy to fully understand a program — in order to explain its code in a book — is, perhaps surprisingly, to try to port it to another language. During the porting process, it is tempting to skip features initially judged to be accessory. Discovering what is truly essential already helps understand the original code. Moreover, the final port very often does not work at first because those "accessory" features turned out to be essential, which reveals hidden subtleties in the original.

Useful on their own

The ports turned out to be useful on their own. OCaml programs are portable — unlike the Plan 9 C code, most XIX programs run on Linux, macOS, and Windows without extra effort. The OCaml code is also roughly 2x more succinct than C, arguably clearer, and safer (no segfaults or buffer overflows). In fact, it became easier to experiment with new features first in OCaml, and only later port them back to the original C code.

The pair omk/orc is mature enough to drive the build process of XIX itself, dogfooding those tools. And orio can substitute the original rio in Plan 9 without any visible difference.

OCaml in Plan 9

Running OCaml programs on Plan 9 required porting OCaml itself. Fortunately, OCaml has a bytecode compiler (ocamlc) whose interpreter (ocamlrun) is a small C program with very few dependencies. By cross-compiling this interpreter with the Plan 9 C compiler, you can then run ocamlc itself to compile other programs. This work is available as ocaml-light, based on an old version of OCaml (1.07) chosen for its minimal dependencies.

Here is orio running on Plan 9, substituting the original C rio windowing system. The terminal windows show various XIX programs in use: omk building code, orc as the shell, and oas/olk compiling and linking programs — all OCaml binaries running on Plan 9 via ocamlrun.

orio running on Plan 9 - the OCaml windowing system replacing rio

The Books

Each OCaml port has its own literate programming book, just like the C originals in the Principia series. The books can be read standalone, but they occasionally reference their C counterparts for comparison.

LOC = lines of code; LOE = lines of explanation; Pages = typeset pages.

Category	Book	Program	Ported from	LOC	LOE	LOE/ LOC	Pages	Version
Core system	Shell	orc	rc	2150	1600	0.74	117	v1
Development toolchain	C compiler	occ	5c	6250	850	0.14	173	v1
	Assembler	oas	5a	1750	350	0.20	76	v1
	Linker	olk	5l	2650	500	0.19	96	v1
	OCaml compiler	ocaml-light	ocamlc/ocamlrun	16300	2250	0.14	500	v3
	Lex and Yacc	olex, oyacc	lex, yacc	3500	800	0.23	111	v3
Developer tools	Editor	oed	ed	1500	350	0.23	55	v1
	Editor (advanced)	efuns	Emacs	5500	4600	0.84	175	v6
	Build system	omk	mk	2750	1400	0.51	112	v1
	Version control	ogit	git	5600	4400	0.79	202	v10
Graphics	Windowing system	orio	rio	3300	550	0.17	123	v1
Networking	Web browser	mmm		22350	2450	0.11	499	v3

Source Code

The OCaml ports are spread across several repositories:

github.com/aryx/xix — shell, toolchain, build system, editor, git, windowing system
github.com/aryx/efuns — Emacs clone in OCaml
github.com/aryx/mmm — web browser in OCaml
github.com/aryx/ocaml-light — simplified OCaml compiler

XIX is part of the Principia Softwarica project.