Arcturus

Arcturus is a programming language and compiler for the Infocom Z-machine.

It is a high-level, readable language for writing interactive fiction: text adventures in the tradition of Zork and the modern works that still run on the Z-machine. You describe a world (rooms, things, verbs, and the behavior that hangs off them) and the compiler produces a standard Z-machine story file that plays on Frotz, Ozmoo, Eris, Vezza and other interpreters, old and new.

Arcturus is designed and written by Stefan Vogt. The compiler is written in Python and depends only on the standard library, so it runs anywhere Python does, with nothing to install. The standard library, Cosmos, is written in Arcturus itself and ships as editable source rather than a black box.

The name is a star, Arcturus, and the narrative arc every story is built on.

Quickstart

The whole toolchain is two self-contained files (a third if your game uses pictures). Download them and you are ready: no installation, no packages, nothing to build. All you need is Python 3.11 or later, which you almost certainly already have.

Component	Version	Download
arcc, the compiler (the Cosmos library is embedded inside it)	0.12.2	build/arcc
Cosmos, the standard library	0.36.6	shipped inside arcc
Actaea, the reference interpreter	1.0.4	build/actaea
arcimg, the arc_image tool (optional, for graphics)	1.10.0	build/arcimg

Each is one self-contained file: download, chmod +x, done. Keeping them current is one command: arcc --update refreshes all three in place (the only time arcc ever touches the network; there is no passive check).

Write a game, compile it, play it:

python3 arcc mygame.storyarc -o mygame.z5    # compile to a Z-machine story
python3 actaea mygame.z5                      # play it in a window,
python3 actaea --console mygame.z5            # or in the terminal

The story file mygame.z5 is a standard Z-machine v5 file: it also plays on Frotz, Ozmoo, and any other interpreter, old or new. Then read the docs and go: start with the syntax reference (the language, with two complete worked games) and, when you want to play or debug, the Actaea guide. The full documentation index is below.

Philosophy

Arcturus is built for one thing and built for it well: writing modern adventure games for the Z-machine. It gives authors the constructs they actually reach for (rooms and things and kinds, containers and doors, scope and light, an NPC conversation model, daemons and timers, computed descriptions, multi-room scenery), each as clean syntactic sugar, so the common cases are short, readable, and hard to get wrong. This is a complete, powerful core, and Arcturus is its own language, not a dialect of any other.

Cosmos, the standard library, is editable Arcturus source you own outright: read it, override any behavior, and reshape it to your game rather than work around a black box. Optional and specialized features are granules you summon, present only where a game wants them and costing nothing where it does not. And because Arcturus owns its whole pipeline (the compiler, the library, and the interpreter), it can be this expressive and still compile small, with whole-program optimization trimming every build to exactly what the game uses.

What's new

Arcturus is in active, healthy development, and the core is solid enough to build real games on today. The three pieces of the toolchain - the compiler, the Cosmos standard library, and the Actaea reference interpreter - are all in stable shape and work together as a real pipeline: write a game, compile it, and play it, start to finish. You will still meet the occasional bug (please report it), but this is no longer use-at-your-own-risk territory. If you write interactive fiction, this is a good time to pick it up.

The most significant recent additions and achievements:

• Kinds without a ceiling. A kind is Arcturus's class sugar, and it used to spend one of the Z-machine's 48 precious attributes even when you only meant to organize objects or span scenery. Not any more: a kind costs an attribute only where you actually test obj is <kind>, so the kinds you use purely to share behavior or distribute scenery are free. Test more kinds than the spare attribute slots hold and the rest fall back automatically to a fast membership scan, built on Arcturus's own catalog feature (what you know as lists in other languages), so you can declare as many kinds as your world wants. This is very efficient on memory and performance on 8-bit hardware compared to Inform's classes. The only real ceiling is now the Z-machine's own, 48 genuine object attributes, and arcc -s shows attributes and kinds separately so you always read your true budget (attributes 26/48, kinds 63). It grew from a large real-world port hitting the old wall, and it is exactly what that port asked for.
• Catalogs: list power without the list tax. catalog declares a fixed, ordered collection, one value per line (the lines of a letter, a roster of suspects, a table of numbers), and the operations read like prose: calculate for how many, entry(x, 3) for the third, last, dice for one at random, position for where something sits, plain in for membership, for each to iterate, and change entry(...) to rewriting one entry in place. Underneath there is no heap and no runtime library, only static tables and single opcodes, so it runs fast on a C64; a game that declares no catalog pays zero bytes. The quote box draws a whole letter from one with one call (worked example).
• SWIM SOUTH parses, and TRANSCRIPT records. A grammar line can end in a direction slot (push noun direction, swim direction), so PUSH CRATE WEST and SWIM SOUTH reach their handlers with the direction riding way, the same slot GO uses; hand the move to the walking machinery whole with perform("go", way). And every game now answers the classic TRANSCRIPT / SCRIPT and TRANSCRIPT OFF / UNSCRIPT, honestly: the library reads the interpreter's own flag back, so a cancelled file prompt never claims a recording. Worded natively in all three languages (MITSCHRIFT AN, TRANSCRIPCION), and the English meta words (SAVE, UNDO, SCORE, TRANSCRIPT and their kin) answer in every pack, so the session is never hostage to a guessed translation. See examples/features/direction-grammar.storyarc.
• arc_image reaches the retro machines. The same numbered pictures a modern build shows now convert to the 8-bit and 16-bit machines' own formats: paint ONE master per scene, and arcimg convert derives the native version for the Commodore 64, ZX Spectrum +3, Amstrad CPC, Amiga, Atari ST, and DOS, resolving each machine's palette and color-cell constraints, with PNG previews to judge without an emulator, an author hint that keeps a moon or sun visible on the narrowest palettes, and a polish loop that round-trips Spectrum art through any .scr editor. Reference loaders are proven on real emulators for four machines so far, and the interpreter blueprints ship with the toolkit (docs/07-arc-image.md for authors).
• perform and appearance: the classic bridges, grown from the field. perform("take", book) runs any action as part of the current turn, refusals, messages, and after-phase included, with the action name checked at compile time (Inform's <<take book>>, Dialog's (try ...)); the appearance property is the paragraph an object always owns in a room description ("The keeper is trimming the wick."), worded by state when computed, beside intro's until-first-taken rule. Both came from early adopters porting real games, and both cost nothing in a game that never uses them; component objects (a lever that is component of its machine) arrived the same way. Worked examples: perform, appearance, components.

The language

The authoritative language definition is the syntax reference, which is enough to start writing Arcturus today:

• docs/01-syntax-reference.md: the language. Grammar, every construct, and two complete worked example games.
• docs/02-cosmos-and-parser.md: the runtime. The Cosmos library, the parser, the action pipeline, scope, light, and the turn loop.
• docs/05-granules.md: the summonable granules. How to summon them, how to fork one, and how to write your own.
• docs/06-actaea.md: Actaea, the reference interpreter. The three ways to play, the tools, saves and transcripts, and conformance.
• docs/07-arc-image.md: pictures in your story. The master art, the arcimg workflow from PNG to every machine, and what plays where today.

For the curious who want to see under the hood, two further documents cover how the compiler itself works:

• docs/03-compiler-pipeline.md: the compiler. The pass pipeline, the command-line interface, how Cosmos is bundled and overridden, the single-file distribution, and the version model.
• docs/04-codegen-mapping.md: the backend. How Arcturus constructs map to Z-machine opcodes and the story-file image, plus the size levers (dead-code elimination and abbreviation text compression).

A fuller wiki will follow as the project matures. For a taste, the two worked games live under examples/ - the Brass Lantern and the classic Cloak of Darkness. Small teaching showcases sit alongside them: examples/features/ isolates core-language features (the container knowledge model, computed properties, kinds and inheritance, doors and locks, multi-room scenery with spans, the intro first-look property, grains, positional grammar, the object catch-all, daemons and timers, Z-machine colours with the self-restoring coloured say, and the player object with its standard self-words, pronouns, and Spanish clitic forms), and examples/granules/ shows the summonable granules (the Infocom-style and menu-driven conversation systems, the status line, verbose exits, the extended verbs, the nautical directions, and the quote box).

File extensions

Arcturus uses three source extensions, named after the star:

• .storyarc is a story: an author's game, the program you compile.
• .prelude is a core Cosmos library file. The library is the prelude loaded before your story; the standard library is editable Arcturus source you can read and override.
• .granule is a summoned module: anything brought in with summon, whether a third-party extension or an optional Cosmos feature or language pack (it loads only when summoned). Granules are the convection cells that tile the Sun's photosphere; since Arcturus is a star, a summoned module is a granule on its surface.

Output

Arcturus emits conformant Z-machine version 5 story files by default. Pass --zversion 8 for a version 8 target for large, modern-only releases (the same code generation, a larger 512KB story-file ceiling).

Getting started

You need Python 3.11 or newer. Nothing else. If you only want to write games, the Quickstart above is the whole story: download build/arcc and build/actaea and go. This section is for working from a clone of the repository, and for rebuilding the standalones from source.

The compiler

The prebuilt standalone is tracked at build/arcc, so a fresh clone can compile immediately:

python3 build/arcc examples/brass-lantern.storyarc -o brass-lantern.z5
python3 build/actaea brass-lantern.z5     # or frotz brass-lantern.z5

To rebuild it from the package after changing the compiler or Cosmos (a single self-contained script, the whole Cosmos library embedded inside it, no dependencies):

python3 tools/amalgamate.py            # writes build/arcc
python3 tools/amalgamate_actaea.py     # writes build/actaea

Useful options:

python3 build/arcc game.storyarc -o game.z5    # compile to a z5 story file
python3 build/arcc game.storyarc --zversion 8 -o game.z8   # target z8 (larger games)
python3 build/arcc game.storyarc -o game.z5 -q # script mode: no banner, no statistics
python3 build/arcc game.storyarc --dump-ast    # show the parsed syntax tree
python3 build/arcc -L /abs/path/cosmos game.storyarc   # use a forked library
python3 build/arcc --make-abbreviations game.storyarc  # tune text compression (below)
python3 build/arcc --version

Every compile prints a statistics ledger: what the story uses of each Z-machine ceiling (attributes, properties, globals, memory, story size), so you can watch the headroom as a game grows; -q silences it for scripts.

Text compression is automatic: every build packs its text against a standard abbreviation set with nothing to configure. To squeeze a large story further, --make-abbreviations computes a set tuned to its own text and writes an abbreviations.granule beside it; add summon abbreviations.granule to the story to use it in place of the default.

Cosmos travels inside arcc, so the compiler works wherever you put it, but it is never locked away. To hack the library:

python3 build/arcc --eject-granule statusline  # write one granule here, to fork it
python3 build/arcc --eject-language .           # write english.prelude (the messages)
python3 build/arcc --extract-library /abs/cosmos   # write the whole library out to edit
python3 build/arcc -L /abs/cosmos game.storyarc    # compile against your edited copy

--eject-granule pulls a single granule out to fork one feature; summon your copy by name (summon statusline.granule) and it wins over the bundled one. --eject-language drops the English language file (where the standard messages live) beside your story for message customization or a translation. --extract-library writes every bundled .prelude and .granule for wholesale forking; then -L (an absolute path) points the compiler at your copy. For a single standard message you need not extract anything: redefine its block (for example block msg_jump()) in your own story and it overrides the library's. A granule's own blocks are not overridable that way - you fork the granule; docs/05 covers the model.

Run from the package (for development)

python3 -m arcturus examples/brass-lantern.storyarc
python3 -m pytest

Editor support

A Visual Studio Code extension provides syntax highlighting for .storyarc, .prelude, and .granule files. Install the packaged extension:

• In VS Code: Extensions view, ... menu, Install from VSIX..., then choose the arcturus-<version>.vsix in editors/vscode/ (exactly one ships there, always the current one); or
• from a terminal: code --install-extension editors/vscode/arcturus-*.vsix

This works on macOS, Windows, and Linux. The extension source is under editors/vscode/; rebuild the .vsix with python3 tools/build_vsix.py.

How Arcturus is built

The language design, the syntax, the Cosmos library, and the compiler are my work, and Arcturus is a human-driven project. Many of you know me: you played my games, like the Hibernated series, Ghosts of Blackwood Manor, or The Curse of Rabenstein, or maybe used one of my tools for your own interactive fiction games. For Arcturus, I work with Anthropic's Claude Code as a coding assistant, and it implements and debugs the toolchain under my direction. Python is my language of choice and I work with it on a daily basis as regional head for one of the departments of a global software company. With that being said, Claude does not do what I couldn't do myself. I know that some of you out there have mixed feelings or even objections against the use of AI. Please consider this: a coding assistant buys me the time and the efficiency to create and maintain a project of this scope and quality for the community. The result is a language full of syntactic sugar, contemporary and easy to use, inspiring and motivating authors to write faster and better, and the outcome is new opportunities for the interactive fiction community and new, wonderful human-made stories. And that is what matters the most to me.

The compiler is developed as a clean, modular Python package under arcturus/ (lexer, parser, AST, and the later semantic and code generation stages). For distribution it is amalgamated into the single arcc script, so the shipped artifact stays identical to the source.

Join the community on Discord

https://discord.gg/JF6YNUTPfT

License

MIT. See the headers in each source file.

Credits

Thanks to Pablo Martínez for the translation of the Spanish language granule.

Special thanks to Charles Moore Jr. for early adoption and bug hunting.