TacitJ

A self-hosting compiler for a tacit-leaning subset of J, written in J.

Quick start | Project layout | Documentation | Contributing

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What is TacitJ?

TacitJ is a compiler for a curated subset of J, the array language known for its terse, point-free, tacit style. The compiler's own source is written in that same tacit style, and the goal is for the compiler to compile itself (a self-hosting bootstrap).

The optimiser uses MDL-inspired compression (grammar induction over J expressions): writing less code makes the compiler smarter at parsing it.

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Why?

J's tacit style (trains, hooks, forks, gerunds, compositions) is what makes it a point-free array language. But the language is large and idiosyncratic; the canonical interpreter is a closed-source commercial product.

TacitJ asks: what is the smallest subset of J you can build a real, useful compiler for, while keeping the result fun to write?

Constraint	Consequence
Compiler is self-hosting	Source must be a strict subset that compiles cleanly under itself
Tacit-first style	Top-level pipeline is one line: compile =: codegen @ opt @ sem @ parse @ lex
Optimiser uses MDL / grammar induction	Less code → lower description length → "smarter" parses
J 9.7 stdlib only	Zero external dependencies; the whole toolchain fits in your $PATH
< 2000 LOC core for Stages 0–3	Forces aggressive reuse of J's built-ins

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Quick start

Prerequisites

• J 9.7+ (Dyalog-compatible subset)

# macOS
brew install --cask j

# Linux / Windows: see https://www.jsoftware.com/

Run

git clone https://github.com/Lulzx/tacitj.git
cd tacitj

# Run the test suite (225 tests)
make test

# Run an example
make run EXAMPLE=examples/mean.ijs

# Start the REPL
make repl

# Run the bootstrap pipeline
make bootstrap          # stage 0 -> 1 -> 2 -> 3
make stage2             # freeze output contract + tacit-density baseline
make stage3             # honest partial self-host milestone
make stage1 INFILE=examples/hello.ijs OUTFILE=bin/hello.ijs

# Run the benchmark suite
make bench              # compile-ms / out-chars / exec-ms per canary

# Run the MDL / grammar-induction demo
make mdl-demo           # mdlScore + grammarInduce + mdlMinimize

# Run the pipeline-trace demo (each stage's output for a sample)
make trace

The Makefile auto-detects the Homebrew J cask (/opt/homebrew/Caskroom/j/*/j*/bin/jconsole). If jconsole isn't found there or on $PATH, override it:

JC=/full/path/to/jconsole make test

Note: /usr/bin/jconsole on macOS is the Java JMX console, not JSoftware's J. The Makefile skips it in favour of the Homebrew cask so make test doesn't hang.

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Project layout

tacitj/
├── src/               compiler source (lex, parse, sem, ir, opt, eval, codegen, tacitj)
├── tests/             J test suite (runtests + test_<module>.ijs)
├── examples/          runnable example programs
├── bootstrap/         Stage 1-3 self-host scripts
├── bench/             benchmark / MDL / trace demos
├── docs/              language reference, examples, architecture, roadmap, design
├── SPEC.md            full technical specification
├── AGENTS.md          operating manual for AI agents
├── Makefile           build / test / smoke / run / repl
├── LICENSE            MIT
└── README.md          this file

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Documentation

• docs/language.md: the TacitJ language reference (verbs, adverbs, conjunctions, forks and hooks).
• docs/examples.md: walkthroughs of every example program.
• docs/architecture.md: the compiler pipeline and IR.
• docs/roadmap.md: roadmap, bootstrap stages, Stage 0 subset, MDL / grammar induction, and the full release history.
• docs/design.md: architecture, decisions, trade-offs.
• SPEC.md: the full technical specification (BNF grammar, component contracts, 5-stage bootstrap strategy, Solon/MDL integration).
• CHANGELOG.md: release notes.

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Contributing

This repo is in early bootstrapping. Two kinds of contributors are welcome:

For humans

The codebase is J 9.7 only, no external dependencies. Edit freely, but:

1. Read SPEC.md for the language subset and architecture.
2. Read src/tacitj.ijs, the canonical composition order.
3. Run make test before sending a patch.
4. Match the existing code style (J banner comments per file, boxed triples for AST nodes).

For AI agents

See AGENTS.md for the operating manual: toolchain, code style, verification rules, what not to do. (TL;DR: don't add dependencies, don't change the AST node shape without updating all consumers, don't commit until asked.)

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Acknowledgments

• JSoftware for the J language and interpreter.
• Kenneth Iverson for inventing APL and J.
• The broader array-programming community.
• The MDL / Solomonoff induction tradition for the compression-as-intelligence idea that drives the optimiser design.

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License

MIT © 2026 TacitJ contributors.

J is a trademark of JSoftware. This project is an independent, community compiler project and is not affiliated with or endorsed by JSoftware.