# AGENTS.md - tricu Project Guide

> For AI agents and contributors working in this repository.

## 0. Test Driven Development

Write and discuss tests with the user before working on implementation code. Do not modify existing tests without explicit permission.

## 1. Build & Test

```bash
# Haskell tests (default check)
nix flake check

# Zig build
nix build .#tricu-zig

# Zig tests (separate target — not part of nix flake check)
nix build .#tricu-zig-tests

# Full build
nix build .#
```

### ⚠️ Never call `cabal` directly

> **Rule of thumb:** if it builds, links, or tests, it goes through `nix`.

## 2. Project Overview

**tricu** (pronounced "tree-shoe") is a programming-language experiment written in Haskell. It implements [Triage Calculus](https://olydis.medium.com/a-visual-introduction-to-tree-calculus-2f4a34ceffc2), an extension of Barry Jay's Tree Calculus, with lambda-abstraction sugar that gets eliminated back to pure tree calculus terms.

### Core types (in `src/Research.hs`)

| Type | Description |
|------|-------------|
| `T = Leaf \| Stem T \| Fork T T` | Tree Calculus term (the runtime value) |
| `TricuAST` | Parsed AST with `SDef`, `SApp`, `SLambda`, etc. |
| `LToken` | Lexer tokens |
| `Node` / `MerkleHash` | Content-addressed Merkle DAG nodes |

### Source modules (Haskell)

| Module | Purpose |
|--------|---------|
| `Main.hs` | CLI entry point (`cmdargs`), three modes: `repl`, `eval`, `decode` |
| `Eval.hs` | Interpreter: `evalTricu`, `result`, `evalSingle` |
| `Parser.hs` | Megaparsec parser → `TricuAST` |
| `Lexer.hs` | Megaparsec lexer → `LToken` |
| `FileEval.hs` | File loading, module imports, `!import` |
| `REPL.hs` | Interactive Read-Eval-Print Loop (haskeline) |
| `Research.hs` | Core types, `apply` reduction, booleans, marshalling (`ofString`, `ofNumber`), output formatters (`toAscii`, `toTernaryString`, `decodeResult`) |
| `ContentStore.hs` | SQLite-backed term persistence |
| `Wire.hs` | Arboricx portable wire format — encode/decode/import/export of Merkle-DAG bundle blobs |

### Multi-language Arboricx ecosystem

Arboricx is the portable executable-object format used by tricu. The project now includes native parsing and execution in multiple languages:

| Language | Location | Capabilities |
|----------|----------|--------------|
| **Haskell** | `src/Wire.hs`, `src/Research.hs` | Reference implementation — bundle encode/decode, content store, full Tree Calculus reduction |
| **tricu (self-hosted)** | `kernel_run_arboricx_typed.dag` | A self-hosting Arboricx parser/executor written in tricu itself. Used as a kernel inside the Zig host for maximum portability ("cool but useless" — ~3s for `append`) |
| **Zig** | `ext/zig/` | **Production host** — native bundle parser, WHNF reducer, C ABI (`libarboricx.so` / `.a`), CLI (`tricu-zig`), Python FFI support |
| **JavaScript (Node)** | `ext/js/` | Native bundle parser, manifest decoder, Merkle DAG verifier, Tree Calculus reducer, CLI runner |
| **PHP** | `ext/php/` | FFI wrapper around `libarboricx.so`, CLI runner |

All hosts share the same bundle format and Merkle hashing scheme.

### File extensions

- `.hs` - Haskell source
- `.tri` - tricu language source (used in `lib/`, `test/`, `demos/`)
- `.arboricx` - Portable executable bundle
- `.dag` - Serialized kernel DAG (used by `gen_kernel.zig` at build time)

## 3. Test Suite

### Haskell tests

Tests live in `test/Spec.hs` and use **Tasty** + **HUnit**.

```bash
nix flake check
```

### Test groups

| Group | What it covers |
|-------|----------------|
| `lexer` | Megaparsec lexer - identifiers, keywords, strings, escapes, invalid tokens |
| `parser` | Parser - defs, lambda, applications, lists, comments, parentheses |
| `simpleEvaluation` | Core `apply` reduction rules, variable substitution, immutability |
| `lambdas` | Lambda elimination, SKI calculus, higher-order functions, currying, shadowing, free vars |
| `providedLibraries` | `lib/list.tri` - triage, booleans, list ops (`head`, `tail`, `map`, `emptyList?`, `append`, `equal?`) |
| `fileEval` | Loading `.tri` files, multi-file context, decode |
| `modules` | `!import`, cyclic deps, namespacing, multi-level imports, unresolved vars, local namespaces |
| `demos` | `demos/*.tri` - structural equality, `toSource`, `size`, level-order traversal |
| `decoding` | `decodeResult` - Leaf, numbers, strings, lists, mixed |
| `elimLambdaSingle` | Lambda elimination: eta reduction, SDef binding, semantics preservation |
| `stressElimLambda` | Lambda elimination stress test: 200 vars, 800-body curried lambda |

### Zig tests

Run separately via:

```bash
nix build .#tricu-zig-tests
```

These are **not** included in `nix flake check`. The test derivation compiles and runs:

| Test | What it covers |
|------|----------------|
| `c_abi_test.c` | Smoke tests — leaf, stem, fork, app, reduce, number/string roundtrip, kernel root |
| `c_abi_append_test.c` | Kernel path — `append.arboricx` with string arguments via Tricu kernel |
| `native_bundle_append_test.c` | Native fast path — `append.arboricx` loaded natively, applied, reduced |
| `native_bundle_id_test.c` | Native fast path — `id.arboricx` |
| `native_bundle_bools_test.c` | Native fast path — `true.arboricx` / `false.arboricx` |
| `python_ffi_test.py` | Python ctypes FFI — tests both kernel and native paths for `id` and `append` |

## 4. tricu Language Quick Reference

```
t                      → Leaf (the base term)
t t                    → Stem Leaf
t t t                  → Fork Leaf Leaf

x = t                  → Define term x = Leaf
id = (a : a)           → Lambda identity (eliminates to tree calculus)
head (map f xs)        → From lib/list.tri

!import "./path.tri" NS  → Import file under namespace

-- line comment
```

CRITICAL:

When working with recursion in `tricu` files:

1. Put consumed data first in recursive workers.
2. Let data shape drive recursion.
3. Do not let counters unroll over abstract input.

## 5. Output Formats

The `eval` command accepts `--form` (shorthand `-t`):

| Format | Value | Description |
|--------|-------|-------------|
| `tree` | `TreeCalculus` | Simple `t` form (default) |
| `fsl` | `FSL` | Full show representation |
| `ast` | `AST` | Parsed AST representation |
| `ternary` | `Ternary` | Ternary string encoding |
| `ascii` | `Ascii` | ASCII-art tree diagram |
| `decode` | `Decode` | Human-readable (strings, numbers, lists) |

## 6. Content Addressing

Each `T` term is content-addressed via a Merkle DAG:

```
NLeaf       → 0x00
NStem(h)    → 0x01 || h (32 bytes)
NFork(l,r)  → 0x02 || l (32 bytes) || r (32 bytes)

hash = SHA256("arboricx.merkle.node.v1" <> 0x00 <> serialized_node)
```

This is stored in SQLite via `ContentStore.hs`. Hash suffixes on identifiers (e.g., `foo_abc123...`) are validated: 16–64 hex characters (SHA256).

## 7. Arboricx Portable Bundles (`.arboricx`)

Portable executable bundles are generated via `Wire.hs`. See `docs/arboricx-bundle-format.md` for the full binary format spec.

```bash
# Export a bundle from the content store
./result/bin/tricu export -o myterm.arboricx myterm

# Run a bundle (requires TRICU_DB_PATH)
./result/bin/tricu import -f lib/list.tri
TRICU_DB_PATH=/tmp/tricu.db ./result/bin/tricu export -o list_ops.arboricx append
```

## 8. Zig Arboricx Host (`ext/zig/`)

The Zig host is a fast implementation for running Arboricx bundles. It provides a native bundle parser and arena-based evaluator.

### Modules

| File | Role |
|------|------|
| `src/main.zig` | CLI entrypoint — default native path, `--kernel` fallback |
| `src/bundle.zig` | Native Arboricx bundle parser — verifies digests, hashes, loads DAG into arena |
| `src/c_abi.zig` | C FFI exports — `arboricx_init`, tree constructors, codecs, reduction, bundle loading |
| `src/reduce.zig` | WHNF reducer (Tree Calculus `apply` rules) |
| `src/arena.zig` | Node arena (`ArrayListUnmanaged`) |
| `src/tree.zig` | `Node` union + iterative `copyTree` |
| `src/codecs.zig` | Number/string/list/bytes encoding + result unwrapping |
| `src/kernel.zig` | Embeds DAG kernel into arena (fallback path only) |
| `src/ternary.zig` | Ternary string parser for Tree Calculus terms |
| `tools/gen_kernel.zig` | Build-time tool: converts `.dag` → `kernel_embed.zig` |
| `include/arboricx.h` | C header for `libarboricx` |

### C ABI

Key functions:

```c
arb_ctx_t* arboricx_init(void);
uint32_t arb_load_bundle(arb_ctx_t*, const uint8_t* bytes, size_t len, const char* name);
uint32_t arb_load_bundle_default(arb_ctx_t*, const uint8_t* bytes, size_t len);
uint32_t arb_reduce(arb_ctx_t*, uint32_t root, uint64_t fuel);
```

`arb_reduce` evaluates in a **fresh scratch arena** so garbage never accumulates.

### Stack size requirement

Tree Calculus reduction is deeply recursive. Assume a segfault is a memory limitation until proven otherwise.

```bash
ulimit -s 32768   # 32 MB
```

### Performance comparison

| Fixture | Native path | Kernel path (`--kernel`) |
|---------|-------------|--------------------------|
| `append "hello " "world"` | **~0.007 s** | ~3.4 s |
| `id "hello"` | **~0.005 s** | ~0.38 s |

The kernel path is kept as a "cool but useless" fallback — the DAG is tiny (~30 KB) so the cost is negligible.

## 9. Nix Flake Outputs

| Output | Description |
|--------|-------------|
| `packages.default` / `packages.tricu` | Haskell tricu package |
| `packages.tricu-zig` | Zig CLI + `libarboricx.a` + `libarboricx.so` + `arboricx.h` |
| `packages.tricu-zig-tests` | **Separate test target** — C ABI + native bundle + Python FFI tests |
| `packages.tricu-php` | PHP source + `libarboricx.so` + `tricu-php` wrapper script |
| `packages.tricu-php-tests` | **Separate test target** — PHP FFI tests against fixture bundles |
| `packages.tricu-container` | Docker image |
| `checks.default` / `checks.tricu` | Haskell test suite via Tasty/HUnit |

`tricu-zig-tests` is deliberately **not** in `checks` so `nix flake check` remains fast.

## 10. Directory Layout

```
tricu/
├── flake.nix           # Nix flake: packages, tests, devShell
├── tricu.cabal         # Cabal package (used via callCabal2nix)
├── AGENTS.md           # This file
├── src/                # Haskell modules
│   ├── Main.hs
│   ├── Eval.hs
│   ├── Parser.hs
│   ├── Lexer.hs
│   ├── FileEval.hs
│   ├── REPL.hs
│   ├── Research.hs
│   ├── ContentStore.hs
│   └── Wire.hs
├── test/
│   ├── Spec.hs         # Tasty + HUnit tests
│   ├── *.tri           # tricu test programs
│   ├── *.arboricx      # Arboricx bundle fixtures
│   └── local-ns/       # Module namespace test files
├── lib/
│   ├── base.tri
│   ├── list.tri
│   └── patterns.tri
├── demos/
│   ├── equality.tri
│   ├── size.tri
│   ├── toSource.tri
│   ├── levelOrderTraversal.tri
│   └── patternMatching.tri
├── ext/                # Multi-language Arboricx hosts
│   ├── js/             # Node.js bundle parser + reducer
│   │   ├── src/
│   │   │   ├── bundle.js
│   │   │   ├── manifest.js
│   │   │   ├── merkle.js
│   │   │   ├── tree.js
│   │   │   ├── codecs.js
│   │   │   └── cli.js
│   │   └── test/
│   ├── php/            # PHP FFI host for libarboricx.so
│   │   ├── src/
│   │   │   └── ffi.php
│   │   └── run.php
│   └── zig/            # Zig production host
│       ├── build.zig
│       ├── build.zig.zon
│       ├── kernel_run_arboricx_typed.dag
│       ├── include/arboricx.h
│       ├── src/
│       │   ├── main.zig
│       │   ├── bundle.zig
│       │   ├── c_abi.zig
│       │   ├── codecs.zig
│       │   ├── kernel.zig
│       │   ├── reduce.zig
│       │   ├── arena.zig
│       │   ├── tree.zig
│       │   └── ternary.zig
│       ├── tests/
│       │   ├── c_abi_test.c
│       │   ├── c_abi_append_test.c
│       │   ├── native_bundle_append_test.c
│       │   ├── native_bundle_id_test.c
│       │   ├── native_bundle_bools_test.c
│       │   └── python_ffi_test.py
│       └── tools/
│           └── gen_kernel.zig
└── docs/
    └── arboricx-bundle-format.md
```

## 11. Content Store Workflow (Custom DB)

The content store location is controlled by the `TRICU_DB_PATH` environment variable. When set, `eval` mode automatically loads all stored terms into the initial environment, so you can call any previously imported/evaluated term by name.

```bash
# Use a local DB
export TRICU_DB_PATH=/tmp/tricu-local.db

# Import terms from the standard library
./result/bin/tricu import -f lib/list.tri

# Now use them in eval mode
echo "not? (t t)" | ./result/bin/tricu eval -t decode
# Output: t

echo "not? (t t t)" | ./result/bin/tricu eval -t decode
# Output: Stem Leaf

echo "equal? (t t) (t t t)" | ./result/bin/tricu eval -t decode
# Output: t

# Check what's in the store
./result/bin/tricu
t> !definitions
```

Without `TRICU_DB_PATH` set, `eval` uses only the terms defined in the input file(s).

## 12. Development Tips

- **REPL:** `nix run .#` starts the interactive tricu REPL.
- **Evaluate files:** `nix run .# -- eval -f demos/equality.tri`
- **Zig host:** `nix build .#tricu-zig` then `./result/bin/tricu-zig <bundle> [args...]`
- **Zig tests:** `nix build .#tricu-zig-tests`
- **GHC options:** `-threaded -rtsopts -with-rtsopts=-N` for parallel runtime. Use `-N` RTS flag for multi-core.
- **Upx** is in the devShell for binary compression if needed.

## 13. Viewing Haskell Dependency Docs from Nix

When you need Haddock documentation for a Haskell dependency available in Nixpkgs, build the package's `doc` output directly with `^doc`.

Example:

Replace `megaparsec` with the dependency name you need:

```sh
nix build "nixpkgs#haskellPackages.${pkg}^doc"
```

View the available documentation files:

```sh
find ./result-doc -type f \( -name '*.html' -o -name '*.haddock' \) | sort
```