# ADR-001: Nickel Plugin CLI Wrapper Architecture\n\n## Status\n\n**Accepted** - 2025-12-15\n\n## Context\n\nThe nu_plugin_nickel project provides Nushell integration for Nickel configuration language. The core decision was whether to implement this as:\n\n1. **Pure Rust Implementation** (using `nickel-lang-core` crate directly)\n2. **CLI Wrapper** (using `Command::new("nickel")` to invoke external binary)\n\n### Technical Constraints\n\nNickel is a **configuration language with module system**:\n\n- Import system: `import "path/to/module"`\n- Module resolution with search paths\n- Standard library (`builtins`, stdlib packages)\n- Complex evaluation context\n- Package management system\n\n### User Requirements\n\nConfiguration files often use Nickel's module system for:\n\n- Code organization\n- Reusable configurations\n- Standard library access\n- External module dependencies\n\n## Decision\n\nImplement nu_plugin_nickel as a **CLI Wrapper** that invokes the external `nickel` binary.\n\n### Architecture\n\n```plaintext\nNushell Script\n    ↓\nnickel-export (plugin command)\n    ↓\nhelpers.rs: run_nickel_command()\n    ↓\nstd::process::Command::new("nickel")\n    ↓\nNickel CLI (official binary)\n    ↓\nModule Resolution (guaranteed correct)\n    ↓\nJSON/YAML Output\n    ↓\nPlugin: serde_json::Value → nu_protocol::Value\n    ↓\nNushell Records/Lists\n```\n\n### Implementation Details\n\n**Core Functions** (`helpers.rs`):\n\n```rust\npub(crate) fn run_nickel_command(\n    file: &str,\n    format: &str,\n    output: Option<&str>,\n) -> Result<String>\n```\n\n**Plugin Commands** (`main.rs`):\n\n1. `nickel-export` - Export/evaluate Nickel files (JSON/YAML)\n2. `nickel-eval` - Evaluate with automatic caching (primary config loader)\n3. `nickel-format` - Format Nickel files\n4. `nickel-validate` - Validate Nickel files/directories\n5. `nickel-cache-status` - Show cache information\n\n**Output Processing**:\n\n- Invokes: `nickel export /file.ncl --format json`\n- Captures: stdout (JSON string)\n- Parses: serde_json::Value\n- Converts: `json_value_to_nu_value()` recursive function\n- Returns: nu_protocol::Value (records/lists, not strings)\n\n**Caching** (non-blocking, graceful degradation):\n\n- SHA256 content-addressed cache\n- Location: `~/.cache/provisioning/config-cache/`\n- Key: SHA256(file_content + format + context)\n- Hit rate: Expected 80-90% in typical workflows\n\n### Type System\n\nCommand signatures declare `Type::Any` output:\n\n```rust\n.input_output_type(Type::Any, Type::Any)\n```\n\nThis allows:\n\n- Plugin returns: nu_protocol::Value::Record\n- Nushell receives: proper record (not string)\n- Cell path access works: `nickel-export json /file.ncl | .config | .servers`\n\n## Rationale\n\n### Why CLI Wrapper Over Pure Rust\n\n| Aspect | Pure Rust (nickel-lang-core) | CLI Wrapper (chosen) |\n|--------|----------------------------|----------------------|\n| **Module resolution** | ❓ Undocumented | ✅ Works automatically |\n| **Import system** | ❌ Unclear how to use | ✅ Built-in |\n| **Standard library** | ❌ Access unclear | ✅ Automatic |\n| **Search paths** | ❓ How to configure? | ✅ CLI handles |\n| **Maintenance** | ❌ Track CLI changes | ✅ No maintenance |\n| **Error handling** | ❌ Different from CLI | ✅ Same as CLI |\n| **Complexity** | 🔴 High (undocumented) | 🟢 Low |\n| **External CLI** | ✅ None needed | ✅ Requires nickel binary |\n\n### Why Not Pure Rust\n\nUsing `nickel-lang-core` directly would require:\n\n1. **Understand module resolution**:\n   - How does it find imported modules?\n   - What are the search paths?\n   - How does it resolve `import "base/package"`?\n\n2. **Access standard library**:\n\n   ```rust\n   // Where is the stdlib?\n   let stdlib_path = find_nickel_stdlib()?;\n   // Is it version-dependent?\n   // How to verify?\n   ```\n\n3. **Handle evaluation context**:\n   - Build context configuration\n   - Search path management\n   - Module caching\n   - Dependency resolution\n\n4. **Match CLI behavior exactly**:\n   - Error messages\n   - Validation rules\n   - Output formatting\n   - Export modes\n\nThis requires **deep understanding of Nickel internals** and **maintaining parity with CLI**.\n\n### Single Source of Truth\n\nDelegating to the CLI ensures:\n\n- ✅ Official implementation handles all cases\n- ✅ Nickel updates automatically available\n- ✅ No maintenance burden\n- ✅ Guaranteed compatibility\n\n## Consequences\n\n### Positive\n\n- **Correctness**: Module resolution guaranteed correct by official CLI\n- **Simplicity**: No need to reverse-engineer Nickel internals\n- **Maintenance**: Updates to Nickel automatically available\n- **Features**: All CLI features automatically supported\n- **Compatibility**: Works with all Nickel versions\n- **Reliability**: Single point of truth (official implementation)\n- **Error Handling**: Consistent with CLI user expectations\n\n### Negative\n\n- **External Dependency**: Requires `nickel` binary in PATH\n- **Performance Overhead**: Process fork (~100-200ms vs direct call)\n- **Process Management**: Spawns subprocess for each execution\n- **Error Output**: Subprocess stderr handling required\n\n### Mitigations\n\n**For External Dependency**:\n\n- Clear documentation: setup guide with Nickel installation\n- Error messages: helpful if `nickel` not found\n- Distribution: Nickel included in provisioning distributions\n\n**For Performance Overhead**:\n\n- Caching: 80-90% hit rate in typical workflows\n- Cache hits: ~1-5ms (not 100-200ms)\n- Lazy evaluation: Only runs when needed\n\n## Alternatives Considered\n\n### Alternative 1: Pure Rust with nickel-lang-core\n\n**Rejected**: Module system undocumented, high maintenance cost\n\n### Alternative 2: Pure Rust with manual module implementation\n\n**Rejected**: Duplicates official CLI, maintenance nightmare\n\n### Alternative 3: Hybrid (pure Rust + CLI fallback)\n\n**Rejected**: Adds complexity, two implementations to maintain\n\n### Alternative 4: Use Nickel LSP (Language Server)\n\n**Rejected**: LSP not designed for programmatic evaluation\n\n## Implementation Status\n\n### Completed\n\n- ✅ Plugin command infrastructure (5 commands)\n- ✅ CLI invocation via `Command::new("nickel")`\n- ✅ Correct command syntax: `nickel export /file --format json`\n- ✅ JSON output parsing (serde_json → nu_protocol)\n- ✅ Recursive value conversion (records, lists, primitives)\n- ✅ Caching system (SHA256, filesystem-based)\n- ✅ Error handling (CLI errors → Nushell errors)\n- ✅ Type system (Type::Any for proper output types)\n\n### Key Fix\n\n**Command Syntax**: Changed from positional to flag-based:\n\n```rust\n// BEFORE (WRONG):\ncmd.arg("export").arg(format).arg(file);\n// Result: "nickel export json /file" → auto-imports nonexistent JSON module\n\n// AFTER (CORRECT):\ncmd.arg("export").arg(file).arg("--format").arg(format);\n// Result: "nickel export /file --format json" → works correctly\n```\n\n### Files\n\n- `src/main.rs` - Plugin commands and JSON parsing (95 lines of logic)\n- `src/helpers.rs` - CLI invocation and caching (300+ lines)\n- `tests/` - Test suite for all commands\n\n## Testing\n\n**Manual Testing**:\n\n```bash\n# Test basic execution\nnickel-export json /path/to/file.ncl\n\n# Test with configuration\nnickel-export json /workspace/config.ncl | .database\n\n# Test cache\nnickel-cache-status\n```\n\n**Verification**:\n\n- ✅ Module imports work correctly\n- ✅ Output is proper records (not strings)\n- ✅ Cell path access works\n- ✅ Cache hits are fast\n- ✅ Error messages are helpful\n\n## References\n\n- [Nickel Official Documentation](https://nickel-lang.org/)\n- [nickel-lang-core Crate](https://crates.io/crates/nickel-lang-core/)\n- [Module System Design](./MODULE_SYSTEM.md)\n- [Caching Strategy](./CACHING.md)\n- [JSON Output Format](./OUTPUT_FORMAT.md)\n\n---\n\n**Author**: Architecture Team\n**Date**: 2025-12-15\n**Decision Made By**: Technical Review