Koa - A Modal Code Editor

An experimental fully extensible modal code editor for the terminal. The Rust codebase provides a minimal bootstrap layer while Janet handles all editor logic, enabling maximum extensibility and live customization.

Key Features:

• Janet-based Core: Editor logic written in Janet for ultimate flexibility
• Live Reconfiguration: Hot-swap key bindings, behaviors, and plugins
• Rust Bootstrap: Minimal, efficient native layer for TUI and system integration
• Tree-sitter Integration: Syntax highlighting, indentation and more via native bindings
• Unlimited Extensibility: Complete editor customization without recompilation

Current Status

Bootstrap Foundation:

• Janet Integration: janetrs library integrated for embedding Janet
• Basic TUI Interface: ratatui integration for terminal UI
• Janet-Rust Bridge System: File system, TUI, and syntax bridges (minimal implementation)
• Janet Event Loop Foundation: Event loop infrastructure in Janet (basic key forwarding only)
• File Operations: Bridge functions for file I/O (not implemented)
• Event Loop Handoff: Control handed from Rust to Janet main loop (partial)

Editor Core:

• Janet Editor State: Core state management in Janet
• Modal System Framework: Basic modal system structure in Janet
• Janet Event Handling: Input processing framework in Janet
• Complete Event Loop: Actual event processing
• Text Editing in Janet: Text operations in Janet
• High-Level UI Bridge: Rust renders editor state from Janet

Architecture

Rust-Janet Hybrid Design:

┌─────────────────────────────────────────────────────────────┐
│                    Janet Layer (Editor Logic)               │
├─────────────────────────────────────────────────────────────┤
│ • Modal system & state management                           │
│ • Text operations & buffer management                       │
│ • Key bindings & command system                             │
│ • Plugin system & user configuration                        │
│ • File I/O coordination                                     │
└─────────────────────────────────────────────────────────────┘
                            ↕ High-Level Bridge
┌─────────────────────────────────────────────────────────────┐
│                Rust Layer (UI & System Core)                │
├─────────────────────────────────────────────────────────────┤
│ • Complete UI rendering                                     │
│ • TUI operations & terminal management                      │
│ • File I/O and system calls                                 │
│ • Tree-sitter integration                                   │
│ • Performance-critical operations                           │
└─────────────────────────────────────────────────────────────┘

Current Structure:

.
├── src/                # Rust source code
│   ├── main.rs         # Bootstrap: Janet init + event loop
│   ├── app.rs          # Application state and main loop
│   ├── event.rs        # Event handling system
│   ├── ui.rs           # UI rendering
│   └── janet/          # Rust-Janet bridge modules
│       ├── mod.rs      # Module definitions
│       ├── keymap.rs   # Key handling bridge
│       └── state.rs    # State management bridge
└── runtime/            # Janet runtime source code
    └── koa/            # Koa Janet modules
        ├── koa.janet   # Main entrypoint
        ├── keymap.janet # Keybindings (minimal)
        └── stubs.janet # Placeholder functions

Roadmap

Phase 1: Strategic Hybrid Foundation

• Janet Integration & Bootstrap

  • Integrate janetrs library for Janet embedding
  • Set up Janet environment initialization in Rust
  • Create minimal Rust bootstrap that hands control to Janet
  • Design Janet API for editor state management

• High-Level UI Bridge (Current Priority)

  • Create semantic rendering bridge functions
  • Maintain ratatui performance and features
  • Bridge accepts editor state from Janet, renders in Rust
  • Replace primitive ratatui calls with high-level operations

• Editor State Migration to Janet

  • Port current editor state management to Janet
  • Implement modal system (Normal/Insert/Select/Command) in Janet
  • Create Janet text operations (insert, delete, cursor movement)
  • Implement Janet-based key binding system

Phase 2: Advanced Editor Operations (Janet-Driven)

• Dynamic Configuration System

  • Implement live configuration system foundation
  • Create event-driven architecture for editor events
  • Design plugin architecture foundation

• Movement Commands

  • Word navigation (w, b, e) with customizable word boundaries
  • Paragraph navigation ({, })
  • Go to line (G, gg) with jump lists
  • Page scrolling (Ctrl-D, Ctrl-U) with smooth scrolling options
  • Search and jump (/, ?, n, N) with regex support

• Text Operations

  • Delete operations (dd, D, dw, etc.) with custom operators
  • Yank/Copy operations (yy, Y, yw, etc.) with multiple registers
  • Paste operations (p, P) with register selection
  • Undo/Redo (u, Ctrl-R) with branching undo tree
  • Replace mode (R) with preview

• Extensible Command System

  • Command mode with Janet expression evaluation
  • Custom command definitions in Janet
  • Command history and completion
  • Macro recording and playback

Phase 3: Language Support & Syntax Highlighting

• Tree-sitter Integration (via Rust bridge)

  • Integrate tree-sitter library
  • Create Janet API for syntax highlighting
  • Parse and highlight common languages:
    • Rust, Zig, C/C++
    • JavaScript/TypeScript
    • Python, Go, Java
    • Janet, Lua, Lisp dialects
    • Markdown, JSON, YAML

• Semantic Features (Janet-configurable)

  • Bracket matching with custom rules
  • Indentation awareness per language
  • Comment toggling with language-specific styles
  • Code folding with custom fold markers

Phase 4: Advanced Editor Features

• Multiple Buffers & Windows

  • Buffer management system in Janet
  • Split windows with customizable layouts
  • Tab interface with user-defined styling
  • Buffer-local configurations and hooks

• Language Server Protocol (LSP)

  • LSP client implementation in Janet
  • Code completion with customizable sources
  • Go to definition and references
  • Error diagnostics with user-defined handlers
  • Custom LSP server configurations

Phase 5: Ultimate Extensibility & Community

• Advanced Plugin System

  • Plugin manager with dependency resolution
  • API versioning and compatibility layer
  • Sandboxed plugin execution
  • Community plugin registry integration

• Performance & Optimization

  • Janet code JIT compilation exploration
  • Lazy loading for large files
  • Async operations for non-blocking UI
  • Memory optimization for large codebases

• Community & Ecosystem

  • Plugin development documentation
  • Example configurations and themes
  • Migration tools from other editors
  • Community contribution guidelines

Architecture Philosophy

Rust-Janet Hybrid Approach: This architecture balances flexibility with performance:

Rust Layer (Performance Core): Handles operations where performance matters:

• Complete UI rendering using ratatui
• TUI operations with proper terminal management
• File I/O and system calls
• Tree-sitter integration for syntax highlighting
• Memory-intensive operations

Janet Layer (Logic Core): Provides maximum extensibility:

• All editor logic and behavior
• Complete customization and extensibility
• Hot-swappable configurations and key bindings
• Plugin system and user extensions
• Domain-specific languages for configuration

High-Level Bridge: Clean interface between layers:

• Janet computes editor state and decisions
• Rust renders state efficiently using native ratatui features
• Minimal bridge calls with semantic operations
• Clear separation of concerns

This approach maximizes both flexibility and performance, avoiding the overhead of primitive bridge operations while maintaining the extensibility benefits of Janet scripting.

Building and Running

# Build the editor (add --release for production build)
cargo build

# Run with a file
cargo run -- demo.rs

# Run without arguments for empty buffer
cargo run

Testing

cargo test

Key Bindings

Note: Most key bindings are not yet implemented. Current functionality is limited to basic character input.

Normal Mode

• hjkl or arrow keys - Movement (undo current selections)
• ^ or Home - Beginning of line
• $ or End - End of line
• i - Enter insert mode
• v - Enter select mode (preserves current selections)
• : - Enter command mode
• w - Move to next word
• b - Move to previous word
• PageUp - Move to top
• PageDown - Move to bottom
• o - Create line below and enter insert mode
• O - Create line above and enter insert mode
• x - Select whole line or next line if the current one is already fully selected
• d - Delete character under cursor or the current selection if one exists
• D - Duplicate selected line
• Delete - Delete character under cursor
• Backspace - Delete character before cursor

Insert Mode

• arrow keys - Movement
• Escape - Return to normal mode
• Backspace - Delete character before cursor
• Delete - Delete character under cursor
• Enter - Insert newline
• Tab - Insert 4 spaces
• Home - Beginning of line
• End - End of line

Select Mode

• hjkl or arrow keys - Movement (extends selection)
• ^ or Home - Beginning of line
• $ or End - End of line
• d or Delete - Delete selection
• i - Delete selection and enter insert mode
• x - Select whole line or next line if the current one is already fully selected
• w - Expand selection to next word
• b - Shorten selection to previous word
• Escape, v - Return to normal mode

Command Mode

• Escape - Return to normal mode
• w - Save file
• q - Quit editor (does not handle unsaved changes confirmation yet!)

Contributing

This is a learning project exploring modal editor design with Rust and Janet. Contributions and suggestions are welcome!

License

GPLv2 License - Feel free to use this code for learning and experimentation.