mould

mould is a modern Terminal User Interface (TUI) tool designed for interactively generating and editing configuration files from templates. Whether you are setting up a .env file from an example, creating a docker-compose.override.yml, or editing nested JSON, YAML, TOML, XML, INI, or Properties configurations, mould provides a fast, Vim-inspired workflow to get your environment ready.

Features

• Universal Format Support: Read, edit, and write .env, JSON, YAML, TOML, XML, INI, and Properties files seamlessly.
• Tree View Navigation: Edit nested data structures in a beautiful, depth-colored tree view.
• Smart Template Discovery: Rule-based resolver automatically discovers relationships (e.g., .env.example vs .env, config.template.properties vs config.properties) and highlights missing keys.
• Strict Type Preservation: Intelligently preserves data types (integers, booleans, strings) during edit-save cycles, ensuring your configuration stays valid.
• Add Missing Keys: Instantly pull missing keys and their default values from your template into your active configuration with a single keystroke.
• Advanced Undo/Redo Engine: Features a tree-based undo/redo history that ensures you never lose changes during complex branching edits. It properly tracks all modifications, including nested renames and item additions/deletions.
• Vim-inspired Workflow: Navigate with j/k, gg/G, edit with i, search with /, and save with :w.
• Modern UI: A polished, rounded interface featuring a semantic Catppuccin Mocha palette with support for terminal transparency.
• Highly Configurable: Customize keybindings and semantic themes via a simple TOML user configuration.
• Neovim Integration: Comes with a built-in Neovim plugin for seamless in-editor configuration management.

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Installation

1. CLI Application

Ensure you have Rust and Cargo installed, then run:

# Install directly from the local path
cargo install --path .

Alternatively, you can build from source:

git clone https://git.narl.io/nvrl/mould-rs
cd mould-rs
cargo build --release
# The executable will be located in target/release/mould

2. Neovim Plugin

If you want to use mould directly inside Neovim, the repository includes a built-in Lua plugin that opens mould in a floating terminal buffer and synchronizes the file upon exit.

Using mini.deps:

add({
	source = 'https://git.narl.io/nvrl/mould-rs',
	checkout = 'main',
})

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Usage

Provide an input template file to start editing. mould is smart enough to figure out if it's looking at a template or an active file, and will search for its counterpart to provide live diffing.

mould .env.example
mould docker-compose.yml
mould config.template.json -o config.json

If you just run mould in a directory, it will automatically look for common template patterns like .env.example or docker-compose.yml.

Inside Neovim

Open any configuration file in Neovim and run :Mould. It will launch a floating window targeting your active buffer.

Keybindings (Default)

• Normal Mode

  • j / Down: Move selection down
  • k / Up: Move selection up
  • gg: Jump to the top
  • G: Jump to the bottom
  • i: Edit value (cursor at start). If selected is a group, enters rename mode.
  • a: Edit value (cursor at end)
  • s / S: Substitute value (clear and edit)
  • r: Rename the current key or group. (Cannot rename array indices).
  • o: Append a new item (as a sibling or array element). Enters rename mode for non-array items.
  • O: Prepend a new item (as a sibling or array element). Enters rename mode for non-array items.
  • alt+o: Append a new group/object as a child. Enters rename mode for the new group.
  • alt+O: Prepend a new group/object as a child. Enters rename mode for the new group.
  • t: Toggle the selected item between a group/object and a standard variable.
  • dd: Delete the currently selected variable or group (removes all nested children).
  • u: Undo the last change
  • U: Redo the reverted change
  • a: Add missing value from template to active config
  • /: Search for configuration keys
  • n / N: Jump to next / previous search match
  • :w or Enter: Save the current configuration
  • :q or q: Quit the application
  • :wq: Save and quit
  • Esc: Clear current command prompt

• Insert / Rename Modes

  • Type your value/key string.
  • Arrow keys: Navigate within the input field
  • Enter: Commit the value/key and return to Normal Mode. If renaming, checks for key collisions.
  • Esc: Cancel edits and return to Normal Mode. Reverts changes to the current field.

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Configuration

mould can be configured using a config.toml file located in your user configuration directory:

• Linux/macOS: ~/.config/mould/config.toml
• Windows: %AppData%\mould\config.toml

Example configuration:

[theme]
transparent = false
bg_normal = "#1e1e2e"
bg_highlight = "#89b4fa"
bg_active = "#a6e3a1"
bg_search = "#cba6f7"
fg_normal = "#cdd6f4"
fg_dimmed = "#6c7086"
fg_highlight = "#1e1e2e"
fg_warning = "#f38ba8"
fg_modified = "#fab387"
fg_accent = "#b4befe"
border_normal = "#45475a"
border_active = "#a6e3a1"
tree_depth_1 = "#b4befe"
tree_depth_2 = "#cba6f7"
tree_depth_3 = "#89b4fa"
tree_depth_4 = "#fab387"

[keybinds]
down = "j"
up = "k"
edit = "i"
edit_append = "a"
edit_substitute = "s"
save = ":w"
quit = ":q"
normal_mode = "Esc"
search = "/"
next_match = "n"
previous_match = "N"
jump_top = "gg"
jump_bottom = "G"
append_item = "o"
prepend_item = "O"
delete_item = "dd"
undo = "u"
redo = "U"
rename = "r"
append_group = "alt+o"
prepend_group = "alt+O"
toggle_group = "t"

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Development & Architecture

mould is written in Rust and architected to decouple the file format parsing from the UI representation. This allows the TUI to render complex, nested configuration files in a unified tree-view.

Core Architectural Principles:

• Separation of Concerns: Clear boundaries between UI rendering, application state, input handling, and file format logic.
• Unified Data Model: All parsed configuration data is normalized into a single Vec<ConfigItem> structure, simplifying application logic across different file types.
• Vim-inspired Modality: The application operates in distinct modes (Normal, Insert, InsertKey, Search), each with specific keybinding behaviors, enabling efficient interaction.
• Non-linear Undo/Redo: A robust undo tree allows users to revert and re-apply changes across complex branching edit histories.

Key Components

1. State Management & Undo Tree (src/app.rs & src/undo.rs)

   • The central state is maintained in the App struct, which tracks the currently loaded configuration variables, application modes, and user input buffer.
   • It integrates an UndoTree, providing non-linear, branching history tracking for complex edits (additions, deletions, nested renaming). Each significant state change (save_undo_state) pushes a snapshot to this tree.

2. Unified Data Model (src/format/mod.rs)

   • Regardless of the underlying file format (JSON, YAML, .env, etc.), all data is translated into a flattened Vec<ConfigItem>.
   • A ConfigItem represents a single configuration entry. It contains:
     • key: The display key (e.g., port or [0]).
     • path: A Vec<PathSegment> (composed of PathSegment::Key(String) for object keys and PathSegment::Index(usize) for array indices) that defines its full hierarchical location.
     • value: Option<String> holding the actual value.
     • is_group: A boolean indicating if this item is a structural node (object or array).
     • status: (ItemStatus::Present, MissingFromActive, Modified) reflecting comparison with a template.
     • value_type: (ValueType::String, Number, Bool, Null) to ensure type preservation during writes.

3. Format Handlers (src/format/*)

   • Each file format has a dedicated handler (EnvHandler, IniHandler, HierarchicalHandler, PropertiesHandler) implementing the FormatHandler trait.
   • HierarchicalHandler: A generalized processor leveraging serde_json::Value as an intermediary layer to parse and write nested JSON, YAML, TOML, and even XML (via quick-xml transposition). This allows complex structures to be flattened for editing and then re-nested accurately.
   • These handlers are responsible for translating between the file's native format and the Vec<ConfigItem> internal representation.

4. Template Resolver (src/resolver.rs)

   • Automatically determines template-active file pairings without explicit user instruction.
   • Uses hardcoded exact rules (e.g., compose.yml -> compose.override.yml) and generic fallback rules to strip .example or .template suffixes to find target output files dynamically.

5. Terminal UI & Event Loop (src/ui.rs & src/runner.rs)

   • UI Rendering (src/ui.rs): Powered by the ratatui library. Renders a flexible layout consisting of a styled hierarchical list, an active input field for editing, and a dynamic status bar.
   • Event Runner (src/runner.rs): Powered by crossterm. It intercepts raw keyboard events, resolves multi-key sequences (like dd or gg), delegates character input to the tui-input backend during active editing, and dispatches actions to mutate the App state. It includes logic to prevent "one-key-behind" issues and manage complex keybindings like alt+o.

6. Neovim Plugin (lua/mould/init.lua)

   • Implements a Lua wrapper that calculates terminal geometries and launches the CLI mould binary inside an ephemeral, floating terminal buffer, ensuring automatic Neovim checktime synchronization upon mould's exit.

Development Process Highlights:

• Iterative Refinement: Features like key renaming, group creation, and advanced undo/redo were developed iteratively, responding to user feedback and progressively enhancing the core data model and interaction logic.
• Robust Error Handling: Key functions (commit_input, enter_insert_key) include collision detection and validation to ensure data integrity during user modifications.
• Modality-driven Design: The introduction of InsertKey mode and careful handling of InsertVariant demonstrates a commitment to a precise, context-aware user experience, minimizing ambiguity during editing.
• Debugging and Performance: Issues like UI "hangs" were traced to subtle interactions in key event processing and input buffer management, leading to refactored key sequence logic for improved responsiveness.

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License

This project is licensed under the MIT License. See the LICENSE file for details.