# Template Filters -- Technical Specification

**Version**: v1.0.0 | **Status**: Active | **Last Updated**: March 2026

## Overview

Placeholder submodule for template filter and transform functions. Intended to provide reusable formatters (e.g., date formatting, string escaping, number rendering) that can be applied within template expressions.

## Architecture

This submodule is currently a namespace placeholder. The `__init__.py` exports an empty `__all__` list. Filter functionality is expected to follow a registry pattern where named filters map to callable transforms.

## Current State

- `__init__.py`: Module docstring and empty `__all__` -- no filter functions implemented yet.
- Future scope: built-in filter library and custom filter registration.

## Planned Interface

When implemented, the module would expose:

| Class / Function | Purpose |
|-----------------|---------|
| `FilterRegistry` | Named registry mapping filter names to callables |
| `register_filter(name, fn)` | Register a custom filter function |
| Built-in filters | `upper`, `lower`, `title`, `truncate`, `date`, `escape_html` |

## Dependencies

- **Internal**: `templating.engines` (applies filters during rendering)
- **External**: none

## Constraints

- Not yet implemented; importing yields an empty namespace.
- Zero-mock: `NotImplementedError` for unimplemented paths.

## Navigation

- [README.md](README.md) | [AGENTS.md](AGENTS.md)
- Parent: [templating](../README.md)
