# Comment Preservation - Implementation Complete! 🎉
**Date**: 2025-10-20
**Status**: ✅ All 4 phases complete
## Summary
SQL comment preservation is now fully functional in both the CLI and Neovim plugin. When formatting SQL queries with `\sf` in Neovim or using `--format --preserve-comments` from the CLI, comments are preserved in the output.
## What Was Implemented
### Phase 1: Dual-Mode Lexer (Completed 2025-10-18)
- Added `LexerMode` enum (SkipComments, PreserveComments)
- Lexer tokenizes comments as `Token::LineComment` and `Token::BlockComment`
- Backward compatible - default mode skips comments
### Phase 2: Opt-In Parser Mode (Completed 2025-10-19)
- Added `ParserMode` enum (Standard, PreserveComments)
- `Parser::new()` defaults to Standard mode (backward compatible)
- `Parser::with_mode()` allows opting into comment preservation
- Comment collection guarded by mode checks
- 7 parser mode tests passing
### Phase 3: Formatter Integration (Completed 2025-10-20)
- Added `format_comment()` and `format_inline_comment()` methods to `AstFormatter`
- Leading comments emitted before SELECT keyword
- Trailing comments emitted at end of statement
- 6 new comment preservation tests passing
- All 457 tests passing (zero regressions)
### Phase 4: Neovim Plugin Integration (Completed 2025-10-20)
- Added `--preserve-comments` CLI flag
- Updated Neovim plugin to use new flag by default
- Removed manual comment preservation workaround
- `\sf` formatter now preserves comments automatically
## Usage
### CLI Usage
```bash
# Preserve comments when formatting
echo "-- Important query
# Output:
# -- Important query
# SELECT id, name
# FROM users
# Without --preserve-comments, comments are stripped (default)
echo "-- Comment
# Output:
# SELECT id
# FROM users
```
### Neovim Plugin Usage
Simply use `\sf` as before - comments are now preserved automatically!
```sql
-- Before formatting
-- User activity query
-- Find active users
SELECT u.id,u.name,u.email FROM users u WHERE u.active=true
-- After \sf (comments preserved!)
-- User activity query
-- Find active users
SELECT u.id, u.name, u.email
FROM users u
WHERE u.active = TRUE
```
## Technical Details
### Files Modified
1. **src/sql/parser/lexer.rs**
- Added `LexerMode` enum
- Dual-mode lexer implementation
2. **src/sql/recursive_parser.rs**
- Added `ParserMode` enum
- `Parser::with_mode()` constructor
- Comment collection logic
3. **src/sql/parser/ast.rs**
- Added `Comment` struct
- Comment fields in `SelectStatement` and `SelectItem`
4. **src/sql/parser/ast_formatter.rs**
- `format_comment()` - formats comments on own line
- `format_inline_comment()` - formats trailing comments
- Comment emission in `format_select()`
5. **src/main_handlers.rs**
- Added `--preserve-comments` flag support
- Uses `ParserMode::PreserveComments` when flag present
6. **nvim-plugin/lua/sql-cli/formatter.lua**
- Passes `--preserve-comments` flag to CLI
- Removed manual comment preservation workaround
### Test Coverage
**6 new comment preservation tests:**
- `test_standard_mode_skips_comments` - Backward compatibility
- `test_preserve_mode_captures_leading_comments` - Comment capture
- `test_formatter_emits_comments` - Formatter output
- `test_full_round_trip_with_comments` - Parse → format cycle
- `test_backward_compatibility` - Existing code unchanged
- `test_block_comment_preservation` - Block comments work
**All tests passing:**
- 457 tests pass (including 6 new ones)
- Zero regressions
- Backward compatibility verified
## Limitations & Known Issues
### What Works
- ✅ Leading comments before SELECT statement
- ✅ Trailing comments at end of statement
- ✅ Line comments (`-- comment`)
- ✅ Block comments (`/* comment */`)
- ✅ Multiple consecutive comments
- ✅ Backward compatibility (Standard mode unchanged)
### Current Limitations
- ⚠️ Inline comments within SELECT items not yet supported
- Example: `SELECT id, /* inline */ name FROM users`
- This is a known limitation documented in Phase 5 of the plan
- ⚠️ Comments within WHERE/JOIN clauses not yet preserved
- Future enhancement (Phase 5)
## Configuration
### Neovim Plugin
Comment preservation is **enabled by default**. To disable it, you would need to modify the plugin configuration (though this is not currently exposed as a user option).
### CLI
Use the `--preserve-comments` flag with `--format`:
```bash
# With comment preservation
sql-cli --format --preserve-comments < query.sql
# Without comment preservation (default)
sql-cli --format < query.sql
```
## Examples
### Example 1: Simple Query with Comments
**Input:**
```sql
-- Get all active users
-- Filter by last login date
SELECT id, name, email
FROM users
WHERE active = true
AND last_login > '2025-01-01'
```
**Output (with --preserve-comments):**
```sql
-- Get all active users
-- Filter by last login date
SELECT id, name, email
FROM users
WHERE active = TRUE
AND last_login > '2025-01-01'
```
### Example 2: Block Comments
**Input:**
```sql
/*
* Important: This query is used for reporting
* Author: John Doe
* Date: 2025-10-20
*/
SELECT COUNT(*) FROM orders
```
**Output (with --preserve-comments):**
```sql
/* Important: This query is used for reporting
* Author: John Doe
* Date: 2025-10-20 */
SELECT COUNT(*)
FROM orders
```
## Performance Impact
- **Standard mode (default)**: Zero performance impact
- **PreserveComments mode**: Minimal overhead
- Comment collection is fast (simple token iteration)
- Only active when explicitly requested
- All 457 tests complete in ~6 seconds
## Future Enhancements (Phase 5)
Potential improvements documented in `COMMENT_PRESERVATION_PHASED_PLAN.md`:
1. **SELECT Item Comments**
```sql
SELECT
id, name, email FROM users
```
2. **WHERE Clause Comments**
```sql
WHERE
status = 'active'
AND verified = true
```
3. **CTE Comments**
```sql
WITH user_stats AS (
SELECT ...
)
SELECT * FROM user_stats
```
## Success Metrics
✅ All success criteria met:
1. **Functionality**
- `\sf` in Neovim preserves top-level comments
- CLI `--format --preserve-comments` works
- Backward compatibility maintained
2. **Quality**
- All 457 tests passing
- Zero regressions
- Comprehensive test coverage for new features
3. **Documentation**
- Implementation plan documented
- Usage examples provided
- Limitations clearly stated
## Related Documents
- `COMMENT_PRESERVATION_PHASED_PLAN.md` - Detailed implementation plan
- `COMMENT_PRESERVATION_DESIGN.md` - Original design document
- `src/sql/parser/comment_preservation_tests.rs` - Test suite
## Conclusion
Comment preservation is now production-ready and can be used in both CLI and Neovim workflows. The implementation follows the tree-sitter approach of maintaining comments in the AST, enabling formatters to preserve them while keeping the parser and execution engine unchanged.
🎉 Mission accomplished!