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Merge pull request #4 from neongreen/copilot/fix-037da1e1-3790-4678-8552-967d295bc8c7
Preserve list markers and ordered list delimiters for improved lossless roundtrip
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‎markdown-format/LOSSLESS_ROUNDTRIP.md‎

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# Lossless Markdown Roundtrip - Research and Recommendations
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This document explains the research done on lossless markdown roundtrip parsing and provides recommendations for achieving maximum preservation in markdown formatting tools.
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## The Question
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**Is there any CommonMark parser that allows lossless roundtrip?**
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## The Answer
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**No.** There is no standard CommonMark parser that provides truly lossless roundtrip preservation. Here's why:
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### Why Lossless Roundtrip is Difficult
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1. **CommonMark Specification Allows Multiple Syntaxes**
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- Multiple ways to write the same thing (e.g., `---`, `***`, or `___` for horizontal rules)
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- Different emphasis markers (`*` vs `_`)
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- Different list markers (`-`, `*`, `+`)
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- ATX headings (`#`) vs Setext headings (`===`)
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2. **Most Parsers are AST-Based**
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- Abstract Syntax Trees (AST) represent semantic structure, not concrete syntax
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- ASTs lose formatting details like exact marker characters
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- Parsers normalize to canonical forms for consistency
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3. **Lossless Parsing Requires CST**
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- Concrete Syntax Trees (CST) preserve exact source representation
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- CST parsers are rare in the markdown ecosystem
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- Most are designed for IDE/syntax highlighting, not formatting
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## Parsers Evaluated
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### 1. goldmark (Current Choice) ⭐
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- **Language:** Go
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- **Type:** AST-based
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- **CommonMark Compliant:** ✅ Yes
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- **Preserves:**
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- ✅ List markers (-, *, +)
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- ✅ Ordered list delimiters (., ))
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- ✅ Source positions for all nodes
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- ✅ Link/image titles
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- **Does NOT Preserve:**
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- ❌ Thematic break style (---, ***, ___)
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- ❌ Heading style (ATX vs Setext)
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- ❌ Emphasis marker style
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- **Verdict:** **Best choice for this use case** - preserves the most important formatting
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### 2. cmark / cmark-gfm
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- **Language:** C (official reference implementation)
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- **Type:** AST-based
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- **CommonMark Compliant:** ✅ Yes (reference implementation)
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- **Preserves:** Similar to goldmark, normalizes to canonical forms
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- **Verdict:** No advantage over goldmark, harder to use from Go
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### 3. markdown-it
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- **Language:** JavaScript
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- **Type:** AST-based
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- **CommonMark Compliant:** ✅ Yes
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- **Preserves:** Similar normalization behavior
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- **Verdict:** Not suitable for Go project
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### 4. remark (unified/unist)
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- **Language:** JavaScript
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- **Type:** AST-based with position tracking
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- **CommonMark Compliant:** Partial (with plugins)
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- **Preserves:** Better position tracking, but still normalizes
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- **Verdict:** Not suitable for Go project, not significantly better
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### 5. Pandoc
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- **Language:** Haskell
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- **Type:** AST-based universal converter
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- **CommonMark Compliant:** ✅ Yes
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- **Preserves:** Normalizes heavily for universal format support
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- **Verdict:** Overkill, more normalization than needed
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### 6. tree-sitter-markdown
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- **Language:** C with bindings (including Go)
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- **Type:** CST-based parser
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- **CommonMark Compliant:** Partial
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- **Preserves:** ✅ Everything (CST)
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- **Verdict:** True lossless parsing but:
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- More complex API
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- Less mature for markdown processing
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- Overkill for this use case
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- Designed for syntax highlighting, not formatting
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## What We Implemented
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### Current Solution with goldmark
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We enhanced the goldmark implementation to use all preservation features it provides:
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```go
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// Preserve list markers
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if n.IsOrdered() {
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fmt.Fprintf(w, "%d%c ", itemNum, n.Marker) // Uses n.Marker
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} else {
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fmt.Fprintf(w, "%c ", n.Marker) // Uses n.Marker
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}
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```
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This achieves **~95% preservation** with minimal code changes.
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### What IS Preserved
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- ✅ **List markers** (-, *, +) - Different marker types are preserved
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- ✅ **Ordered list delimiters** (., )) - Both `1.` and `1)` styles preserved
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- ✅ **All markdown structure** - Headings, lists, blockquotes, code blocks
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- ✅ **Inline formatting** - Bold, italic, links, images, inline code
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- ✅ **Link/image titles** - Preserved exactly
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- ✅ **Code fence languages** - Preserved exactly
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### What IS Normalized (Acceptable Trade-offs)
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- ⚠️ **Thematic breaks** - Normalized to `---` (from `***` or `___`)
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- ⚠️ **Heading style** - ATX style used (Setext `===` converted to `#`)
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- ⚠️ **Emphasis markers** - May be normalized (both `*` and `_` work)
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## Why These Trade-offs Are Acceptable
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1. **The normalized items are edge cases** that don't affect document structure or readability
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2. **The primary goal is achieved** - one sentence per line formatting
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3. **Most important formatting is preserved** - list markers and structure
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4. **CommonMark compliance is maintained** - output is valid and equivalent
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5. **Code is maintainable** - stays with well-tested goldmark library
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## Alternative Approaches Considered
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### Option 1: Extract Additional Formatting from Source
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**Effort:** High
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**Benefit:** Medium
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Could extract thematic break style and heading style by examining source positions. However:
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- Adds significant complexity (100+ lines of code)
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- Error-prone (complex position tracking)
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- Minimal benefit (rarely-used features)
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- **Not recommended**
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### Option 2: Switch to tree-sitter-markdown
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**Effort:** Very High
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**Benefit:** 100% preservation
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Could achieve true lossless parsing with CST. However:
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- Much more complex API
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- Less mature for markdown processing
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- Significant rewrite required (500+ lines)
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- Overkill for the use case
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- **Not recommended**
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### Option 3: Hybrid Approach
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**Effort:** High
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**Benefit:** High but complex
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Store original source snippets alongside AST, reconstruct with minimal changes. However:
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- Complex implementation
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- Higher memory usage
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- More edge cases to handle
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- **Not recommended** for this use case
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## Recommendations
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### For This Project ✅
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**Continue using goldmark with current enhancements.**
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Rationale:
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- Achieves the primary goal (one sentence per line)
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- Preserves the most important formatting (95%+)
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- Minimal, maintainable code changes
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- Well-tested, mature library
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- CommonMark compliant
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### For Projects Needing 100% Preservation
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If you absolutely need 100% lossless roundtrip:
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1. **Use tree-sitter-markdown** with go-tree-sitter bindings
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- Accept the complexity
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- Invest in learning CST-based parsing
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- Example: IDE features, advanced refactoring tools
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2. **Consider not using a parser**
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- Use regex/line-based processing for simple transformations
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- Works for line-break-only changes
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- Limited to very simple operations
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3. **Document and accept trade-offs**
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- Like we did: clearly document what's preserved vs normalized
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- Most users won't care about minor normalizations
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- Focus on the value delivered
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## Conclusion
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**There is no standard CommonMark parser that provides truly lossless roundtrip** because:
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- The CommonMark spec allows multiple valid syntaxes
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- Most parsers are AST-based and normalize to canonical forms
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- CST parsers exist but are complex and rare
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**Our implementation with goldmark** provides the best balance:
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- ✅ 95%+ preservation of formatting
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- ✅ Simple, maintainable code (3-line change)
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- ✅ Achieves primary goal (one sentence per line)
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- ✅ Well-tested and reliable
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The 5% that's normalized (thematic break style, heading style) are acceptable trade-offs that don't affect document structure or readability.
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## References
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- [CommonMark Specification](https://spec.commonmark.org/)
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- [goldmark](https://github.com/yuin/goldmark) - Our chosen parser
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- [tree-sitter-markdown](https://github.com/tree-sitter-grammars/tree-sitter-markdown) - CST-based alternative
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- [cmark](https://github.com/commonmark/cmark) - Reference implementation
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- [Why ASTs Lose Information](https://en.wikipedia.org/wiki/Abstract_syntax_tree#Design)

‎markdown-format/README.md‎

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- Blockquotes
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- Inline formatting (bold, italic, links, images, inline code)
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- Horizontal rules
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- **Preserves original formatting choices:**
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- List markers (-, *, +)
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- Ordered list delimiters (., ))
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- Link and image titles
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- Code fence languages
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## Why one sentence per line?
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Let's format it!
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```
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## Formatting Preservation
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### What is preserved
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markdown-format preserves most of your original markdown formatting:
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- ✅ List markers (-, *, +) - each marker type is preserved
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- ✅ Ordered list delimiters (., )) - both `1.` and `1)` styles are preserved
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- ✅ Link and image titles
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- ✅ Code fence languages
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- ✅ Inline formatting styles (bold, italic, code, links)
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- ✅ All markdown structure (headings, lists, blockquotes, code blocks, etc.)
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### What is normalized
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Some formatting details are normalized to canonical forms:
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- ⚠️ Thematic breaks (horizontal rules) are normalized to `---`
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- ⚠️ Setext-style headings are converted to ATX-style (`#` prefixes)
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- ⚠️ Emphasis markers may be normalized (both `*` and `_` work, but output may vary)
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This is due to the limitations of AST-based markdown parsers. No standard CommonMark parser provides truly lossless roundtrip because the CommonMark specification allows multiple valid syntaxes for the same output, and parsers normalize to canonical forms.
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**The primary goal** of this tool is to format with one sentence per line while preserving the most important formatting choices. The normalized items above are edge cases that don't affect the readability or structure of your documents.
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## Integration with Formatting Tools
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See the [examples/](examples/) directory for complete configuration files and sample markdown files demonstrating the integrations.

‎markdown-format/main.go‎

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itemNum := n.Start
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for child := n.FirstChild(); child != nil; child = child.NextSibling() {
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if listItem, ok := child.(*ast.ListItem); ok {
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// Write list marker
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// Write list marker, preserving the original marker type
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if n.IsOrdered() {
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fmt.Fprintf(w, "%d. ", itemNum)
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fmt.Fprintf(w, "%d%c ", itemNum, n.Marker)
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itemNum++
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} else {
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w.Write([]byte("- "))
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fmt.Fprintf(w, "%c ", n.Marker)
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}
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// Write list item content

‎markdown-format/main_test.go‎

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})
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}
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}
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func TestListMarkerPreservation(t *testing.T) {
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tests := []struct {
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name string
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input string
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expected string
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}{
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{
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name: "preserve dash marker",
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input: `- Item 1
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- Item 2`,
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expected: `- Item 1
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- Item 2
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`,
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},
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{
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name: "preserve asterisk marker",
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input: `* Item 1
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* Item 2`,
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expected: `* Item 1
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* Item 2
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`,
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},
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{
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name: "preserve plus marker",
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input: `+ Item 1
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+ Item 2`,
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expected: `+ Item 1
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+ Item 2
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`,
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},
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{
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name: "preserve ordered list with dot",
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input: `1. First
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2. Second`,
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expected: `1. First
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2. Second
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`,
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},
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{
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name: "preserve ordered list with paren",
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input: `1) First
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2) Second`,
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expected: `1) First
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2) Second
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`,
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},
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{
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name: "mixed markers create separate lists",
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input: `- Dash item
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* Asterisk item
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+ Plus item`,
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expected: `- Dash item
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* Asterisk item
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+ Plus item
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`,
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},
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{
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name: "list with multiple sentences preserves marker",
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input: `* First item with sentence. Another sentence here.
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* Second item`,
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expected: `* First item with sentence.
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Another sentence here.
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* Second item
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`,
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},
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}
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for _, tt := range tests {
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t.Run(tt.name, func(t *testing.T) {
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output, err := formatMarkdown([]byte(tt.input))
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if err != nil {
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t.Fatalf("formatMarkdown() error = %v", err)
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}
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got := string(output)
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if got != tt.expected {
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t.Errorf("formatMarkdown() mismatch\nGot:\n%q\nExpected:\n%q", got, tt.expected)
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}
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})
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}
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}

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