Ast Grep

ast-grep Community Best Practices

Comprehensive best practices guide for ast-grep rule writing and usage, maintained by the ast-grep community. Contains 46 rules across 8 categories, prioritized by impact to guide automated rule generation and code transformation.

When to Apply

Reference these guidelines when:

• Writing new ast-grep rules for linting or search
• Debugging patterns that don't match expected code
• Optimizing rule performance for large codebases
• Setting up ast-grep projects with proper organization
• Reviewing ast-grep rules for correctness and maintainability

General Workflow

Follow this workflow when creating ast-grep rules for code search:

Step 1: Understand the Query

Clarify what you want to find:

• Target programming language
• Edge cases to handle
• What to include vs exclude

Step 2: Create Example Code

Write a sample code snippet representing the desired match pattern.

Step 3: Write the ast-grep Rule

Choose the right approach:

• Use pattern for simple structures
• Use kind with has/inside for complex structures
• Combine with all, any, or not for compound queries
• Always use stopBy: end for relational rules (inside, has) to ensure complete search

Step 4: Test the Rule

# Inspect AST structure
ast-grep run --pattern '[code]' --lang [language] --debug-query=ast

# Test inline rule
echo "[code]" | ast-grep scan --inline-rules "[rule]" --stdin

# Test from file
ast-grep scan --rule [file.yml] [path]

Step 5: Search the Codebase

Deploy the validated rule:

# Search with pattern (simple matches)
ast-grep run --pattern '[pattern]' --lang [language] [path]

# Search with rule file (complex queries)
ast-grep scan --rule [file.yml] [path]

# Apply fixes interactively
ast-grep scan --rule [file.yml] --interactive [path]

Quick Tips

1. Always use stopBy: end - Ensures complete subtree traversal for relational rules
2. Start simple, add complexity - Begin with patterns, progress to kinds, then relational rules
3. Debug with AST inspection - Use --debug-query=ast to verify structure matching
4. Escape in inline rules - Use \$VAR or single quotes for shell commands
5. Test in playground first - Use https://ast-grep.github.io/playground.html for rapid iteration

Rule Categories by Priority

Quick Reference

1. Pattern Correctness (CRITICAL)

• pattern-valid-syntax - Use valid parseable code as patterns
• pattern-language-aware - Account for language-specific syntax differences
• pattern-context-selector - Use context and selector for code fragments
• pattern-avoid-comments-strings - Avoid matching inside comments and strings
• pattern-strictness-levels - Configure pattern strictness appropriately
• pattern-kind-vs-pattern - Choose kind or pattern based on specificity needs
• pattern-debug-ast - Use debug query to inspect AST structure
• pattern-nthchild-matching - Use nthChild for index-based positional matching
• pattern-range-matching - Use range for character position matching

2. Meta Variable Usage (CRITICAL)

• meta-naming-convention - Follow meta variable naming conventions
• meta-single-node - Match single AST nodes with meta variables
• meta-reuse-binding - Reuse meta variables to enforce equality
• meta-underscore-noncapture - Use underscore prefix for non-capturing matches
• meta-named-vs-unnamed - Use double dollar for unnamed node matching
• meta-multi-match-lazy - Understand multi-match variables are lazy

3. Rule Composition (HIGH)

• compose-all-for-and-logic - Use all for AND logic between rules
• compose-any-for-or-logic - Use any for OR logic between rules
• compose-not-for-exclusion - Use not for exclusion patterns
• compose-inside-for-context - Use inside for contextual matching
• compose-has-for-children - Use has for child node requirements
• compose-matches-for-reuse - Use matches for rule reusability
• compose-precedes-follows - Use precedes and follows for sequential positioning
• compose-field-targeting - Use field to target specific sub-nodes

4. Constraint Design (HIGH)

• const-kind-filter - Use kind constraints to filter meta variables
• const-regex-filter - Use regex constraints for text patterns
• const-not-inside-not - Avoid constraints inside not rules
• const-pattern-constraint - Use pattern constraints for structural filtering
• const-post-match-timing - Understand constraints apply after matching

5. Rewrite Correctness (MEDIUM-HIGH)

• rewrite-preserve-semantics - Preserve program semantics in rewrites
• rewrite-meta-variable-reference - Reference all necessary meta variables in fix
• rewrite-transform-operations - Use transform for complex rewrites
• rewrite-test-before-deploy - Test rewrites on representative code
• rewrite-syntax-validity - Ensure fix templates produce valid syntax

6. Project Organization (MEDIUM)

• org-project-structure - Use standard project directory structure
• org-unique-rule-ids - Use unique descriptive rule IDs
• org-severity-levels - Assign appropriate severity levels
• org-file-filtering - Use file filtering for targeted rules
• org-message-clarity - Write clear actionable messages

7. Performance Optimization (MEDIUM)

• perf-specific-patterns - Use specific patterns over generic ones
• perf-stopby-boundaries - Use stopBy to limit search depth
• perf-thread-parallelism - Leverage parallel scanning with threads
• perf-avoid-regex-heavy - Avoid heavy regex in hot paths

8. Testing & Debugging (LOW-MEDIUM)

• test-valid-invalid-cases - Write both valid and invalid test cases
• test-snapshot-updates - Use snapshot testing for fix verification
• test-playground-first - Test patterns in playground first
• test-edge-cases - Test edge cases and boundary conditions

How to Use

Read individual reference files for detailed explanations and code examples:

• Section definitions - Category structure and impact levels
• Rule template - Template for adding new rules

Full Compiled Document

• AGENTS.md - Complete compiled guide with all rules

Reference Files