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

pattern-

meta-

compose-

const-

rewrite-

org-

perf-

test-

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