Mojo Lint Syntax

Lint Mojo Syntax

Validate Mojo code against v0.26.1+ syntax standards.

When to Use

• Writing new Mojo code before testing
• Reviewing Mojo code for syntax issues
• Migrating code from older Mojo versions
• Checking for deprecated patterns
• Pre-commit validation of Mojo files

Quick Reference

# Validate single executable file (with main())
mojo build -I . file.mojo

# Build entire package (for library files)
mojo package shared

# Format code (fixes many syntax issues)
pixi run mojo format .

# Check for deprecated patterns
grep -r "inout self\|@value\|DynamicVector\|->" *.mojo | grep -v "result\|fn"

IMPORTANT: Library files with relative imports CANNOT be validated using mojo build - use mojo package instead.

Common Syntax Issues

Deprecated Patterns:

• ❌ inout self → ✅ out self in __init__, ✅ mut self in methods
• ❌ @value → ✅ @fieldwise_init with trait list
• ❌ DynamicVector → ✅ List
• ❌ -> (T1, T2) → ✅ -> Tuple[T1, T2]

Constructor Issues:

• Wrong parameter type in __init__ (must be out self)
• Missing trait conformances (Copyable, Movable)
• Incorrect initialization order

Type Issues:

• Missing type annotations (required in fn declarations)
• Mismatched types in assignments
• Invalid type parameters

Ownership Issues:

• Missing transfer operator ^ for non-copyable types
• Using var parameter incorrectly
• Copy/move semantics violations

Validation Workflow

1. Identify file type: Determine if file is library (has relative imports) or executable (has main())
2. Check syntax: Run appropriate command:
   • Executable files: mojo build -I . file.mojo
   • Library files: Skip standalone compilation (part of package)
3. Fix format: Run pixi run mojo format to auto-fix style
4. Verify patterns: Check for deprecated patterns
5. Type check: Ensure all types are correct
6. Ownership check: Verify ownership semantics
7. Package validation: Use mojo package shared for library files
8. Report issues: List all problems found

Output Format

Report syntax issues with:

1. File - Which file has the issue
2. Line - Line number of error
3. Error - Syntax error message
4. Pattern - What deprecated/wrong pattern was used
5. Fix - How to correct it
6. Severity - Critical (won't compile) or warning

Error Handling

Validation Checklist

Before committing Mojo code:

• File compiles with mojo build
• No syntax errors in compiler output
• No deprecated patterns (inout, @value, DynamicVector)
• All __init__ use out self (not mut self)
• All non-copyable returns use ^ operator
• All type annotations present in fn declarations
• Zero compiler warnings

References

• See CLAUDE.md for v0.26.1+ syntax standards
• See validate-mojo-patterns for pattern validation
• See mojo-format skill for code formatting