Stage 2: Type Checker

This stage implements type inference using Algorithm W (Hindley-Milner type system) for the bootstrap compiler.

Overview

Stage 2 builds upon the parsed AST from Stage 1 and adds type inference capabilities:

• Type environment management
• Unification algorithm with occurs check
• Algorithm W implementation
• Self-typing validation

Tickets

• BOOTSTRAP-010: Type Environment
• BOOTSTRAP-011: Unification Algorithm
• BOOTSTRAP-012: Algorithm W Implementation
• BOOTSTRAP-013: Self-Typing Test

Success Criteria

✅ Type inference working on all bootstrap stages ✅ Soundness property: well-typed programs don't crash ✅ Self-typing: type checker can type its own code ✅ O(n log n) complexity achieved

Technical Highlights

• Hindley-Milner type system: Automatic type inference
• Let-polymorphism: Generalization at let bindings
• Occurs check: Prevents infinite types
• Constraint solving: Unification-based inference