EZC: Type checker implementation

[SchoolyB]

Type Checker

The type checker is the single biggest missing piece in EZC. It enables:

• Correct string interpolation with any type (currently broken for string variables)
• Type mismatch errors with helpful messages
• Unused variable/import detection
• Missing return analysis
• Implicit narrowing warnings
• "Did you mean?" suggestions (requires symbol table)

Checklist

Core infrastructure

• Scope/symbol table — nested scopes with variable name → type mappings
• Type representation — primitives, arrays, maps, structs, enums, functions, void
• Type registry — register struct/enum types during declaration pass
• Function signature registry — register function params and return types

Type checking passes

• Pass 1: Register declarations — collect all struct, enum, and function declarations
• Pass 2: Check statements — validate types in function bodies

Expression type resolution

• Literals — int, float, string, char, bool, nil have known types
• Variables — look up type from scope
• Binary expressions — validate operand types, determine result type
• Comparison expressions — operands must be compatible, result is bool
• Function calls — validate argument types against parameter types, return type resolution
• Member access — look up struct field types, enum member types
• Index access — array element type, map value type
• String interpolation — determine format specifier from resolved expression type

Statement type checking

• Variable declarations — validate value type matches declared type
• Assignments — validate value type matches variable type, check mutability
• Return statements — validate return value types match function signature
• If conditions — must be boolean
• For range — iterator variable is int
• When/is — case values must match switch value type

Integration with codegen

• Annotate AST nodes with resolved types (side table keyed by node pointer)
• Codegen reads resolved types for string interpolation format specifiers
• Codegen reads resolved types for println dispatch

References

• Go type checker: pkg/typechecker/typechecker.go (9497 lines — the logic to port)
• Go scope: embedded in typechecker