Type Checking

After parsing, the compiler walks the AST and checks types.

Symbol table per scope:

Type system (simplified C):

Type checking rules:

• Variable use: look up in scope; type is the declared type.
• Function call: check arg count + types match; result is return type.
• Binary op +: both sides must be int (or pointer + int for ptr arith); result is int.
• Assignment: lhs must be lvalue; rhs type compatible.
• Pointer dereference *p: p must be pointer; result is base type.
• Array index a[i]: a is array or pointer; i is integer; result is base type.

Implicit conversions in C:

• char ↔ int: char promoted to int in arithmetic.
• int ↔ pointer: NOT (cast required, except 0 → null pointer).
• arrays decay to pointers in most contexts.

Type errors caught here would be runtime crashes otherwise. Modern languages (Rust, Go) do MORE checking; old C is permissive.