09-02: Scope

• Matching identifier declaration with uses;
• The scope of an identifier is the potion of a program in which that identifier is accessible;
• Static scope depends only on the program text;
• Dynamic scope depends on execution of the program.

09-03: Symbol Tables

• Much of semantic analysis can be expressed as a recursive descent of an AST;
• When performing semantic analysis on a portion of the AST, we need to know which identifiers are defined;
• A symbol table is a data structure that tracks the current bindings of identifiers;
• Semantic analysis requires multiple passes.

09-04: Types

1. The goal of type checking is to ensure that operations are used only with the correct types;
2. Three kinds of languages:
   • Statically typed: All or almost all checking of types is done as part of compilation;
   • Dynamically typed: Almost all checking of types is done as part of program execution;
   • Untyped: No type checking.

09-05: Type Checking

• The appropriate formalism for type checking is logical rules of inference.

09-06: Type Environments

1. A type environment gives types for free variables:
   • A type environment is a function from ObjectIdentifiers to Types;
   • A variable is free in an expression if it not defines within the expression.