Class 26: Type Checking (2): Type Equivalence

Held: Friday, 2 April 2004

Summary: Today we continue to consider issues of type equivalence. That is, when two types can be treated as equivalent for purposes of assignment and function/procedure calls.

Related Pages:

• EBoard.
• Lab on Type Checking.

Assignments

• Project, Phase 3: The Type Checker.

Notes:

• Are there lingering questions from yesterday's lab?
• We are simplifying Pascal for the remaining stages of the compiler. Read the details.

Overview:

• Summary of current type knowledge
• Types of types
• Type equivalence

Status

• Just to keep you posted, here are some of the thigns we've decided about type checking.
• Type checking has many uses, although there are two big ones.
  • Verifying that code is correct (e.g., that function calls have the correct number and types of parameters).
  • Deciding which code to generate (e.g., for arithmetic operations).
• We want to do two activities hand-in-hand:
  • Determining the type of each expression.
  • Determining whether that expression is used in a type-appropriate way.

Reflection on Type Checking Lab

• I don't want to pre-bias you.
• What did you learn?
• What will be different for Pascal?

Types of Types

• We know about many different types. These include the following.
• Basic types: integer, reals, strings, Booleans (in Pascal)
• Procedure or Function types (from list of types to type)
• Record types (list of name/type pairs)
• Arrays (might be functions, might be something else)
• Enumerated types (subrange or explicit list)
• Pointers
• We'll need to think about what kinds of type checking we do for each. (At least in Pascal.)