Class 25: Type Checking (1): Introduction

Held: Wednesday, 31 March 2004

Summary: Today we begin our consideration of a key semantic step: type checking. That is, we consider where and how one might verify types in a program. We also consider related issues, such as the definition of type.

Related Pages:

• EBoard.

Assignments

• Homework 2: Lexical Analysis.

Notes:

• Mr. Walker is speaking on pair programming today at 4:15. Extra credit for attending.

Overview:

• Introduction to Type Checking
• Compile-time vs. Run-time Type Checking
• Common Types

An Introduction to Type Checking

• The problem of type checking is one of the first important semantic aspects of compilation. (What we've been doing so far is more syntactic.)
• Essentially, type checking
  • allows the programmer to limit what types may be used in certain circumstances;
  • assigns types to values;
  • determines whether those values are used in an appropriate way.
• Type checking is also useful because it has relevance to generated code (e.g., you will often generate a different assembly instruction for adding reals and adding ints)
• There are many variants of type checking. For example, if the type of an object is incorrect, you may coerce it to a different type.
• The definition of types is subtle; it's often worth asking when two values should be considered as having the same type.

When Should You Type Check?

• In many modern languages, type checking is done at compile time.
• In some, it's done at run time.
• What are some advantages for type checking at compile time?
  • Catches many common errors.
  • Can result in faster code.
  • ...
• What are some advantages for type checking at run time?
  • Frees the programmer.
  • Can result in more robust code.
  • Can result in clearer code.
  • ...
• Pascal type checking is traditionally done at compile time.

Common Types

• Built-in types (integers, floating-point numbers, strings, ...)
• Compound types (arrays, lists, records, classes)
• User-defined enumerated types.
• ...

Computing Types

• In Pascal, the types of variables are declared in advance.
• However, we still need to type other things (such as functions and ...)
• We'll consider as a class what kinds of things should be typed and where we use types.