1.
Data Types

2. Purpose of Types in PLs Provide implicit context
Example: a + b is
integer addition if a and b are integers
floating-point addition if a and b are floats
etc.
Limit operations (to prevent errors)
Example: Prevent programmer from passing a string to a function that expects an integer

3. Parts of a Type System
Mechanism to define types and associate them with constructs that have values
Examples: constants, variables, parameters, subroutines
Set of rules for
Type equivalence
Are two values of same type?
Type compatibility
Can value of this type be used in this context?
Type inference
What type is this expression, given the types of its parts?

4. Type Checking
Process of ensuring program obeys type compatibility rules
Strongly typed lang.: Prohibits invocation of any operation on any object that doesn’t support the operation
Statically typed lang.: Strongly typed and type checking performed at compile time
Many languages are mostly, but not entirely

5. Two Approaches to Type Equivalence
Structural equivalence: Use structure of objects
Example of structurally equivalent types
Name equivalence: Use names given by programmer
More popular

6. What should you do if…
… you want to use a value of one type in a context that requires a different type?
C++ Example: float f = 5.5; int i = f % 2; // Error!
Cast! int i = (int)f % 2;

7. The method could return a Foo or a class that is derived from Foo
Type Compatibility
Most languages do not require equivalence in every context – just compatibility
What are Java’s type compatibility rules?
What compatible types could this method return? Foo myFooRef = someMethod();
The method could return a Foo or a class that is derived from Foo

8. Another Approach: Duck Typing
If it walks like a duck and swims like a duck and quacks like a duck, call it a duck
JavaScript Example:

9. Type Coercion
When value of one type used in context where another is expected, conversion of value to expected type
May be trivial or may actually require computation
Coercion controversial in lang. design because may lead to subtle errors
Consider loss of precision coercing double to float

10. Universal Reference Type
Give programmer a way to reference any type
void* in C/C++
Object in Java

11. Type Inference C++ Example: C# Example:
Types sometimes need to be inferred from expression: cout << x + y + z << endl;
C# Example:
Implicit type var: used as if you declared a type, but the compiler figures it out

12. Type Inference
Another C# Example:

13. Activity: Java Type Checking
X x = new X();
Y y = new Y();
Z z = new Z();
X xy = new Y();
X xz = new Z();
Y yz = new Z();
Y y1 = new X();
Z z1 = new X();
X x1 = y;
X x2 = z;
Y y1 = (Y) x;
Z z1 = (Z) x;
Y y2 = (Y) x1;
Z z2 = (Z) x2;
Y y3 = (Y) z;
Z z3 = (Z) y;
Object o = z;
Object o1 = (Y) o;
Given
Base class X
Class Y extends X
Class Z extends X
For each statement, tell
Which is involved?
Type equivalence
Type compatibility
Type inference
Static typing
Dynamic typing
What is the result? X Z Y

14. Discussion Question

15. Discussion Question

16. What’s next?
Homework 3 due next class
Exam 2 in one week