Types and Type Equivalence in Programming Languages

1. http://flic.kr/p/3aKUAq 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. 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 ora 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: • 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. 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; Activity: Java Type Checking X • 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? Y Z

14. Discussion Question

15. Discussion Question

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