1. Overview: Lecture 6: Dolores Zage

2. What is a program? n Operations that are to be applied to certain data in a certain sequence (definition holds for all languages) n Basic differences among languages exist in: u types of data allowed u types of operations available u mechanisms for controlling the sequence of the operations applied to the data

3. Focus: n Chapter 4 concentrates data, types (highlighting imperative languages) n Chapter 5 procedures (extensions to the usual built in types defined by the programmer) encapsulation (a higher level abstraction or representation) actually defining the syntax and operations on this construct!

4. Type Checking n Computer hardware data storage representations usually include no type information n 1110000101010….111 n Can you tell if this is an integer, real, sequence of chars, an instruction? n Common error in assembly languages is to invoke an operation such as integer addition on arguments of the wrong type

5. Type Checking n These errors are difficult to locate because the operation works since the result (usually meaningless) is just a bit string. n A major advantage of using a higher level language is that the language implementation can provide type checking. n Type checking means that each operation executed by a program receives the proper number of arguments of the proper data type

6. Dynamic type checking n At run time n done immediately before operation n usually implemented by storing a type tag in each data object n each operation must attach the appropriate type tags to the results n done in languages that do not require declarations n dynamic types advantage is flexibility in design - programmer is “freed” from most concerns about data types

7. Disadvantages of Dynamic Type Checking n Errors lurk in the program until they are executed (in large programs can be several hours of execution) n not all program paths are executed n type information can be substantial n implemented in software and each operation must be checked before -> speed of execution is likely to be greatly reduced

8. Static Type Checking n Eliminate or minimize dynamic checking-> perform type checking at compile time. n Collect info in symbol table n In most languages, static type checking is not possible for some constructs u dynamic type checking u leaving the operations unchecked

9. Typing n Strong - all type errors can be detected statically (strongly typed) n Text book definition - so only one language can fit in this category - ML n Notice that when a language has either arrays, enumeration, subranges -> it cannot be strong n How do you know that A[1+number] will be in the range of your array? n or that Monday + Sunday will still be part of the enumerated set?

10. Typing n So the question is what will you give up to get a strongly typed language !(or to get a safe run-time program) n Another question you may have … How does a strongly typed language do any operations? Every “overflow” condition is defined in the language. n There is much confusion on strong typing. You either catch ALL errors or you are weak. PERIOD. But I guess there are scales of weakness.. n So there is much confusion out in the “PUBLIC”. And there is a difference between strong type and structured type!

11. Typing n what-is definition what-is definition F by what-is.com n what is a “Merry Strongly-Typed Christmas” n Garbage Collection for Strongly-Typed Languages Garbage Collection for Strongly-Typed Languages n typing and binding, typing and binding, F page by Carl Erickson n PERL PERL for slow connections, I simply downloaded the page and made a local copy

12. Other PL sites n Hotprog.com Hotprog.com

13. Weak - Safe Weak Safe All Programs There are weak programs that are safe (type errors are caught at run time)