1. Five Generations

2. The five generations We generally count five "generations" of programming languages The generations aren't formally defined Each generation represents technological advances "Advances" may just reflect changing preferences Don't take the distinctions too seriously –but they do provide a good framework for discussion

3. First generation Examples: FORTRAN, COBOL, BASIC Key concept: Language designs were based directly on available hardware Efficiency was everything Language names are acronyms and are typically spelled with all capital letters

4. 1G: Primitive data types Data types reflect types defined by the hardware Multiple types of numbers (fixed, floating, various sizes) Characters and booleans are represented as numbers No user-defined types Identifiers have types, but type safety is not a big concern Identifiers bound directly to storage locations--no dynamic storage

5. 1G: Data structures Data structures are based on those of machine language –that is, single contiguous blocks of storage No nesting of data structures (just arrays, records) –Arrays represent a block of locations (FORTRAN, BASIC) –Records represent layout of data on a punched card (COBOL)

6. 1G: Control structures Control structures are based on those of machine language –Multiple kinds of GOTOs –Little nesting of control structures (exception: DO loops in FORTRAN) –No recursion (exception: BASIC) –A single way to pass parameters (usually by reference)

7. 1G: Syntax One statement per line (line = punched card), fixed columns Hardware doesn't yet support lowercase characters Pre-BNF, so syntax is irregular and inconsistent Keywords not reserved, but context-dependent Scopes are disjoint (because the is only enough memory to compile one subprogram at a time)

8. Second generation Algol 60 was the premier 2G language Key concepts: Abstraction and generalization –Algol 60 introduced the notion of a "virtual machine," not tied to particular hardware –Algol 60 introduced real and integer data types not tied to a particular bit representation, and generalized loop and selection (if) statements Alan Perlis: "Algol was indeed an achievement; it was a significant advance on most of its successors."

9. 2G: Data structures Machine independence seen as a valid concern Simplification and abstraction of numeric types (real, integer) Arrays with user-defined lower bounds (not just 1) Dynamic (expandable) arrays Booleans introduced, but still not characters –Strings could be used for output only Strong typing

10. 2G: Control structures True hierarchical (nested) control structures Generalized control structures –Often overly complex and baroque (e.g. for, switch) "Blocks" provide fine control over nesting, scopes Could pass parameters by value (good idea) Could pass parameters by name (bad idea)

11. 2G: Syntax BNF used to simplify and regularize syntax Free-format programs (not line-oriented) Indentation used to show hierarchical structure of program Typically used reserved keywords rather than keyword-in-context Lowercase letters used (but not supported by hardware)

12. Third generation Example: Pascal Key concepts: Simplicity and generality –More but simpler control structures –Expanded and generalized data types

13. 3G: Data Structures Recognition that not everything is a number Language support for strings (or at least characters) New data types introduced: –sets –subranges –enumeration types User-defined data types Hierarchical nesting of data types User-controllable memory allocation

14. 3G: Control structures More but simpler control structures –three kinds of loop replace Algol's single for loop Case statements introduced –called "switch" in C, Java Simpler control structures can be more efficient as well as easier to understand "Call by name" parameter transmission was discarded

15. Syntax No significant improvements over Algol 60

16. First generation features of C Efficiency is primary concern Based on PDP-7 hardware "Flat" program space--no nested scopes or functions –This weakness leads to need for make Some first generation syntax ("=" for assignment)

17. Second generation features of C Abstractions of numeric types, pointers Pointer and array abstractions are lower-level than some machine hardware –Wrong abstraction level can hinder optimization Seen on a button: "C combines the flexibility of assembly language with the power of assembly language"

18. Third generation features of C Hierarchical data structures (if not programs) Support for strings (sort of), and enumeration types (almost) User-controllable memory allocation Bottom line: C mixes characteristics of three generations

19. Fourth generation Examples are Ada, Modula Key concept: Emphasis on data abstraction

20. 4G: Data structures Separation of specification and definition –Other programmers and program parts can access the specification, not the code –This gives better support for information hiding –But this leads to duplication, which is error-prone Name access by mutual consent Generic or polymorphic subprograms Exceptions (errors) attain the status of a data type

21. 4G: Control structures Concurrent programming, that is, multiple communicating processes Protected or synchronized types Functions can be overloaded (operators, too) Exception handlers result in a new flow of control

22. 4G: Syntax Fully bracketed control structures replace begin...end or {...} Examples: –if...end if; –loop...end loop; –function foo ( )... end foo;

23. Fifth generation, I Key concept: No general agreement on what 5G is Prolog was the basis of the Japanese "Fifth- Generation Project" –The Fifth-Generation Project was not a success O-O languages are now the dominant paradigm –They are the de facto fifth generation But logic programming has a prior claim to the name Functional programming is also a contender for 5G

24. Fifth generation, II O-O languages are Simula 67, Smalltalk, Java Yes, the first O-O language appeared in 1967! Almost all surviving languages now have O-O extensions –C has Objective C, C++ –Pascal has Object Pascal –Ada has Ada 95 –There are O-O versions of FORTRAN, BASIC, Prolog, etc.

25. Fifth generation, III Fifth generation (Smalltalk, Java) Key concept: Object orientation Three essential features: –Abstract data types (grouping of data and methods) –Inheritance (of types and methods, with overriding) –Messages (dynamic binding to methods)

26. 5G: Data structures Everything (or almost everything) is an object All behaviors (methods) are encapsulated within objects Objects are arranged in a hierarchy But object space is still "flat," with little or no nesting –Java's inner classes are too little, too late Data and methods are inherited Data and methods can be overridden

27. 5G: Control structures Instead of calling functions, send messages to objects –Variables can hold different kinds of objects at different times –Therefore, messages sent to the variable may go to different kinds of objects Most O-O languages have support for GUI events –An event is a message sent "to whom it may concern" –Event-handling may be container-based (Mac, Java 1.0) –Better event-handling is listener-based (Java 1.1)

28. 5G: Syntax First support for international alphabets No other improvements in syntax In fact, Java syntax is a throwback to the C era

29. Advantages of O-O languages Most significant: best solution (so far) to code reuse –You can inherit data and methods from predefined objects –It's easy to override methods or to adapt them This changes completely the way programmers work –Don't write from scratch; find something similar and adapt it –Along with syntax, you must now learn vast libraries Additional advantage: first real support for GUIs and for event handling

30. Summary: Data structures First generation: Flat (non-nested) blocks of storage Second generation: Generalized numbers, strong typing Third generation: New data types, user-defined data structures, dynamic memory allocation Fourth generation: Non-lexical control of data access Fifth generation: Objects encapsulate their methods

31. Summary: Control structures First generation: Based on machine instructions, with heavy use of GOTOs Second generation: Machine independent, nested, but jack-of-all-trades Third generation: More but simpler control structures Fourth generation: Concurrent programming, exception handling Fifth generation: Messages to objects, event handling

32. Summary: Syntax First generation: Line-oriented, inconsistent syntax, disjoint scopes Second generation: Free form, uniform syntax, nested scopes Third generation: No advances Fourth generation: Fully bracketed control structures Fifth generation: No advances, some losses

33. But wait...there's more! Simula 67 was the first true O-O language – This shows that it takes time to get good ideas into mainstream languages Logic languages (Prolog) and purely functional languages (ML) still have some very good ideas Giant, all-inclusive languages (PL/I, Algol 68) are not the answer Finally, don't take this classification scheme too seriously!

34. The End