Programming Languages (CS )

Transcription

1 Programming Languages (CS ) Dr. Sara Porat IBM Haifa Research Lab Spring 2011 Based on slides by Prof. Yossi Gil, Prof. Ron Pinter and Dr. Tal Cohen

2 Administration Course material Most material is provided in class and tutorials Lectures are mostly based on Watt s book Programming Language Concepts and Paradigms, by David A. Watt. Prentice Hall, Chapters 1-8,10,13-14 A list of other text books appear on the course web site Prerequisites and co-requisites are strictly enforced Running announcements will be sent through the course mailing list; please subscribe from the course s webpage Course staff Dr. Sara Porat Mr. Yuval Shimron (head TA) Requests and questions: shimron@cs.technion.ac.il porat@il.ibm.com

3 Assignments: Assignments and Grading Every 1-2 weeks Total weight is up to 20% (not optional) Teams of 2 (strict!) - matching services provided by teaching assistants Final exam: Date(s) set by the authorities Weight is 80% or more Final Grade: If exam > 50 then 0.8*exam + 0.2*assignments Else 1.0*exam

4 Few words on myself Background in programming languages

5 What are you going to study in this course? Explain the concepts underlying programming languages, and how languages use these concepts in different ways Examples will be given through a wide range of languages There are over 2,000 different languages; but there are basically few paradigms Theoretical part will give you a new way of seeing things Systematic approach for studying new languages Study (mostly during the tutorials) few (more) languages PASCAL (well designed imperative language) Java (leading OO language) ML (neat functional language) Prolog (elegant logic programming) Will prove useful for other courses: OOP Declarative Programming Compilation Software Engineering Semantics of Programming Languages

6 What are the goals of this course? Understand the design space of programming languages Conflicts and trade-offs between language constructs Implementation costs Develop a better understanding of the languages that you use by comparing them with other languages The study of programming languages is, in part, the study of conceptual frameworks for problem solving and software development

7 At the end of the course you ll know What distinguishes different programming languages from one another A variety of mechanisms in familiar and less familiar programming languages Programming in the functional language ML and in the logic language Prolog Some basic concepts in Pascal, Java and others Two main skills: How to learn a new language easily How to use languages correctly and productively There is nothing worse than writing JavaScript code like a C programmer

8 What kind of a beast is JavaScript? Imperative, With prototypes (object-based, but not object-oriented), Functions are first-class entities, Has lambda functions, With closures, Is weakly typed, Has dynamic typing, Has dynamic scoping, By the end of these course, most of these terms will be covered in depth. and a must-know for any modern website developer!

9 Possible approaches Define and compare paradigms of programming languages Present formal approaches to syntax and semantics Present ways of implementing and analyzing programs in various programming languages Show the concepts that must be dealt with by any programming language, and the possible variety in treatment

10 1 Introduction Course Topics *) 2 Values 3 Storage 4 Bindings 5 Abstractio n 6 Encapsulation 7 Type Systems 8 Sequencers 9 Concurrency 10 Imparative Paradigm 12 Object Paradigm 13 Functional Paradigm 11 Concurrent Paradigm 14 Logic Paradigm C Pascal ML Prolog *) according to Watt's book chapters

11 Who Needs Programming Languages? Computers' native tongue is machine language Programmers need higher level languages, because: They can't write machine language correctly They can't read machine language fluently They can't express their ideas in machine language efficiently Life is too short to program in machine language. A formal language is not only a man-machine interface but also a person-to-person language! Conclusion: Programming languages are a compromise between the needs of humans and the needs of machines

12 What is a Programming Language? A linguistic tool with formal syntax and semantics The syntax is concerned with the form of programs The semantics is concerned with the meaning Programming languages cannot be compared with natural languages in terms of their range, expressiveness or subtlety The natural linguist is restricted to analyzing existing languages Programming languages are consciously designed, and can be implemented on computers The syntax influences how programs are written, read and parsed The semantics determines how programs are composed, understood and interpreted

13 Compile Time vs. Run Time A programmer would like to find errors at compile time But some errors may not surface until run-time Methods that detect program errors at compile time are usually conservative Static analysis of source code Compile-time optimization Legacy modernization Impact analysis

14 Aren t All Languages Pretty Much the Same? The move from C to C++ isn t insurmountable Moving from C++ to Java is trivial And if you know Java, you pretty much know C#, too Even if the syntax isn t C-style used (e.g., Eiffel), it can t be that difficult, right? Why make such a fuss about it?

15 SQL: What s the Difference? SELECT first_name, last_name, age FROM user WHERE first_name = David ORDER BY age vs. SELECT first_name, last_name, age FROM user WHERE age > 18 AND age < 65 ORDER BY age Think about implementing these queries in C++. Would the code look just the same in both cases? The difference between the Declarative paradigm and the Imperative paradigm

16 What is a Paradigm? par a digm (Merriam-Webster Collegiate Dictionary) a philosophical and theoretical framework of a scientific school or discipline within which theories, laws, and generalizations and the experiments performed in support of them are formulated Thomas Kuhn ( ) A set of universally recognized scientific achievements that for a time provide a model for a community practitioners. Programming Paradigm (wikipedia) A programming paradigm is a fundamental style of computer programming. Paradigms differ in the concepts and abstractions used to represent the elements of a program (such as objects, functions, variables, constraints, etc.) and the steps that compose a computation (assignment, evaluation, data flows, etc.) Paradigms enrich the way we think about solving problems A family of languages with similar basic constructs and mental model of execution

17 Programming Language Paradigms Imperative Pascal C Logic Procedural bash C++ Object-Oriented Java Eiffel Prolog Datalog SQL Declarative CLOS Functional ML Lisp (CLOS: Common Lisp Object System)

18 Requirements from a Programming Language Universal: every problem (that can be solved at all by a computer) must have a solution that can be programmed in the language Express recursive functions Exception: domain-specific languages No recursion in standard SQL Natural: application domain specific The programming language should be reasonably natural for solving problems within its intended application area It s not reasonable to solve a GUI problem in FORTRAN or a compiler in Cobol Implementable: Neither mathematical notation Nor natural language Efficient Open to debate

19 Desiderata for a Programming Language Expressiveness Turing-completeness But also a practical kind of expressiveness: how easy is it to program simple concepts? Efficiency Open to debate: What is acceptably efficient? Recursion in functional languages is expressive but sometimes inefficient Strongly influenced by currently available computer architectures Also, efficiency in development and testing

20 Wirth's law is a computing adage made popular by Niklaus Wirth in 1995: Software is getting slower more rapidly than hardware becomes faster

21 Desiderata for Programming Languages (cont d) Simplicity - as few basic concepts as possible Sometimes a trade-off with convenience (C has for, who needs while and do-while?) Uniformity and consistency of concepts Why does for in Pascal require a single statement (including begin.. end construct) while repeat allows any number of statements? Abstraction - language should allow to factor out recurring patterns Clarity to humans The distinction = vs. == in C is a bit confusing Information hiding (encapsulation) and modularity Safety - possibility to detect errors at compile time

22 Less is More Two program fragments to find the n th Fibonacci number in Algol 68 x,y := 1; to n do (if x<y then x else y) := x+y; x := max(x,y); x,y := 1; to n do begin x,y := y,x; x := x+y end;

23 What Characterizes a Programming Language 1. Concepts How it handles values How it checks types How it stores values How it manipulates values How it manages control How it attaches names to values How it allows generalization => values, types and expressions => typing systems => storage => commands => sequencers => binding => abstraction

24 What Characterizes a Programming Language (cont d) 2. Paradigms Imperative programming: Fortran, Cobol, Algol, PL/I, C, Pascal, Ada, C++, Icon, Modula-2, Modula-3, Oberon, Basic Concurrent programming: Ada, Occam, Par-C. Object-oriented programming: Small-talk, Self, C++, Objective-C, Object Pascal, Beta, CLOS, Eiffel Functional programming: Lisp, Scheme, Miranda, ML Logic programming: Prolog, Prolog-dialects, Turbo-Prolog, Icon

25 The Imperative Paradigm Fortran, Algol, C, Pascal, Ada The program has a state reflected by storage and location It comprises commands (assignments, sequencers, etc.) that update the state of the program They can be grouped into procedures and functions There are also expressions and other functional features Models real-world processes, hence still dominant Lends itself to efficient processing (optimizing compilers etc.)

26 The Functional Paradigm Lisp, Scheme, ML, Haskell Everything is a function that takes arguments and returns results Don t really need assignment operation or sequencers Use recursive activation of functions instead of iteration Moreover, the functions are just another kind of value that can be computed (created), passed as a parameter, etc. Elegant, extensible, few basic concepts Used for list manipulation Requires a truly different perception Using an imperative programming style in ML is even worse than a word-for-word translation among natural languages

27 The Logic Programming Paradigm Prolog, constraint languages, database query languages Declaring some facts about objects and their relationships Defining some rules about objects and their relationships Predicates as the basis of execution: Asking questions about objects and their relationships A computation is implicit it shows what follows from the given facts and rules Emphasizes what is needed, rather than how to compute it Used for expert systems Will see the basics of Prolog later in the course

28 The Object-Oriented Paradigm C++, Smalltalk, Eiffel, Java The world has objects that contain both fields with values and operations (called methods) that manipulate the values Objects communicate by sending messages that ask the object to perform some method on its data Types of objects are declared using classes that can inherit the fields and methods of other classes Polymorphic calls: the ability to call a variety of functions using exactly the same interface Has become the primary paradigm because it seems to treat large systems better than other approaches Treated mainly in the follow-up course Object-Oriented Programming (236703) Will do a little bit of Java in the recitations

29 Program Development Requires a lot of administration 1. Use of some text editor or an Interactive Development Environment (IDE) to prepare the files that constitute the program 2. Some acquaintance with the underlying Operating System to manage the files 3. Handling compiler errors 4. Running the program

30 Preparing a program from GNU/Linux OS % rm -f hello.c; cat << EOF > hello.c /* ** hello.c: My first C program; it prints ** "Hello, World!", and dies. */ #include <stdio.h> int main(int argc, char *argv[], char *envp[]) { printf("hello, World!\n"); return 0; } EOF

31 Editing a file using gvim gtksourceview library in GNU/Linux to support editing of programs in more than 70 programming languages

32 Basic Syntax Elements The Language Tokens Literals Comments Keywords Identifiers Operation symbols (+, = ) Punctuation symbols

33 #include <stdio.h> int main(int argc, char *argv[], char *envp[]) { printf( Hello, World!\n ); return 0; } One comment (lines 1-4) Three keywords Two identifiers Two literals Eight symbols Hello World in C

34 Hello World in Pascal {hello.p: My very first Pascal program. It does not do too much --- it prints the string Hello, World! (followed by a new line), and then terminates.} program HelloWorld(output); begin WriteLn( Hello World! ) end. One comment Three kewords Three identifiers (two of them, marked in blue are pre-defined) One literal Four symbols

35 IDENTIFICATION DIVISION. PROGRAM-ID. HELLO-WORLD. * Hello, World! in COBOL ENVIRONMENT DIVISION. CONFIGURATION SECTION. SOURCE-COMPUTER. RM-COBOL. OBJECT-COMPUTER. RM-COBOL. DATA DIVISION. FILE SECTION. PROCEDURE DIVISION. MAIN-LOGIC SECTION. BEGIN. DISPLAY LINE 1 POSITION 1 ERASE EOS. DISPLAY Hello world! LINE 15 POSITION 10. STOP RUN. MAIN-LOGIC-EXIT. EXIT. Notice the heavy use of the character - in the identifiers Hello World in Cobol

36 #!/usr/bin/gawk -f # # Hello, World! in AWK # BEGIN { print( Hello, World! ) exit } Hello World in AWK Notice the keyword print to call an I/O routine

37 Types of Identifiers print AWK WriteLn printf C HelloWorld

38 // Hello world in Go package main import fmt func main() { fmt.printf( Hello World\n ) } Hello World in Go Printf is a library identifier

39 Hello World in Ada -- Hello World in Ada with Text_IO; procedure Hello_World is begin Text_IO.Put_Line( Hello World! ); end Hello_World; Put_Line is an identifier defined in the library Text_IO The library is imported through the keyword with

40 Hello World in Java // Hello.java: my first Java program. It prints // the string Hello World on the standard output // stream, and terminates. class HelloWorld { static public void main(string args[]) { System.out.println( Hello World! ); } } The library is imported automatically (by the compiler)

41 Symbols for Denoting Blocks The {} family of languages Java, C++, C#, AWK, GO, No delimiters, see Occam program (next foil) begin end (e.g., Pascal) Attaching a reference to the starting point e.g., end Hello_World in Ada if fi, case esac (Algol 68)

42 Hello World in Occam -- Hello world in Occam #INCLUDE hostio.inc #USE hostio.lib PROC hello.world (CHAN OF SP fs, ts) SEQ so.write.string.nl(fs, ts, Hello World! ) SEQ i = 1 FOR 10 SEQ so.write.int(fs, ts, i, 0) so.write.nl(fs, ts)

43 note Hello World in Eiffel class HELLO create run feature run do print ( Hello World!%N ) end end HELLO Hello World in Eiffel Notice the use of the keyword note to indicate a one-line comment

44 Extended Backus-Naur Form (EBNF) A meta-notation for describing the grammar of a language Terminals = actual legal strings, written as is, or inside Non-terminals = concepts of the language, written construct or construct or construct Rules = expanding a non-terminal to a series of NTs and Ts One non-terminal is designated as the start of any derivation A sequence of terminals not derivable from start symbol by rules of the grammar is illegal is choice among several possibilities [ ] encloses optional constructs { } encloses zero or more repetitions Example: if-stmt = if expression then statement [ else statement ]

45 Example of EBNF expression = term { add-op term } term = factor { mult-op factor } factor = variable-name number add-op = + - mult-op = / Is a + 2/ b - c7 a legal expression? Yes, because there is a sequence of rule applications from expression that yields this string (these can be drawn as a syntax tree )

46 Observations on Syntax If there are several possible syntax trees for the same sequence of terminal symbols, the sequence is syntactically ambiguous, and that often leads to semantic ambiguity (several possible ways to understand the meaning) Good programming language design avoids ambiguity There are some syntactic rules that cannot be expressed just using EBNF (which gives context-free grammars) Every variable used is previously declared The number of arguments in a procedure call equals the number of arguments in the declaration of the procedure Much more on grammars and identifying legal programs you will learn in the courses Automata and Formal Languages and Compilation

47 Development Environments and The Program Starting Point The program code refers to the code written by the programmers (user code) and a set of imported (standard) libraries Defining which files constitute a program and the starting point Autarchic approach Metaphysic approach Holistic approach

48 Autarchic approach One of the keywords denotes the starting point The keyword program in Pascal The keyword BEGIN in AWK A program in Pascal and AWK contains one single file No standard libraries

49 Metaphysic Approach The program starts from a function with a specific name The name is not a keyword The starting point is not part of the language definition Most implementations of C use the name main to denote the starting function The implementation of C for Windows uses the name WinMain The compiler denotes which files constitute the program

50 /* Hello world in C for MS-Windows */ #include <windows.h> C for MS-Windows int PASCAL WinMain(HINSTANCE hinstance, HINSTANCE hprevinstance, LPSTR CmdLine, int Show) { MessageBox( GetActiveWindow(), Hello World!, Hello Windows World, MB_OK); return 0; }

51 Not part of the grammar Holistic Approach Yet, the language defines how to identify the files that constitute the program and what is the starting point These definitions are independent on the development environments (platforms) Eiffel (through the Cluster file), Java

52 Language Processors Systems/applications that process programs (code): Compiler Interpreter Syntax directed editor Program checker Program verifier Studied in other courses: Compilation Program verification Software engineering

53 Relations to Other Fields in Computer Science Databases and Information Retrieval: Query languages - languages for manipulating databases Human-Computer Interaction: Programming Languages are designed to be written and read by humans Operating Systems: Input-Output support. Storage management. Shells are in fact Programming Languages. Computer Architecture: Programming Language design is influenced by architecture and vice versa. Instructions sets are Programming Languages. Hardware design languages.

54 Closely Related Topics Automata and Formal Languages, Computability: Provide the foundation for much of the underlying theory. Compilation: The technology of processing programming languages. Software engineering: The process of building software systems.

55 Language Inception and Evolution Initial definition by a single person: Lisp (McCarthy), APL (Iverson), Pascal (Wirth), REXX (Cowlishaw), C++ (Stroustrup), Java (Gosling) small team: C (Kernighan and Ritchie), ML (Milner et al.), Prolog (Clocksin and Mellish), Icon (Griswold and Griswold) committee: FORTRAN, Algol, PL/1, Ada Some survived, many more perished usability compilation feasibility dependence on platform politics and sociology Most successful languages were taken over by standards committees (ANSI, IEEE, ISO, )

56 1950 Language Genealogy (till 1990) 1960 Algol-60 Fortran Cobol Lisp 1970 Simula Smalltalk Pascal Algol-68 PL/I C Prolog ML 1980 Ada Miranda 1990 OO lang. C++ Imperative and Concurrent lang. Haskell Functional lang.

57 Historical Background Until early 1950s: no real programming languages, but rather a mixture of assembly languages and other aids for machine code programming Early 1950s: the Laning and Zierler System (MIT): a simple algebraic language, a library of useful functions. 1954: Definition of FORTRAN (FORmula TRANslator). Originally for numerical computing. Symbolic expressions, subprograms with parameters, arrays, for loops, if statements, no blocks, weak control structures 1957: first working compiler

58 Early 1960s: COBOL: Data processing. Means for data description. Algol 60: Blocks, modern control structures One of the most influential imperative languages Gave rise to the Algol-like languages for two decades (Pascal, PL/1, C, Algol 68; Simula, Ada) Lisp (list processing language): symbolic expressions (rather than numerical), computation by list manipulation, garbage collection; the first functional language Mid 1960s: Historical Background (cont d) Foundation for S-expressions: defined recursively as either single data objects called "atoms" or lists of S-expressions (= 4 (+ 2 2)) PL/1: an attempt to combine concepts from numerical computation languages (FORTRAN, Algol 60) and data processing languages (Cobol). Simula: object oriented, abstract data types

59 : Several OO languages: Smalltalk, C++, Eiffel Logic Programming: Prolog Functional Programming: ML, Miranda, Haskell Ada: Another attempt, more successful than PL/I, for a general purpose language, including concurrency. Specific usages: SNOBOL, Icon, Awk, REXX, Perl: String manipulation and scripting SQL: Query language for relational databases Mathematica, Matlab, Python: Mathematical applications... Historical Background

60 1990-present: Historical Background Object oriented + WWW: Java, C# Scripting + [OO] + WWW: Perl, Python, PHP, Ruby Client-Side scripting: JavaScript Reuse and design patterns become useful and popular Flexibility in choice of language and moving among languages We shall NOT refer to: Rule languages HyperText Markup Language (HTML) Extensible Markup Language (XML)

61 Program Execution Model A language design must be specific about how all basic operations are done Predict program running time Examples of execution models: Fortran: Flat register machine No stacks, no recursion Memory arranged as linear array Algol family: Stack of activation records Heap storage: an area of memory used for dynamic memory allocation Smalltalk: Objects, communicating by messages