Lecture 2 Concepts of Programming Languages Arne Kutzner

Lecture 2 Concepts of Programming Languages Arne Kutzner Hanyang University / Seoul Korea

Topics • • Reasons for Studying Concepts of Programming Languages Programming Domains Language Evaluation Criteria Influences on Language Design Language Categories Language Design Trade-Offs Implementation Methods Programming Environments Concepts of Programming Languages 2

Reasons for Studying Concepts of Programming Languages • Improved background for choosing appropriate languages – Reduction of the risk of wrong decisions • Better use of languages that are already known • Better understanding of significance of language implementations Concepts of Programming Languages 3

Programming Domains • Business applications – E. g. Middleware that implements some business process – Java, COBOL (still popular here!) • Web programming – Languages: markup (e. g. , HTML), scripting (e. g. , PHP), general-purpose (e. g. , Java) • General purpose applications – Examples: Photoshop, Autocad, Word …. – Reliability and efficiency are important • Systems programming / Operating System implementation – Low level, code efficiency is very important – C, Assembler • Scientific applications – E. g. Simulations; computational expensive tasks – Fortran, (C, C++) • Artificial intelligence – Experimental work with languages like LISP (Scheme) and Prolog – However, still on the level artificial stupidity Concepts of Programming Languages 4

Language Evaluation Criteria • Readability: Is this code easily readable? • Writability: Do you like this coding? ++++[>++++[>++>+++>+<<<<-]>+>+>->>+[<]<-]>>. > ---. +++++++. >>. <-. <. +++. --------. >>+. >++. – Hello World in Brainfuck (http: //esolangs. org/wiki/Brainfuck) • Reliability: unexpected crashes, blue screen of death • Cost: $ $ $ … Concepts of Programming Languages 5

Evaluation Criteria: Readability / Writability • Overall readability/writability – Are the constructs of the language self describing/intuitive/well human readable … – Syntax considerations • Special symbols and their meaning (e. g. creation of compound statements) • Special words, meaningful keywords • Data types and structures – Adequate predefined data types and structures – The presence of adequate facilities for defining new data structures Concepts of Programming Languages 6

Evaluation Criteria: Readability / Writability • Support for abstraction – The ability to define and use complex structures or operations in ways that allow details to be ignored • Expressivity – A set of relatively convenient ways of specifying operations – Strength and number of operators and predefined function Concepts of Programming Languages 7

Evaluation Criteria: Reliability • Static type checking vs. dynamic type checking – Recognition of type errors during compile time / runtime • Exception handling – Intercept run-time errors and take corrective measures Concepts of Programming Languages 8

Evaluation Criteria: Cost • Training programmers to use the language • Language implementation system: Availability of free compilers • Reliability: poor reliability leads to high costs • Maintenance costs • Deployment costs Concepts of Programming Languages 9

Further Evaluation Criteria … • Portability – The ease with which programs can be moved from one implementation to another • Generality – The applicability to a wide range of applications • Well-definedness – The completeness and precision of the language’s official definition Concepts of Programming Languages 10

Influences on Language Design • Computer Architecture – Architecture as driving factor of language design. – E. g. Von Neumann Architecture – Open. CL/CUDA for computing on GPUs • Programming Methodologies – Abstract data types – concept of object orientation • Computational models / Mathematical models for computation – Lambda Calculus, Predicate Logic Concepts of Programming Languages 11

Von Neumann Architecture Concepts of Programming Languages 12

Programming Methodologies History / Mainstream developments • 1950 s and early 1960 s: Simple applications; worry about machine efficiency • Late 1960 s: People efficiency became more important; readability, better control structures – structured programming – top-down design and step-wise refinement • Late 1970 s: Process-oriented to data-oriented – data abstraction • Middle 1980 s: Object-oriented programming – Data abstraction + Inheritance + Polymorphism – Appearance of C++, Eiffel … Concepts of Programming Languages 13

Imperative Languages • Inspired by von Neumann computers – Data and programs stored in memory – Memory is separate from CPU – Instructions and data are piped from memory to CPU • Characteristics of imperative languages – Variables model memory cells – Assignment statements used for assigning values to memory cells – Iteration represents central concept – Popular examples: C, Pascal Concepts of Programming Languages 14

Language Categories / Families • Imperative – Comprises languages that support object-oriented programming – Comprises scripting languages – Examples: C, Java, Perl, Java. Script, Visual BASIC. NET, C++, C# • Markup/programming hybrid – Markup languages extended to support some programming – Examples: HTML, XML, PHP, XSLT • Functional – Main means of making computations is by applying functions to given parameters – Examples: LISP, Scheme, Haskell • Logic – Rule-based (rules are specified in no particular order) – Example: Prolog Concepts of Programming Languages 15

Language Design Trade-Offs • Reliability vs. Cost of execution – Example: Java demands all references to array elements be checked for proper indexing, which leads to increased execution costs • Writability (flexibility) vs. Reliability – Example: C pointers are powerful and very flexible but they are unreliable Concepts of Programming Languages 16

Implementation Characteristics of languages • Compilation – Programs are translated directly into machine language • Pure Interpretation – Programs are interpreted by another program known as an interpreter • Hybrid Implementations – A compromise between compilers and pure interpreters Concepts of Programming Languages 17

Compilation • Translate high-level program (source code) into machine code (executable file) • Slow translation, fast execution • Phases of compilation process: 1. Lexical analysis: converts characters in the source program into lexical units 2. Syntax analysis: transforms lexical units into parse trees which represent the syntactic structure of program 3. Semantics analysis: generate intermediate code 4. Optimization: automatically apply improvements 5. Code generation: machine code is generated Concepts of Programming Languages 18

Additional Compilation Terminology • Linking: the process of collecting “objects files” for creating an executable file as output. Concepts of Programming Languages 19

Pure Interpretation • Advantages – No translation/compilation required • Disadvantages – Errors are recognized during runtime – Slow execution speed (10 to 100 times slower than compiled programs) – No static type-check, because of the absence of compilation • Significant in the area of Web scripting languages (e. g. Java. Script, PHP) Concepts of Programming Languages 20

Hybrid Implementation Systems • A compromise between compilers and pure interpreters • A program code is first translated to an intermediate code (called byte code) for later execution on a virtual machine • Faster than pure interpretation • More portable than compiled code • Examples – Java, C# Concepts of Programming Languages 21

Hybrid Implementations Write Source Code Compile Source Code program text in human readable form all 5 steps of compilation Byte Code represents an intermediate code Execute Byte Code using a byte-code interpreter Concepts of Programming Languages 22

Just-in-Time Compilation • Optimization for hybrid implementations • Instead of interpreting the byte-code is first compiled into machine code and this machine code is executed direct on processor level • Higher performance compared to interpretation • JIT-compilation requires initially extra time. So it delays code execution / program start • Nowadays standard with most hybrid implementations Concepts of Programming Languages 23

Preprocessors • A preprocessor processes/changes source code before it is compiled – Works like a macro mechanism and implements a text to text transformation • C, C++ preprocessor – expands #include, #define, and similar macros • Not popular outside C and C++ • Main disadvantage: Compiler error messages can become quite cryptic/strange Concepts of Programming Languages 24

Integrated Development Environments • Not part of the programming language itself; only supportive tools for convenient software development • Popular examples: – Microsoft Visual Studio tools: • Supports MS-compiler for C#, Visual BASIC. NET, Jscript, J#, and C++ – Eclipse • Open programming environment that supports many programming languages – Net. Beans • An integrated development environment for Java provided by Oracle Concepts of Programming Languages 25