COS 301                                                   Topics
                                Programming Languages                                                  • Reasons for Studying Concepts of Programming 
                                                                                                           Languages
                        Preliminaries                                                                  • Programming Domains
                                                                                                       • Language Evaluation Criteria
                                                                                                       • Influences on Language Design
                                                                                                       • Language Categories
                                                                                                       • Language Design Trade-Offs
                                                                                                       • Implementation Methods
                                                                                                       • Programming Environments
                         Why Study Concepts of Programming                                             Why Study Concepts of Programming 
                         Languages?                                                                    Languages?
                          • Increased ability to express ideas                                         • To improve expressiveness
                          • Improved background for choosing appropriate                                   – Expressiveness increases with vocabulary 
                            languages                                                                      – Ability to visualize solutions increases with new 
                          • Increased ability to learn new languages                                         programming constructs and depth of abstraction
                          • Better understanding of significance of                                        – If the only tool you know is a hammer, the only 
                                                                                                             solutions you can see involve nails
                            implementation                                                                 – Even if a language does not directly support a 
                          • Better use of languages that are already                                         particular abstract feature (e.g. associative array) 
                            known                                                                            you can simulate it as part of solution
                          • Overall advancement of computing
                         Gain improved background for choosing                                         Improve your ability to learn new 
                         appropriate languages                                                         languages
                         • A fundamental result of theory is that all                                  • First language or two are difficult and time-
                            languages are equivalent in computing power                                  consuming to learn
                         • Anything that can be computed in Java or                                    • These languages also constrict your view of 
                            Python can also be computed in BASIC, COBOL or                               languages
                            FORTRAN                                                                       – As you learn the abstract concepts underlying 
                             – But some languages are better in terms of                                    programming languages you acquire a framework for 
                                                                                                            understanding a new language
                               expressiveness for a given problem                                         – Eventually you can read many unfamiliar languages 
                             – Learning just one or two languages in great depth can                        with little or minimal effort
                               be hazardous to your career                                                – Current trend is to combine relatively simple 
                         • Let’s consider some problems…                                                    language with a huge library. Why re-invent the 
                                                                                                            wheel?
                   The Tiobe Index                                            Gain a better understanding of significance 
                                                                              of implementation
                   • See                                                      • Programming languages provide a virtual 
                     http://www.tiobe.com/index.php/content/pap                 machine with which we express problems and 
                     erinfo/tpci/index.html                                     their solutions.
                   • Counts top languages by counting hits of the             • The virtual machine has to execute on a real one
                     most popular search engines                              • Understanding the implementation of a 
                   • See                                                        programming language helps to
                     http://www.tiobe.com/index.php/content/pap                  – Achieve more efficient problem solutions
                     erinfo/tpci/tpci_definition.htm                             – Avoid subtle bugs caused by particular language 
                                                                                  implementations
                   Gain better use of current languages                       Programming Paradigms
                   • As languages evolve, they become large and               • A programming paradigm is a pattern of 
                     complex compared to earlier languages                      problem-solving thought that underlies a 
                   • Many modern languages are multi-paradigm                   particular genre of programs and languages.
                     languages – procedural, functional, object               • Many people classify languages into four main 
                     oriented                                                   paradigms:
                   • A broad understanding of concepts and                       – Imperative (Procedural)
                     paradigms allows you to better leverage your                – Object-oriented
                     knowledge of current languages                              – Functional
                                                                                 – Logic (declarative)
                   Imperative or Procedural Paradigm                          A Von Neumann Computer
                   • The oldest programming model
                   • Follows the classic von Neumann-Eckert model:
                      – Program and data are indistinguishable in memory
                      – Program = a sequence of commands
                      – State = values of all variables when program runs
                   • Large programs use procedural abstraction
                   • Language has assignments, loops, conditionals, 
                     procedure and functional calls, control flow
                   • Example imperative languages: 
                   •    Cobol, Fortran, C, Ada, Perl, …
                          Object Oriented (OO) Paradigm                                                 Functional Paradigm
                          • An OO Program is a collection of objects that interact by                   • Functional programming models a computation as a 
                            passing messages that transform the state.                                    collection of mathematical functions.
                          • Concepts of OOP:                                                               – Input = domain
                             – Sending Messages                                                            – Output = range
                             – Inheritance                                                              • Functional languages are characterized by:
                             – Polymorphism                                                                – Functional composition
                          • Example OO languages:                                                          – Recursion
                             – Smalltalk, Java, C++, C#, and Python                                        – Conditional evaluation
                          • We will look at Smalltalk in detail later. Languages such                   • Example functional languages:
                            as Java and C++ are OOP concepts grafted onto a                                – Lisp, Scheme, ML, Haskell, …
                            procedural core                                                                – Surprisingly Javascript supports functional programming very 
                                                                                                             well!
                          Logic Paradigm                                                                Declarative Languages
                          • Logic programming declares what outcome the                                 • Prolog is a unique language with its basic ideas 
                            program should accomplish, rather than how it                                 of facts, rules and atoms combined into a 
                            should be accomplished.                                                       program often called a database 
                          • When studying logic programming we see:                                     • There are other declarative languages where a 
                             – Programs as sets of constraints on a problem                               “program” consists only of statements declaring 
                                                                                                          what the result should be, not how to do it
                             – Programs that achieve all possible solutions                             • SQL (Structured Query Language) is the prime 
                             – Programs that are nondeterministic                                         example
                          • Example logic programming languages:                                           – But all SQL implementations have procedural 
                             – Prolog                                                                        extensions (PL/SQL, T-SQL, etc.)
                                                                                                        • XPath is another example
                          Markup Hybrid Languages                                                       Markup Hybrids
                          • Pure markup languages are mostly based on                                   • XSLT (Extensible Stylesheet Language 
                            Standardized General Markup Language (SGML)                                   Transformations) is a declarative XML based 
                          • HTML and XML are examples of two pure markup                                  language used for transforming from XML to XML 
                            languages                                                                   • Often used to transform XML documents into 
                             – No processing is done by the language                                      HTML (a subset of XML) for presentation in 
                             – Markup provides directions to an external tool                             browsers
                          • But markup languages have been extended to                                  • Requires a processing engine that takes XML and 
                            support some programming                                                      XSLT as input and outputs XML (or HTML)
                          • Examples: JSTL, XSLT                                                        • Capabilities overlap XQuery
                                         Programming Domains                                                                                                             Programming Domains
                                         • Scientific applications                                                                                                              • Computer software operates in a large number of 
                                               –  Large numbers of floating point computations; use of arrays                                                                        different domains
                                               –  Fortran                                                                                                                             –  Scientific applications
                                         • Business applications                                                                                                                      –  Business applications
                                               –  Produce reports, use decimal numbers and characters                                                                                 –  Artificial intelligence
                                               –  COBOL                                                                                                                               –  Systems programming
                                         • Artificial intelligence                                                                                                                    –  The Internet
                                               –  Symbols rather than numbers manipulated; use of linked lists                                                                        –  Multimedia
                                               –  LISP                                                                                                                                –  Embedded Systems
                                         • Systems programming                                                                                                                        –  Industrial Control
                                               –  Need efficiency because of continuous use
                                               –  C                                                                                                                             • Some programming languages have been designed 
                                         • Web Software                                                                                                                              for specific domains
                                               –  Eclectic collection of languages: markup (e.g., XHTML), scripting                                                             • Others were explicitly design as general purpose 
                                                  (e.g., PHP), general-purpose (e.g., Java)
                                         Scientific Applications                                                                                                         Scientific Applications
                                         • AKA Number Crunching. The first computer                                                                                      • Earliest high-level languages (FORTRAN, from 
                                              applications (1940’s)                                                                                                          FORmula TRANslation) were designed for 
                                         • Normally involve simple data structures, simple                                                                                   scientific computing
                                              control structures and large numbers of floating                                                                           • Efficiency is primary concern  -- competition 
                                              point computations                                                                                                             with assembler
                                         • Abstraction is usually applied OUTSIDE the                                                                                    • Algol 60 was designed to provide better 
                                              language before the computational problem is                                                                                   expressiveness and abstraction but failed to 
                                              expressed in a language                                                                                                        catch on 
                                                                                                                                                                         • FORTRAN is still very much in use – only recently 
                                                                                                                                                                             has C begun to be adopted in the domain
                                         Business Applications                                                                                                           COBOL
                                         • Early 1950’s people realized that computers                                                                                   • More lines of COBOL are in existence and still 
                                              were very good at doing tedious and boring tasks                                                                               running than any other computer language 
                                         • Computers were specifically designed for                                                                                      • If you want job security learn COBOL – there’s 
                                              business (high I/O capabilities) as were                                                                                       always demand for people who can maintain the 
                                              languages                                                                                                                      COBOL corpus
                                         • COBOL (COmmon Business Oriented Language)                                                                                     • Other developments in business applications are 
                                              was first designed in 1959 (based on Grace                                                                                     of little interest to computer science; e.g., RPG 
                                              Hopper’s Flow-Matic language)                                                                                                  (Report Program Generator)
                                         • In 2002 object-oriented extensions were grafted                                                                               • Not discussed much in text
                                              onto the language