Introduction%20to%20Programming%20Languages

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Introduction to Programming Languages

• CS331

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What is a Programming Language?

• Formally, a set of finite sequences of elements
  of the alphabet
• E.g., abcd
• It has a syntax and a semantics
• We will define these in great detail shortly
• Communicates only an algorithmic idea
• A programming language makes it easier and
  efficient for people to communicate with
  computers

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Principles of Language Design

• The major principles that underlie the design of
  modern programming languages are
• Syntax
• Type systems (semantics)
• Memory Management
• Exception Handling
• We treat these categories formally instead of
  just comparing features of different languages to
• Apply more rigor (needed for example in writing a
  compiler)
• Understand future languages better (formalisms
  universal)

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Syntax

• Tells us what constitutes a correctly written
  program
• Vocabulary
• Alphabet
• Not unlike work with spoken languages
• Borrows from linguistic theory, grammar
• Most programming languages use a context-free
  grammar

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Type Systems and Semantics

• If the syntax is correct
• What kind of values can we manipulate?
• Is a 32 bit value an integer, a float, 4 ASCII
  characters, etc.
• Semantics applies meaning to the data values
• What is the exact effect of a statement on the
  state of computation?
• What happens next?
• These must all be defined somehow we will use a
  formal abstract definition for semantics

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Memory Management

• Collection of techniques to map values and data
  structures to memory
• Static vs. Dynamic memory
• Use of the heap
• Garbage collection and reclamation
• Numerous algorithms and strategies have been
  designed to manage memory

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Exception Handling

• An exception is an unexpected event
• Division by zero
• Overflow
• Illegal memory access
• How can these be trapped and handled?
• (will mostly skip this chapter in the class)

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Tower of Babel of Programming Languages

• What programming languages do you already know or
  can think of the names?

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Why so many languages?

• Application domains
• Levels of Languages
• Different philosophies to solve problems
• Language improvement
• Standards

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Application Domains

• Scientific Computing
• Mathematical, Parallel, Historically FORTRAN
• Management Information Systems
• Database, SQL
• COBOL for business systems
• Client/Server approach popular
• Artificial Intelligence
• Intelligent behavior, deduction
• Roots of functional and logical programming
• Systems
• Compilers, OS, network, etc.
• Typically low level languages, Assembly/C
• Web
• Overlap with other applications
• Client/Server, often interpreted
• Education
• PASCAL, ALICE

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Programming Paradigms

• Paradigm Model or mental framework for thinking
  about something
• Procedural/Imperative
• Programs are a series of steps assignments,
  loops, sequences, conditional statements
• Object-Oriented
• Collection of interacting objects inheritance,
  polymorphism
• Functional
• Program is a collection of mathematical functions
  using functional composition
• Logic
• Program is a collection of logical declarations
  states what the outcome should be rather than how
  to accomplish the outcome
• Event-Driven
• Continuous loop responding to events
• Concurrent
• Collection of cooperating processes issues for
  parallelism

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Pragmatic Considerations

• Architecture constraints
• Stuck with the von Neumann model?
• Instruction set matches with Imperative model
• Contextual constraints
• Relates back to application area, levels of
  languages
• General purpose vs. special purpose
• Compiled vs. interpreted

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Virtual Machine

• This text uses the virtual machine model for a
  language called Clite
• Interpreter is Java, so Clite is really
  doubly-interpreted! (or Just-In-Time Compiled)
• By doing so, we avoid details of mapping a
  language to a specific machine architecture

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Standards

• Standardization process of defining a
  machine-independent definition to which all
  implementors of the language must adhere
• Used to be bottom-up and standardization only
  took place if a language was used enough
• Process is becoming top-down with larger
  companies (e.g. Microsoft, Sun)
• Major standards organizations, ANSI, ISO, ECMA
• Slow process, e.g. ANSI/ISO C (1999)

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Brief history of PLs
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FORTRAN

• FORmula TRANslation
• IBM 1957 John Backus
• First high level language
• Designed to support numerical computation
• Still used today in scientific computing
• Code snippet
• 10 IF (NUMBER .LT. 0) GO TO 20
• READ (,) NUMBER
• GO TO 10
• 20
• Early versions used GOTO
• A simple FORTRAN error resulted in errors for
  NASAs Project Mercury sub-orbital flights (myth
  for Mariner Venus probe)
• DO 3 I 1,3 was mistyped as DO 3 I 1.3 which
  was accepted by the compiler as assigning 1.3 to
  the variable DO3I

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COBOL

• Common Business Oriented Language
• Designed for business needs payroll, accounting,
  etc.
• Responsible for huge amounts of legacy code
• Designed to be English-Like to make it easy to
  program
• ADD A TO B GIVING SUM

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Pascal

• 1970 Nicklaus Wirth
• Designed to enforce good programming techniques
  as a teaching language, easy to learn
• Taught in most schools/universities in the 80s
• Not embraced as an industrial language instead
  they used C
• But offshoot language Delphi (Borland) is used
  commonly and commercially today competitor to
  Microsofts Visual Basic

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C

• 1970 Dennis Ritchie, Bell Labs
• Originally designed to write UNIX
• Close to assembly, allows speed efficiencies
• Led to its popularity today, the basis for just
  about every operating system (including Windows)
• One of the most popular choices when it comes to
  real-time systems

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C

• 1980 Bjarne Stroustrup, Bell Labs
• Superset of C
• Adds object-oriented programming on top of C
• Standardized by ANSI/ISO
• One of the most popular commercial languages
  today, allows low-level access of C together with
  OOP
• Well examine C in this class

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C

• Relatively new language developed by Microsoft
• ECMA standard, on fast track for ISO standard
• Combines aspects of C/Java/Visual Basic
• OOP
• Uses concept of virtual machine (CLI) - .NET
• Easy integration with web services
• Easy development of GUI via IDE
• Cleans up some messiness of previous languages
• We will also examine C in this class

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ADA

• Named after Ada Lovelace
• Daughter of Lord Byron
• Mathematician, Scientist
• 1979 US Armed Services
• Intended as a common language for all government
  contractors
• Powerful OOP, Multiprocessing, structures for
  efficiency and re-usability
• Criticized for being difficult to learn
• Dont see much ADA around nowadays

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Java

• 1994 James Gosling, Sun Microsystems
• OOP
• Virtual machine
• Both interpreted and compiled
• Rise of the web has been attributed to its
  success
• No ISO/ECMA standard of Java
• Sun pulled out of the standards process in 1999
  for more control
• Does have Java Community Process

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Lisp

• 1958 John McCarthy
• LISt Processing
• Based on Alonzo Churchs Lambda Calculus
• Computing with symbolic expressions rather than
  numbers
• List structure as the key data structure
• Composition of functions as a tool for forming
  more complex functions
• Sample
• (defun list-of (n elt)
• (if (zerop n)
• nil
• (cons elt (list-of (- n 1) elt))))
• Well study Scheme, an offshoot of Lisp

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SML

• 1980s Standard Meta Language
• Edinburgh Lab, Robin Milner
• Fully formal definition
• Close to mathematics (sets, tuples, functions)
• No declarations as in imperative languages
• No side effects
• Type of variables is computed by inference,
  depending on context
• 1,4,5 list of integers
• (1,1) is a tuple of integers
• fun square(x) xx

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Prolog

• 1972 - Alan Colmeraurer
• PROgramming LOGic
• ISO standard 1995
• A Prolog program is a set of facts and a set of
  logical rules
• speaks(mary, russian).
• speaks(bob, english).
• speaks(mary,english).
• talkswith(P1,P2) - speaks(P1,L), speaks(P2,L),
  P1\P2.
• Program that defines people that speak Russian
  and that you talk to someone that speaks the same
  language.

• Queries speaks(mary,X). Xenglish, russian

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Special Purpose Languages

• Languages designed for specific tasks
• SQL
• Retrieval from relational databases
• PERL
• Practical extraction and report language
• Processing strings, regular expressions
• HTML
• Markup of hypertext
• When we get to PHP we will assume you know some
  HTML

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Programming Language Qualities

• Simplicity and clarity
• How easy is it to write in the language?
• Different definitions some minimize keystrokes
• Binding
• How are values bound to variables?
• Definition, compilation, runtime, early vs. late
  binding
• Orthogonality
• Do features combine in a predictable way? E.g.
  meaning of
• Reliability
• Behave the same way each time it is run?
• Applicability
• How well does the language map to its
  applications?
• Abstraction
• Low or high level?
• Efficiency
• Practical and efficient on a von Neumann machine?

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Conclusions

• There are lots of programming languages out there
• Different paradigms
• Different applications
• Different levels of computing
• There will probably continue to be even more
  programming languages in the future
• By studying formal properties of programming
  languages and a sampling of paradigms, you will
  be well prepared to face these new languages