CSE

Principles of Programming Languages

UNIT - I
PRELIMINARY CONCEPTS
  • Topics
    1. Reasons for Studying Concepts of Programming Languages
    2. Programming Domains
    3. Language Evaluation Criteria
    4. Influences on Language Design
    5. Language Categories
    6. Language Design Trade-Offs
    7. Implementation Methods
    8. Programming Environments
    1.1 Reasons for Studying Concepts of Programming Languages
    1.Increased ability to express ideas
    1.The depth at which people can think is influenced by the expressive power of the language in which they communicate their thoughts
    2.The language in which programmers develop software places limits on the kinds of control structures, data structures, and abstraction they can use
    1. Awareness of a wider variety of programming language features can reduce such limitations in software development
    3. Languages constructs can be simulated in other languages that do not support those constructs directly;
    however, the simulation is often
    1. less elegant
    2. more cumbersome
    3. less safe
    2. Improved background for choosing appropriate languages
    3. Increased ability to learn new languages
    4. Better understanding of significance of implementation
    1. Program bugs
    2. Performance
    5. Overall advancement of computing
    1.2 Programming Domains
    • Scientific applications
    – Large number of floating point computations
    – Fortran
    • Business applications
    – Produce reports, use decimal numbers and characters
    – COBOL
    • Artificial intelligence
    – Symbols rather than numbers manipulated
    – LISP
    • Systems programming
    – Need– C
    • Web Software
    – Eclectic collection of languages: markup (e.g., XHTML), scripting (e.g., PHP), general-purpose (e.g., Java)
    1.3 Language Evaluation Criteria
    • Readability: the ease with which programs can be read and understood
    • Writability: the ease with which a language can be used to create programs
    • Reliability: conformance to specifications (i.e., performs to its specifications)
    • Cost: the ultimate total cost
    Evaluation Criteria: Readability
    • Overall simplicity
    – A manageable set of features and constructs
    – Few feature multiplicity (means of doing the same operation)
    – Minimal operator overloading
    • Orthogonality
    – A relatively small set of primitive constructs can be combined in a relatively small number of ways
    – Every possible combination is legal
    • Control statements
    – The presence of well-known control structures (e.g., while statement)
    • Data types and structures
    – The presence of adequate facilities for defining data structures
    • Syntax considerations
    – Identifier forms: flexible composition
    – Special words and methods of forming compound statements
    – Form and meaning: self-descriptive constructs, meaningful keywords
    Evaluation Criteria: Writability
    • Simplicity and orthogonality
    – Few constructs, a small number of primitives, a small set of rules for combining them
    • 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
    – Example: the inclusion of for statement in many modern languages
    Evaluation Criteria: Reliability
    • Type checking
    – Testing for type errors
    • Exception handling
    – Intercept run-time errors and take corrective measures
    • Aliasing
    – Presence of two or more distinct referencing methods for the same memory location
    • Readability and writability
    – A language that does not support ―natural‖ ways of expressing an algorithm will necessarily use ―unnatural‖ approaches, and hence reduced reliability
    Evaluation Criteria: Cost
    • Training programmers to use language
    • Writing programs (closeness to particular applications)
    • Compiling programs
    • Executing programs
    • Language implementation system: availability of free compilers
    • Reliability: poor reliability leads to high costs
    • Maintaining programs
    Evaluation Criteria: Others
    • 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
    1.4 Influences on Language Design
    • Computer Architecture
    – Languages are developed around the prevalent computer architecture, known as the von Neumann architecture
    • Programming Methodologies
    – New software development methodologies (e.g., object-oriented software development) led to new programming paradigms and by extension, new programming languages
    Computer Architecture Influence
    • Well-known computer architecture: Von Neumann
    • Imperative languages, most dominant, because of von Neumann computers
    – Data and programs stored in memory
    – Memory is separate from CPU
    – Instructions and data are piped from memory to CPU
    – Basis for imperative languages
    • Variables model memory cells
    • Assignment statements model piping
    • Iteration is efficient

    The Von Neumann Architecture

    Programming Methodologies Influences
    • 1950s and early 1960s: Simple applications; worry about machine efficiency
    • Late 1960s: People efficiency became important; readability, better control structures
    – structured programming
    – top-down design and step-wise refinement
    • Late 1970s: Process-oriented to data-oriented
    – data abstraction
    • Middle 1980s: Object-oriented programming
    – Data abstraction + inheritance + polymorphism
    1.5 Language Categories
    • Imperative
    – Central features are variables, assignment statements, and iteration
    – Examples: C, Pascal
    • Functional
    – Main means of making computations is by applying functions to given parameters
    – Examples: LISP, Scheme
    • Logic
    – Rule-based (rules are specified in no particular order)
    – Example: Prolog
    • Object-oriented
    – Data abstraction, inheritance, late binding
    – Examples: Java, C++
    • Markup
    – New; not a programming per se, but used to specify the layout of information in Web documents
    – Examples: XHTML, XML
    1.6 Language Design Trade-Offs

    • Reliability vs. cost of execution
    – Conflicting criteria
    – Example: Java demands all references to array elements be checked for proper indexing but that leads to increased execution costs

    • Readability vs. writability
    – Another conflicting criteria
    – Example: APL provides many powerful operators (and a large number of new symbols), allowing complex computations to be written in a compact program but at the cost of poor readability
    • Writability (flexibility) vs. reliability
    – Another conflicting criteria
    – Example: C++ pointers are powerful and very flexible but not reliably used
    1.7 Implementation Methods
    • Compilation
    – Programs are translated into machine language
    • Pure Interpretation
    – Programs are interpreted by another program known as an interpreter
    • Hybrid Implementation Systems
    – A compromise between compilers and pure interpreters
    Layered View of Computer
    The operating system and language implementation are layered over Machine interface of a computer

    Compilation
    • Translate high-level program (source language) into machine code (machine language)
    • Slow translation, fast execution
    • Compilation process has several phases:
    – lexical analysis: converts characters in the source program into lexical units
    – syntax analysis: transforms lexical units into parse trees which represent the syntactic structure of program
    – Semantics analysis: generate intermediate code
    – code generation: machine code is generated

    The Compilation Process


    Additional Compilation Terminologies
    • Load module (executable image): the user and system code together
    • Linking and loading: the process of collecting system program and linking them to user program
    Execution of Machine Code
    • Fetch-execute-cycle (on a von Neumann architecture)
    initialize the program counter
    repeat forever
    fetch the instruction pointed by the counter
    increment the counter
    decode the instruction
    execute the instruction
    end repeat
    Von Neumann Bottleneck
    • Connection speed between a computer‘s memory and its processor determines the speed of a computer
    • Program instructions often can be executed a lot faster than the above connection speed; the connection speed thus results in a bottleneck
    • Known as von Neumann bottleneck; it is the primary limiting factor in the speed of computers
    Pure Interpretation
    • No translation
    • Easier implementation of programs (run-time errors can easily and immediately displayed)
    • Slower execution (10 to 100 times slower than compiled programs)
    • Often requires more space
    • Becoming rare on high-level languages


    Significant comeback with some Web scripting languages (e.g., JavaScript)
    Hybrid Implementation Systems
    • A compromise between compilers and pure interpreters
    • A high-level language program is translated to an intermediate language that allows easy interpretation
    • Faster than pure interpretation
    • Examples
    – Perl programs are partially compiled to detect errors before interpretation
    – Initial implementations of Java were hybrid; the intermediate form, byte code, provides portability to any machine that has a byte code interpreter and a run-time system (together, these are called Java Virtual Machine)


    Just-in-Time Implementation Systems
    • Initially translate programs to an intermediate language
    • Then compile intermediate language into machine code
    • Machine code version is kept for subsequent calls
    • JIT systems are widely used for Java programs
    • .NET languages are implemented with a JIT system
    Preprocessors
    • Preprocessor macros (instructions) are commonly used to specify that code from another file is to be included
    • A preprocessor processes a program immediately before the program is compiled to expand embedded preprocessor macros
    • A well-known example: C preprocessor
    – expands #include, #define, and similar macros
    SUBJECTIVE
    1) What are the reasons of studying programming languages?
    2) What are the significant characteristics of programming languages?
    3) What makes a language portable?
    4) Explain language evaluation criteria and characteristics that affect them?
    5) Explain about Von-Neumann computer architecture?
    6) Explain different aspects of the cost of a programming language?
    7) Compare procedure oriented and object oriented programming. Explain the object oriented features supported by C++?
    8) Explain fundamental features of object oriented programming language with examples?
    9) What do you mean by imperative programming language?
    10) What are the differences between special purpose and general purpose programming languages?
    11) A programming language can be compiled or interpreted. Give relative advantages and disadvantages of compilation and interpretation. Give examples of compiled and interpreted languages?

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