Main Talk

  • Barbara Likov @ http://www.infoq.com/presentations/programming-abstraction-liskov
  • March - 2013, MIT

Software is Complex

  • Systems are big
  • Distributed systems

Addressing Complexity

  • Algorithms

  • Programming Methodology
    • Programming structure
  • Programming Languages
    • Some programming methodologies have become programming languages

Programming Methodology

  • how should programs be designed - process
  • how should programs be structured - underlying the design process

Go To statment considered harmful - E.W. Dijkstra

  • no proper control structures

  • software programs are hard to reason

  • program code is static

  • program execution is dynamic.

  • reasoning about static is different from reasoning when in execution.
  • when static and dynamic paths are similar it is to trasfer static reasoning to dynamic behaviour
  • the goto statement kind of spoils relationship

Program Development by stepwise refinement - N. Wirth

  • top down design
  • problem spec ..to line by line statements

Modularity

  • A program is collection of modules
  • each has an interface, described by specification
  • a using module depends only on the interface
  • benefits
    • local reasoning
    • independent development

The situation

  • procedures were only type of module
  • not enough - e.g. file system

Partitions

  • no globals
  • hid state and access only by functions

Idea

  • Connect partitions to data types

Programming Languages supporting Abstract Data Types

  • Syntactic - extension - terrible idea (factorial ! symbol)
  • Difference between abstraction and encapsulation(java)
    • Abstraction is implemented using interface and abstract class while Encapsulation is implemented using private and protected access modifier.

Protection in Programming Languages - J.H. Morris

Programming with Abstract Data Types - B. Liskov

  • Abstract data types
    • a set of opertions
    • a set of objects
    • operations provide the ONLY way to use the objects

Why a programming language

  • Communicating to programmers - providing language constructs
  • getting a precise definition

Facts about CLU

  • static type
  • heap based
  • separate compilation units - object files
  • no concurrancy, no gotos, no inheritance

CLU Mechanism

  • new data types -> Cluster -> class in java
  • polymorphism

Clusters

  • operations belong to type (instead of object)
  • pick the operation of type and invoke it on an instance of that type

Polymorphism (generics)

  • Set

Iterators

  • For all x in C do S
    • Destroy the collection
    • Complicate the abstraction

Alphard

  • Generators - Iterators(java)
  • Iterators - Yield - (c#)
  • d

Exception Handling

  • first class procedures and iterators

Type Hierarcy

CLU to Object Oriented Programming

  • small talk provided the inheritance

Inheritance was used for

  • Implementation - technique
  • Type hierarchy

Liskov Substitution Principle

Programming Languages Today

  • Java, C#
  • introductory - python