DESIGN PRINCIPLES BEHIND SMALLTALK Dan Ingalls, Byte Magazine, August 1981 Arun Kannan

1
DESIGN PRINCIPLES BEHIND SMALLTALKDan Ingalls,
Byte Magazine, August 1981Arun Kannan
2
Organization of presentation

• What is Smalltalk
• What is the objective of Smalltalk
• What are its language design principles
• How to handle system complexity
• Some implementation information
• Additional exploration Simulation
• Summary

3
What is Smalltalk

• Purity Supports object oriented paradigm
• Simplicity Uniformity of the language
• Intuitive appeal Simulation

4
OBJECTIVE AND DESIGN PRINCIPLES OF SMALLTALK
5
Broad Objective

• Provide computer support for the creative spirit
  in everyone.
• Must be entirely comprehensible to a single
  individual.
• Language to communicate models in human mind and
• User interface to aid in such communication.

6
Objective -gt Design Principles

• Intended for everyone
• Completely comprehensible to a single individual
• Spirit that is creative
• Computer support

• Represent models in human mind
• Orthogonal language with small set of
  unchangeable parts
• Features to build complex systems
• Meaningful user interface

7
Design principles

• Objects Should 'support' a concept of object
  and provide a uniform way to refer to objects.
• Language Should provide framework for all kinds
  of communication between objects.
• Organization Should have a framework in which
  complex systems can be built.
• User Interface Should present components of the
  system in a meaningful way.

8
Pictorially ...
Language
Object1
Object2

• Bare Essentials
• Define objects (shape, color etc)
• Refer to objects (arrow mark)
• Communicate to objects (language)
• Accessories
• Organization (a way to build complex systems)
• User Interface (for the user to interact with
  computer)

9
Objects and reference

• Humans view universe to be comprised of objects.
• There are objects and there are references to
  objects.
• Ex Chair and that chair over there.
• In order to provide models compatible with the
  human mind, the language should
• support the concept of object and
• provide a uniform means for referring to objects
  in its universe.

10
Communication between objects

• Message passing between objects as a uniform way
  of communication.
• Objects send the name of the desired operation,
  along with any arguments, as a message to another
  object.
• The transmission of messages is the only process
  that is carried on outside of objects.

11
Organization

• One of the objective is to build complex systems
  using Smalltalk.
• Several related organizational principles
  contribute to the successful management of
  complexity.
• For example Classification, Modularity,
  Factoring, Polymorphism.

12
User Interface

• Visual presentation of objects through text,
  menus and pictures.
• If a computer language provides storage
  management, file system, display handling,
  keyboard input, access to other subsystems then
  the need for an operating system is eliminated.
• All the above and more are part of Smalltalk.
• Squeak, Squeak NOS

13
SYSTEM COMPLEXITY ORGANIZATIONAL PRINCIPLES
14
Classification and Modularity

• Grouping related components.
• A class describes objects -- their internal
  state, the message protocol they recognize, and
  the internal methods for responding to those
  messages.
• Minimize interdependence between components in
  the system.
• Structural information is protected because all
  access to the internal state of an object is
  through the message interface.

15
Factoring and Polymorphism

• Each independent component in a system would
  appear in only one place.
• Smalltalk encourages well-factored designs
  through inheritance.
• Generic description is crucial to models of the
  real world.
• A program should only specify the behavior of
  objects, not their representation.

16
Example

• Consider an automobile traffic simulation.
• Many procedures in the program will refer to the
  various vehicles involved.
• Suppose one wished to add a new vehicle, say, a
  street sweeper.
• StreetSweeper (and all other vehicle classes)
  would be described as a subclass of a general
  Vehicle class.
• Provided that street sweepers support the same
  protocol as all other vehicles, no changes are
  needed to include them in the simulation.

17
IMPLEMENTATION
18
Virtual Machine

• A virtual machine specification provides the set
  of primitive operations that would be sufficient
  to support an entire computing system.
• The Smalltalk virtual machine establishes an
  object-oriented model for storage, a
  message-oriented model for processing, and a
  bitmap model for visual display of information.

19
Storage Management

• Objects are created when expressions are
  evaluated.
• They can be passed around by uniform reference,
  so that no provision for their storage is
  necessary in the procedures that manipulate them.
  
• When all references to an object have disappeared
  from the system, the object itself vanishes, and
  its storage is reclaimed.

20
COMPUTATION AS SIMULATION
21
Simulation

• Some problems involve collections of interacting
  entities whose low-level pairwise interactions
  with each other are known but whose high-level
  group interactions cannot be grasped.
• Simulation enables one to encode these low-level
  interactions and simulate them, dynamically
  approximating the overall behaviour of the
  system. Drolet, Moodie 91

22
Computation as Simulation

• Object-oriented programming views a program as a
  collection of loosely connected agents, termed
  objects.
• Each object is responsible for specific tasks.
• It is by the interaction of objects that
  computation proceeds.
• In a certain sense, therefore, programming is
  nothing more or less than the simulation of a
  model universe. Timothy Budd, Addison Wesley
  Pub.

23
SUMMARY
24
Summary

• Way to describe objects
• Uniform way to refer to objects
• Uniform way to pass message to objects
• Several organizational principles available to
  manage complexity of system
• Meaningful user interface