2.5. Introduction to Smalltalk

Why are we using Smalltalk?

• Smalltalk is a pure object-oriented language
  • Forces object-oriented design
  • Makes it easier to learn other OO languages
• Smalltalk is in widespread use in industry
• Smalltalk programmers are higher paid than C++ programmers

What is Smalltalk?

• A language
• A programming environment
• A class library

The Language

• Developed at Xerox PARC
• No strong typing
• Uniformly Object Oriented
• Unusual syntax
  • Simple but quirky
  • Object - Message

The Environment

• One of the first modern GUI environments
• Provides a number of tools for programmers
  • text editing
  • browsing all system code
  • incremental compile and debug

VisualWorks

• Direct descendant of Smalltalk-80
• Very powerful application development environment
• Extremely portable
• Available in all Sun clusters & Baird NT cluster
• Can get from OIT - No power macs

The Class Library

• Contains all the functionality
• All inspectible
• Non-standardized
• Includes things like
  • numbers, data structures, UI classes, graphics classes, process control, memory control

Smalltalk Virtual Machine

• 95% of VisualWorks is written in Smalltalk
  • The Image
• There is a small machine specific virtual machine
• Working in VisualWorks involves adding to the image

Literals

• Numbers: 3 13.5 100r2 2.23e5
• Characters: $d $7
• Strings: 'this is a string'
• Symbols: #aSymbol
• Literal Arrays: #(1 2 3 $a $b $c)
• Special Literals:
  • true, false, nil

Variables

• pointer to an object
• name is an expression that refers to the object
• local variables start with lower case
  • aString
• globals start with upper case
  • GlobalVariable

Messages

• <reciver> <message>
• Unary Message
  • 7 negated
• Binary Message
  • 3 * 4
• Keyword Message
  • aDictionary at: course put: 'cs2390'

Precedence Rules

• Parentheses have precedence over everything
• Unary messages (left to right)
• Binary messages (left to right)
• Keyword messages (left to right)
• Multiple expressions separated by a period

Precedence Examples

• 4 cos rounded
• 4 + 5 * 8
• (anArray at: 14) even
• 2 raisedTo: 3 raisedTo: 7
• 2 + 3 negated
• 2 raisedTo: 3 + 7
• 2 raisedTo: 3 + 7 negated

A Muppet example

• Creating a small hierarchy
• Abstract and concrete classes
• variables and messages

Creating a class

• Create a catagory (optional)
• Set NameOfSuperclass
• Set NameOfSubclass
• Create instance variable list
• Create class variable list
• Accept the code

Adding functionality

• Create a protocol (optional)
• type in the box
• accept the code
• test the code

Adding subclass functionality

• Creating new methods in the subclass
• Why a subclass and not an instance?
  • Added functionality
  • Added information
  • different responses

Inspection and debugging

• Opening an inspector
  • <object> inspect
• Opening the debugger
  • self halt