Introduction to Smalltalk

1
Introduction to Smalltalk

• Programming Language Design and Implementation
• (4th Edition)
• by T. Pratt and M. Zelkowitz
• Prentice Hall, 2001
• Section 7.2.4
• Appendix A.12

2
Smalltalk overview

• Smalltalk was developed by Alan Kay at the Xerox
  Palo Alto Research Center (PARC) in the early
  1970s.
• Goal was a total personal computing environment
• In 1972, Dan Ingalls developed the first
  implementation of Smalltalk-72, although
  Smalltalk-80 is the generally accepted definition
  of the language.
• Smalltalk has several features that make it
  unique among most languages
• A total environment design, not just a language
• A minimal language design (comes with a
  predefined set of class definitions)
• Execution model - The execution model for
  Smalltalk is based on a communication model. Data
  in Smalltalk consist of objects, and methods are
  considered to be messages sent to objects.
• Smalltalk uses a dynamic execution sequencing
  model.

3
Language models

• Look at language execution models again
• Imperative - sequences of actions on data C,
  C, FORTRAN, FORTH, Postscript, Ada, Basic,
  Pascal
• Applicative - Develop
• functions that represent
• final transformation of
• data ML, LISP
• Rule-based - Specify
• format of results -
• BNF, Prolog
• Message-based - A
• program is a network
• that dynamically sends
• messages to objects -
• Smalltalk

4
Smalltalk features

• Smalltalk
• 1. Each object is an instance of a class.
• 2. Each class has a set of associated methods
  (functions).
• 3. Execution proceeds by sending a message (which
  is one of these methods) to an object of that
  class.
• Historically Smalltalk was the first language to
  have these features. These were later borrowed
  for C and Java.

5
Smalltalk example

• 1 Array variableSubclass Datastore
• 2 instanceVariableNames ''
• 3 classVariableNames 'DataFile ArrIndex
  Storage Size'
• 4 poolDictionaries ''
• 5 category nil !
• 6 !Datastore class methodsFor 'instance
  creation'!
• 7 new
• 8 DataFile _ FileStream open'data' mode
  'r'.
• 9 Storage _ Array new 99.
• 10 Size _ 99.
• 11 self reset !!
• 12 !Datastore class methodsFor 'basic'!
• 13 asgn aValue
• 14 ArrIndex _ ArrIndex 1.
• 15 Storage at ArrIndex put aValue !
• 16 getval
• 17 ArrIndex _ ArrIndex 1.
• 18 Storage at ArrIndex ! ... Continued ...

Figure A.17 in text
6
Smalltalk example (continued)

• 19 nextval
• 20 ((DataFile next) digitValue - 0
  digitValue)!
• 21 reset
• 22 ArrIndex _ 0 !!
• 23 k j sum
• 24 Datastore new.
• 25 "Initialize k"
• 26 (k _ Datastore nextval) gt 0
• 27 whileTrue 1 to k do (j _ Datastore
  nextval) print.
• 28 Character space
  print.
• 29 Datastore asgn j.
• 30 Datastore reset.
• 31 sum _ 0.
• 32 ' SUM ' print.
• 33 1 to k do sum _ sum Datastore
  getval.
• 34 sum printNl.
• 35 Datastore reset !

7
Smalltalk execution

• Syntax object message !
• 3 printNl ! ? Send printNl message to object 3
• printNl method prints its argument followed by
  newline (Nl).
• print method does same except doesn't add
  newline.
• Note naming convention thisIsAMethodName
• Words strung together, all except first
  capitalized.

8
Smalltalk execution (continued)

• The printNl method is defined for string objects
• 'Hello World' printNl' ! ? Send printNl message
  to
• object Hello World
• Each method returns an object, so can repeat
  process
• object method1 method2 method3 !
• Functions are arbitrary so no precedence - left
  to right evaluation
• 2 3 4 ? 20 (not the expected 14)

9
Methods

• Three kinds of methods
• 1. Unary - Method name only no arguments. E.g.,
  printNl, print.
• 2. Binary - Operator between objects builtin,
  E.g., 23 means
• Send the 3 object to the method of the 2
  object. Very different from (3 2)
• 2 3 4! Means
• - send the 3 object to of 2 object return
  the 5 object.
• - send the 4 object to of 5 object return
  the 20 object.
• What happens 2 3 4 printNl! ???
• 2 3 results in 5 object
• 4 printNl results in 4 being printed.
• Unary operator associated with preceding object.
• 5 4 results in 20 object
• Use parentheses to get desired effect
• (( 2 3) 4 ) printNl !

10
Keyword methods

• 3. Keyword methods methodName argumentObject
• If A is an array object
• A at4 put7 ! - the atput method sets 4th
  element 7
• A at4 - the at method returns the 4th object in
  A
• Note naming conventions.
• Method atput may not be same as putat method.
• Other execution features
• Blocks smalltalkStmt. smalltalkStmt
• ? means return object ?A returns object A

11
True and false in Smalltalk

• Object true as defined in Smalltalk library
• ifTrue trueBlock ifFalse falseBlock
• ?trueBlock value ! (value evaluates block
  argument)
• ifFalse falseBlock ifTrue trueBlock
• ?trueBlock value !
• ifTrue trueBlock
• ?trueBlock value !
• ifFalse falseBlock
• ?nil !
• The ifTrue argument is evaluated in each case.
• Object false as defined in Smalltalk library
• ifTrue trueBlock ifFalse falseBlock
• ?falseBlock value !
• . . . Others in an analogous way
• Always evaluate the ifFalse argument

12
Use of Smalltalk logical expressions

• Example method definition
• hello
• 2 self
• ifTrueHello world' printNl
• ifFalse'Goodbye world' printNl !!
• 2 hello !
• 2 self ? self is the argument, which is 2, so
  returns true
• true passed ifTrueifFalse blocks ? ifTrue block
  evaluated
• printNl passed to 'Hello world' ? 'Hello world'
  printed
• 3 hello !
• 2 self ? self is the argument, which is 3, so
  returns false
• false passed ifTrueifFalse blocks ? ifFalse
  block evaluated
• printNl passed to 'Goodbye world' ? 'Goodbye
  world' printed

13
Smalltalk inheritance

• So far we have discussed method invocation.
• Power of Smalltalk is in inheritance.
• Smalltalk comes with a predefined class library.
• All object classes are subclasses of a parent
  Object class.
• If any method is not defined for an objects, the
  parent of that object is called with the same
  method.
• Earlier example 3 printNl ! not quite right.
• printNl a method defined on Object class.

14
Object.st method definitions

• print
• self printOn stdout !
• printNl
• self print.
• stdout nl !
• printOn aStream
• article nameString (local variables)
• nameString _ self classStringName. (_ is
  assignment)
• article _ nameString firstIsVowel
• ifTrue'an' ifFalse 'a'.
• aStream nextPutAll article
• nextPutAll nameString !
• In Integer.st
• printOn aStream baseb
• aStream nextPutAll (self radixb) !
• printOn aStream
• aStream nextPutAll (self signedstringbase10
  showradixfalse)!

15
Creating methods

• Two types of methods
• Class methods - Applies to class objects
  (Integer).
• ! ClassName class methodsFor 'description' !
• Class instance methods - Applies to instances of
  class objects (2, 3, 42).
• ! ClassName methodsFor 'description' !
• Create a new class - Use following 5-keyword
  method (which is why all 5 keywords are needed)
  to create a new subclass of class Object
• Object subclass newClassName
• instanceVariableNames instanceVariables ? Local
  data
• classVariableNames classVariables ? global data
• poolDictionaries ' ' ? Can generally ignore
• category nil ! ? Can generally ignore

16
Creating methods (continued)

• Example A Thing which is a subclass of an
  Integer
• Integer subclassThing instanceVariableNames
  'aValue classVariableNames ' '
  poolDictionaries ' ' category nil !
• gives the name a global attribute
• ! Thing class methodsFor 'Creation' !
• new r
• r _ super new
• ?r init !
• (super is superclass of self object. Allocate
  for Thing the same thing as an Integer (its
  parent class))
• ! Thing methodsFor 'Using things' !
• init aValue _ 0 !!

17
Method inheritance

• Review previous definitions of print in Object.st
  and show that
• Integer printNl ! Prints an Integer
• 3 printNl ! prints 3