PZ06BX Programming Language design and Implementation -4th Edition Copyright©Prentice Hall, PZ06BX - Introduction to Smalltalk Programming Language Design and Implementation (4th Edition) by T. Pratt and M. Zelkowitz Prentice Hall, 2001 Section Appendix A.12

PZ06BX Programming Language design and Implementation -4th Edition Copyright©Prentice Hall, 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.

PZ06BX Programming Language design and Implementation -4th Edition Copyright©Prentice Hall, 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

PZ06BX Programming Language design and Implementation -4th Edition Copyright©Prentice Hall, 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.

PZ06BX Programming Language design and Implementation -4th Edition Copyright©Prentice Hall, 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: Size _ self reset !! 12 !Datastore class methodsFor: 'basic'! 13 asgn: aValue 14 ArrIndex _ ArrIndex Storage at: ArrIndex put: aValue ! 16 getval 17 ArrIndex _ ArrIndex ^Storage at: ArrIndex !... Continued... Figure A.17 in text

PZ06BX Programming Language design and Implementation -4th Edition Copyright©Prentice Hall, 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) > 0] 27 whileTrue: [1 to: k do: [(j _ Datastore nextval) print. 28 Character space print. 29 Datastore asgn: j]. 30 Datastore reset. 31 sum _ ' SUM =' print to: k do: [ sum _ sum + Datastore getval]. 34 sum printNl. 35 Datastore reset] !

PZ06BX Programming Language design and Implementation -4th Edition Copyright©Prentice Hall, 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.

PZ06BX Programming Language design and Implementation -4th Edition Copyright©Prentice Hall, 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: * 4  20 (not the expected” 14)

PZ06BX Programming Language design and Implementation -4th Edition Copyright©Prentice Hall, Methods Three kinds of methods: 1. Unary - Method name only; no arguments. E.g., printNl, print. 2. Binary - Operator between objects; builtin, E.g., 2+3 means: Send the 3 object to the + method of the 2 object. [Very different from (3 + 2)] * 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: * 4 printNl! ??? 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 !

PZ06BX Programming Language design and Implementation -4th Edition Copyright©Prentice Hall, Keyword methods 3. Keyword methods: methodName: argumentObject If A is an array object: A at:4 put:7 ! - the at:put: method sets 4th element = 7 A at:4 - the at: method returns the 4th object in A Note naming conventions. Method at:put: may not be same as put:at: method. Other execution features: Blocks: [ smalltalkStmt. smalltalkStmt]  means return object:  A returns object A

PZ06BX Programming Language design and Implementation -4th Edition Copyright©Prentice Hall, 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

PZ06BX Programming Language design and Implementation -4th Edition Copyright©Prentice Hall, Use of Smalltalk logical expressions Example method definition: hello 2 = self ifTrue:[`Hello world' printNl] ifFalse:['Goodbye world' printNl] !! 2 hello ! 2 = self  self is the argument, which is 2, so returns true true passed ifTrue:ifFalse: 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 ifTrue:ifFalse: blocks  ifFalse block evaluated printNl passed to 'Goodbye world'  'Goodbye world' printed

PZ06BX Programming Language design and Implementation -4th Edition Copyright©Prentice Hall, 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.

PZ06BX Programming Language design and Implementation -4th Edition Copyright©Prentice Hall, 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 base:b aStream nextPutAll: (self radix:b) ! printOn: aStream aStream nextPutAll: (self signedstringbase:10 showradix:false)!

PZ06BX Programming Language design and Implementation -4th Edition Copyright©Prentice Hall, 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

PZ06BX Programming Language design and Implementation -4th Edition Copyright©Prentice Hall, Creating methods (continued) Example: A Thing which is a subclass of an Integer: Integer subclass:#Thing 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 !!

PZ06BX Programming Language design and Implementation -4th Edition Copyright©Prentice Hall, Method inheritance Review previous definitions of print in Object.st and show that: Integer printNl ! Prints “an Integer” 3 printNl ! prints “3”