Smalltalk Topics  History and Significance of Smalltalk  Object-Oriented Programming the Best Way to Predict the Future Is to  the Smalltalk Language Invent It

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Smalltalk Topics  History and significance of Smalltalk  Object-oriented programming The best way to predict the future is to  The Smalltalk language invent it.  Smalltalk today  Additional Examples Alan Kay, 1971

 Alan Kay coined the term “object History of Smalltalk oriented”

 Smalltalk was developed at the Xerox Palo “…and I can tell you I did not have C++ in mind.” Alto Research Center (PARC) in the early 1970’s  2004 Turing Award Winner  Alan Kay, Dan Engalls, Adele Goldberg, Dave Robson  Delivered the Turing Lecture at OOPSLA 2004

OOP and GUI Steve Jobs, PARC, Dec 1979

 And they showed me really three things. But I was so blinded by the Smalltalk was the first object-oriented first one I didn't even really see the other two. One of the things they programming (OOP) language and the first showed me was object oriented programming; they showed me that but I didn't even see that. The other one they showed me was a graphical user interface (GUI) networked computer system...they had over a hundred Alto computers all networked using email etc., etc., I didn't even see that. I was so blinded by the first thing they showed me which was the graphical user interface. I thought it was the best thing I'd ever seen  Gave Steve Jobs the idea for the in my life. Now remember it was very flawed, what we saw was MacIntosh incomplete, they'd done a bunch of things wrong. But we didn't know that at the time but still though they had the germ of the idea was there and they'd done it very well and within you know ten minutes it was obvious to me that all computers would work like this some day.

Smalltalk 1 Object-Oriented Programming OOP

 Object: encapsulates data describing  Inheritance: reuse of classes by something (often an object in the real specializing general behavior world) as well as methods (or programs)  Polymorphism: many different objects can that manipulate that data respond to the same message  Message: communication between objects;  Dynamic Binding: resolution of message method invocations binding is deferred until runtime  Class: defines structure and common behavior of a group of objects

OOP Smalltalk Implementations

 Combination of inheritance with  Smalltalk is written mostly in Smalltalk polymorphism and dynamic binding provides a powerful form of genericity  The core of any Smalltalk implementation is a small bytecode interpreter (virtual  Allows high levels of reuse, fast development machine)  Smalltalk source code is compiled to intermediate bytecode to be executed on  BUT: “Even if you only want the banana, you have to take the whole gorilla.” the VM

Programming in Smalltalk Objects  have local memory, inherent processing  Programming in Smalltalk involves capability, capability to communicate with – Defining classes other objects. – Implementing class methods  Examples: – Evaluating expressions 2 True FileStream Text Editor  Most Smalltalk languages provide an Class MetaClass interactive GUI as a development  Can be passed as parameters and returned as environment results

Smalltalk 2 Messages and Methods Classes and Instances  Class: The abstract definition of the  Message: a request to an object to perform behavior of a class of objects an operation (analogous to function call)  Method: implementation of an operation  Instance: an individual object defined by a (function) class  Examples of messages: ‘hello, world’ size #(1 12 24 36) includes: 4 factorial 3 < 4 ifTrue: [‘Yes’] ifFalse: [‘No’]

Instance methods and Class methods Instance variables

 Instance method: a method that describes  Instance variables: a variable that stores how an operation is carried out by every data for an instance of a class instance of a class  Class method: a method that describes how  Generally have different values for each an operation is carried out by a class, such instance as creating a new instance of itself. Class methods are sent to the class (an  The collection of instance variables instance of Class MetaClass) rather than to describes the state of the object an instance of the class

Class variables Inheritance  A variable that is shared by a class and all of its instances  Classes are arranged in a hierarchy Superclass: the parent of a class  Available to all instances of a class Subclass: a child of a class

 Can be constants (Float pi) or even  Subclasses inherit the variables and references to existing instances, such as methods of the superclass Students class that maintains a list of Student instances

Smalltalk 3 The Magnitude Class Hierarchy Inheritance in Smalltalk Magnitude Association  Is a strict tree structure: a subclass can Character have one and only one parent class Date Number Float  Subclasses usually “specialize” the more Fraction general behavior of the parent class Integer – Can add new variables or methods LargeInteger – can also hide inherited functionality SmallInteger Time

The Collection Class Hierarchy Another Collection Hierarchy Collection Bag IndexedCollection FixedSizeCollection Array Bitmap ByteArray CompiledMethod Interval String Symbol OrderedCollection Process SortedCollection Set Dictionary … 22

Expressions Literals and Variables  Smalltalk does not have a “statement” in the sense used by procedural languages  Computation is achieved by evaluating  Literals expressions #aSymbol #(1 2 4 16 32 64)  All expressions return an object ‘Hello, World’  Types: – Literals – Variable names  Variables – Message expressions Smalltalk x selectedDictionary – Blocks of code

Smalltalk 4 Variables Public and Private Variables

 Names are syntactically similar to other  Public variables are shared and visible languages globally  All variables are pointers to objects Name begins with upperclass letter  All variables are inherently typeless  Private variables are local to an object, No difference between pointing to the number block or method 42 and pointing to an instance of a text editor – Name begins with lowerclass letter

Message Expressions Message Expression Examples set add: stream next  Sending a message involves: (receiver is set, selector is add, argument is result of stream next, where receiver is stream and next is – object to which message is sent (“receiver”) selector) – additional objects included in message 2 + 3 (“arguments”) (receiver is 2, selector is +, argument is 3) – desired operation to be performed (“message selector”) array at: index + offset put: Bag new

– Accepting the single object returned as the array at: 1 put: self “message answer”

Message Syntax Types Message Syntax Types

 Unary (no parameters)  Keyword (a general extension of infix firstAngle cos syntax) 42 PrintString myArray at: 42 put: 5  Binary (selector is “at: put:”) 2 + 3  In practice very few methods accept more thisCollection = thatCollection than 2 parameters

Smalltalk 5 Selector Evaluation Order Cascading Messages

 A series of messages sent to same object  Rules are a bit odd unless you are used to APL myPen home; up; goto: 100@200; down; home  Unary and binary expressions associate left to right Equivalent to: 12 - 3 * 3 -> 27 myPen home 12 - (3 * 3) -> 3 myPen up 12 - 12 sin * 3 -> 0 myPen goto: 100@200 myPen down myPen home

Method Definitions Self references

 Often an object needs to send a message to itself  General Syntactic Form MessagePattern  Pseudovariable "self" is used to refer to the object itself | local variables | Expressions  Examples:  Return object designated with ^ count = 0  An object can refer to itself with self ifTrue: [self error: "0 items being averaged"] ifFalse: [ ^sum / count]

Self references Unary method example  Fibonacci numbers fibonacci left "Answer the nth fibonacci number, where "return the left subtree of the n is the receiver" receiver" ^self < 3 ifTrue: [1] ^ self left ifFalse: [(self-1) fibonacci + (self-2) fibonacci] Testing fibonacci #(1 2 3 4 5 6 7 10 30) collect: [:m | m fibonacci]

Smalltalk 6 Binary method example index := self size Binary method example index ~= aCollection size = aCollection ifTrue: [^false] "answer true if elements contained by receiver are equal to the elements [index <= 0] contained by aCollection " whileFalse: [ | index | (self at: index) = aCollection at: index) self == aCollection ifFalse: [^false]. ifTrue: [^true]. index := index – 1.] (self class == aCollection class) ^true ifFalse: [^false]. Note: == is equivalence (test for same object) while = is test for equality (equal values)

Keyword method example Assignment

setX: xCoord setY: yCoord setZ: zCoord  Syntactically similar to other languages, but "set coordinates for a threeDpoint" variables are “typeless” x := xCoord y := yCoord z := zCoord  | x | ^self x := 1 x := “A String” x := Pen new.

Control Structures Blocks

 Smalltalk has NO conventional control  Objects consisting of a sequence of structures expressions [index := index + 1. sum := sum + index.]  Expression are evaluated when block  Control structures are formed by passing receives the message “value” block objects as parameters to Boolean [index := index + 1. objects sum := sum + index.] value  Blocks can be assigned to variables and executed by sending the message "value" to the variable.

Smalltalk 7 Iteration Iteration count := 0. sum := 0. "copy a disk file" [ count <= 20 ] | input output | whileTrue: [ sum := sum + count. input : = File pathName: 'go'. count := count + 1 ] output := File pathName: 'junk'. [input atEnd]  WhileTrue: is a method of Class Boolean whileFalse: [output nextPut: input next], value is a method of Class Block input close, whileTrue: sends message "value" to conditional block output close Conditional block returns object true or false If results is true, WhileTrue: evaluates code in parameter block

For loop (to: by:) Selection  Similar to iteration. Common control structures "compute the sum of 1/2, 5/8, 3/4, 7/8, 1" ifTrue: | sum | if False: ifTrue: ifFalse: sum := 0. 1/2 to: 1 by: 1/8 total = 0 do: [ :i | sum := sum + i] . ifTrue: [average := 0] ^sum ifFalse: [average := sum / total]  Message "= 0" sent to object total returns either True or False resulting Boolean object is receiver of ifTrue: message

Block Parameters Classes  Blocks can accept parameters  All Smalltalk objects are instances of [:x :y | (x * x) + (y * y).] classes (including classes: they are instances of class MetaClass) [:x :y | (x * x) + (y * y).]  A Class definition has four parts: value: 2 value: 3 -> 13 – Class name – Superclass name – declaration of local (instance) variables – a set of methods that define how instances will respond to messages

Smalltalk 8 Classes and Message Lookup Polymorphism  A message sent to an object causes lookup  Polymorphism: a specific message may be in class definition sent to different instances of different class  If search fails, then lookup proceeds to at different times superclass  Top of hierarchy is class Object  Method lookup in Smalltalk occurs at  If no method found, error results execution time

 Allows unlimited polymorphism since any class can implement any method

Polymorphism Type Checking  Consider a + b - c  Variables are typeless and can be bound to The methods + and - could be implemented by class any object String with an arbitrary meaning.  Type checking is performed dynamically  a + b when message sent to object – string concatenation-returns a with b concatenated  If object lacks a method to respond to the  b – c message, lookup proceeds to superclass – returns b with all occurrences of c removed  Then a + b - c would first remove all All the way up to class Object occurrences of c from b, then concatenate the Failure at this point results in error result to a (if a, b and c are instances of String)

A Stack class name Stack Inheritance superclass name Object instance variable names stack  A subclass inherits all of the instance variables, instance methods and class methods of the "Class Methods" superclass new – New instance variables can be added "Create an instance" – Names must differ from names of instance variables stack = OrderedCollection new. ^self in ancestor classes  A subclass can define new methods or redefine "Instance Methods" methods implemented in an ancestor class pop "Answer the receiver with last item removed from  Redefinition hides definition in ancestor class stack"  Pseudovariable "super" can be used in methods self size = 0 to refer method search to the superclass ifTrue: [self error: "Attempt to pop empty stack"] ifFalse: [stack removeFirst. ^self]

Smalltalk 9 push: anItem Another Stack "Answer the receiver with anItem added to the class name Stack top" stack addFirst: anItem. superclass name OrderedCollection ^self instance variable names none "Class Methods" top new "Answer the top item on the receiver" "Create an instance" ^self size = 0 ^super new. ifTrue: [self error: "Attempt to take top of pop empty stack"] ^self removeFirst ifFalse: [^stack at: 1] push: anItem empty ^self addFirst: anItem "Answer true if the receiver is empty" top ^self size = 0 ^self at: 1 empty size ^self size = 0 "Answer the size of the receiver" ^stack size

Designing a class hierarchy is not easy… Smalltalk Today

 Problem with stack implementation as  "The best way to predict the future is to subclass of OrderedCollection: invent it.“ (Alan Kay, 1971) myStack:= Stack new. 1 to: 10 do: [ :x | myStack push: x] middle := myStack at: 5  Smalltalk is the origin of two of the most important computing “developments” of the last two decades:  Need to hide “inappropriate” methods of – Object Oriented Programming (OOP) OrderedCollection – Graphical User Interfaces (GUIs)

Smalltalk Today Smalltalk Resources  Smalltalk still has a small but enthusiastic following  Most prominent open-source version is  Often used in industry as a rapid prototyping Squeak Smalltalk environment  See http://www.squeak.org/  IBM (VisualAge Smalltalk) promotes it as  Smalltalk.org( http://www.smalltalk.org ) an e-commerce tool has a comprehensive list of products and  OOVM has developed a Smalltalk VM for many interesting articles and tutorials embedded systems that does not need an OS  Implemented in MS .NET 2003 as S#

Smalltalk 10 Of interest to Ruby users: 99 Bottles of Beer

"Copy into a workspace, highlight the code and choose do  Ruby The Smalltalk Way it." "Tested under Squeak 3.7 and VisualWorks 7.3" http://www.sapphiresteel.com/Ruby-The-Smalltalk-Way | verseBlock | verseBlock := [ :bottles | | verse | verse := WriteStream with: (String new). bottles = 0 ifTrue: [verse nextPutAll: 'No more bottles of beer on the wall. No more bottles of beer...'; cr; nextPutAll: 'Go to the store and buy some more... 99 bottles of beer.'; cr]. bottles = 1 ifTrue:

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[verse nextPutAll: '1 bottle of beer on the wall. 1 bottle of beer...'; cr; nextPutAll: 'Take one down and pass it around, no more bottles of beer on the wall'; cr]. Example: Polynomials bottles > 1 ifTrue: Example: Polynomials [verse nextPutAll: bottles printString; nextPutAll: ' bottles of beer on the wall. '; nextPutAll: bottles printString; nextPutAll: ' bottles  Polynomials of beer...'; cr; nextPutAll: 'Take one down and pass it around, '; 2 nextPutAll: (bottles - 1) printString, ' bottle'; – Represent Polynomials such 3x + 5x - 7 nextPutAll: (((bottles - 1) > 1) ifTrue: ['s '] ifFalse: [' ']); – Representation is a collection of coefficients: nextPutAll: 'of beer on the wall'; cr]. verse contents]. #(-7 5 3) 99 to: 0 by: -1 do: – Subclass of Magnitude [: i | Transcript show: (verseBlock value: i); cr].

 For the true OO version of 99 Bottles, see http://www.99-bottles-of-beer.net/language-smalltalk-1513.html

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degree "Highest non-zero power" Polynomial class ^ coefficient size - 1 Magnitude subclass: #Polynomial coefficient: power instanceVariableNames: 'coefficient' "Coefficient of given power" classVariableNames:: '' (power >= coefficient size) ifTrue: [ ^ 0 ]. poolDictionaries: '' ^ coefficient at: power + 1

new asArray "Unary class constructor: return 0*x^0" ^ coefficient deepCopy ^ self new: #( 0 ) = aPoly new: array ^ coefficient = aPoly asArray "Keyword class constructor" ^ (super new) init: array != aPoly ^ (self = aPoly) not init: array "Private: initialize coefficient" < aPoly coefficient := array deepCopy 65 "not defined" 66 ^ self shouldNotImplement

Smalltalk 11 Complex class Evaluate method Object Subclass: #Complex instanceVariableNames: 'realpart imagpart' classVariableNames:: '' evaluate: aPolynomial x: aNumber poolDictionaries: '' "Return the results of evaluating aPolynomial new for the value aNumber" "Unary class constructor: Invalid" ^ self error 'use real:imaginary:' | index val | new: aComplex index := 1 "Class constructor aComplex" val := 0 ^ (super new) copy: aComplex [index < coefficient size] whileTrue: real: r imaginary: i [ val := val + (coefficient at: index) * "Class Constructor" (aNumber raisedToInteger: (index – 1)) ] ^ (super new) setReal: r setImaginary: i ^ val setReal: r setImaginary: i "Private instance method to initialize self" realpart := r imagpart := i 67 68 ^self

real "Return real part" ^ realpart negated "Return new complex number: - self" imaginary ^ Complex real: realpart negated imaginary: "Return imaginary part" imagpart negated ^ imagpart = val + val "Return self = val" "Return new complex number: self + val" ^ (realpart = val real) & (imagpart = val ^ Complex real: realpart + val real imaginary) imaginary: imagpart + val imaginary < val - val "Not mathemtically defined" "Return new complex number: self - val" ^ self shouldNotImplement ^ Complex real: realpart - val real imaginary: imagpart - val imaginary

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