12. A bit of Smalltalk

© O. Nierstrasz P2 — A bit of Smalltalk 11.2 Roadmap  Some history  Smalltalk syntax & object model  The Smalltalk environment

© O. Nierstrasz P2 — A bit of Smalltalk 11.3 Roadmap  Some history  Smalltalk syntax & object model  The Smalltalk environment

© O. Nierstrasz P2 — A bit of Smalltalk 11.4 Don’t panic! New Smalltalkers often think they need to understand all the details of a thing before they can use it. Try to answer the question “How does this work?” with “I don’t care”. — Alan Knight. Smalltalk Guru

© O. Nierstrasz P2 — A bit of Smalltalk 11.5 Squeak resources Free download — Print-on-demand SqueakByExample.org A modern, open-source Smalltalk One-click image

© O. Nierstrasz P2 — A bit of Smalltalk 11.6 Available Smalltalks  Squeak — —  Cincom VisualWorks Smalltalk —smalltalk.cincom.com  Dolphin Smalltalk —  Gnu Smalltalk —  Others … —en.wikipedia.org/wiki/Smalltalk

© O. Nierstrasz P2 — A bit of Smalltalk 11.7 Object-oriented language genealogy

© O. Nierstrasz P2 — A bit of Smalltalk 11.8 The origins of Smalltalk Alan Kay’s Dynabook project (1968) Alto — Xerox PARC (1973)

© O. Nierstrasz P2 — A bit of Smalltalk 11.9 Origins of Smalltalk  Project at Xerox PARC in 1970s —Language and environment for new generation of graphical workstations (target: “Dynabook”)  In Smalltalk-72, every object was an independent entity —Language was designed for children (!) —Evolved towards a meta-reflective architecture  Smalltalk-80 is the standard  See: The Early History of Smalltalk, by Alan Kay —gagne.homedns.org/~tgagne/contrib/EarlyHistoryST.htmlgagne.homedns.org/~tgagne/contrib/EarlyHistoryST.html

© O. Nierstrasz P2 — A bit of Smalltalk What is Smalltalk?  Guiding principles: —“Everything is an Object” —“Everything happens by sending messages”  Pure OO language —Single inheritance —Dynamically typed  Language and environment —Class browser, debugger, inspector, … —Mature class library and tools  Virtual machine —Objects exist in a persistent image [+ changes] —Incremental compilation

© O. Nierstrasz P2 — A bit of Smalltalk Smalltalk architecture Image Changes + Virtual machine Sources +

© O. Nierstrasz P2 — A bit of Smalltalk Smalltalk vs. C++ vs. Java SmalltalkC++Java Object modelPureHybrid Garbage collectionAutomaticManualAutomatic InheritanceSingleMultipleSingle TypesDynamicStatic ReflectionFully reflectiveIntrospection ConcurrencySemaphoresSome librariesMonitors ModulesCategories, namespaces NamespacesPackages

© O. Nierstrasz P2 — A bit of Smalltalk Roadmap  Some history  Smalltalk syntax & object model  The Smalltalk environment

© O. Nierstrasz P2 — A bit of Smalltalk Three kinds of messages > Unary messages > Binary messages > Keyword messages 5 factorial Transcript cr raisedTo: 10 modulo: 5 Transcript show: 'hello world'

© O. Nierstrasz P2 — A bit of Smalltalk Precedence 2 raisedTo: factorial * (2 * 3) 128 9(!) 7 First unary, then binary, then keyword: Use parentheses to force order:

© O. Nierstrasz P2 — A bit of Smalltalk A typical method in the class Point <= aPoint "Answer whether the receiver is neither below nor to the right of aPoint." ^ x <= aPoint x and: [y <= aPoint y] <= true Method nameArgumentComment Return Binary message Keyword messageInstance variable Block

© O. Nierstrasz P2 — A bit of Smalltalk Statements and cascades | p pen | p := pen := Pen new. pen up. pen goto: p; down; goto: p+p Temporary variables Statement Cascade

© O. Nierstrasz P2 — A bit of Smalltalk Literals and constants Strings & Characters'hello' $a Numbers Symbols#yadayada Arrays#(1 2 3) Pseudo-variablesself super Constantstrue false

© O. Nierstrasz P2 — A bit of Smalltalk Fun with Numbers  Large and exact numbers  Automatic coercion 1000 factorial / 999 factorial 1000 factorial printString size (1/3) + (2/3) 1 class maxVal (1 class maxVal + 1) class LargePositiveInteger

© O. Nierstrasz P2 — A bit of Smalltalk “Natural language” album play album playTrack: 1 album playFromTrack: 5 to: 10

© O. Nierstrasz P2 — A bit of Smalltalk More syntax  Comments are enclosed in double quotes  Use periods to separate expressions  Use semi-colons to send a cascade of messages to the same object "This is a comment." Transcript cr; show: 'hello world'; cr Transcript cr. Transcript show: 'hello world’. Transcript cr "NB: don’t need one here"

© O. Nierstrasz P2 — A bit of Smalltalk Variables  Declare local variables with | … |  Use := to assign a value to a variable [NB: also  (_) ] | x y | x := 1

© O. Nierstrasz P2 — A bit of Smalltalk Method Return  Use a caret to return a value from a method or a block  By default, methods return self max: aNumber ^ self < aNumber ifTrue: [aNumber] ifFalse: [self] 1 max: 2 2

© O. Nierstrasz P2 — A bit of Smalltalk Blocks  Use square brackets to delay evaluation of expressions ^ 1 < 2 ifTrue: ['smaller'] ifFalse: ['bigger'] 'smaller'

© O. Nierstrasz P2 — A bit of Smalltalk Variables  Local variables are delimited by |var| Block variables by :var| OrderedCollection>>collect: aBlock "Evaluate aBlock with each of my elements as the argument." | newCollection | newCollection := self species new: self size. firstIndex to: lastIndex do: [ :index | newCollection addLast: (aBlock value: (array at: index))]. ^ newCollection (OrderedCollection with: 10 with: 5) collect: [:each| each factorial ] an OrderedCollection( )

© O. Nierstrasz P2 — A bit of Smalltalk Control Structures (I)  Every control structure is realized by message sends max: aNumber ^ self < aNumber ifTrue: [aNumber] ifFalse: [self] 4 timesRepeat: [Beeper beep]

© O. Nierstrasz P2 — A bit of Smalltalk Control Structures (II)  Every control structure is realized by message sends |n| n := 10. [n>0] whileTrue: [Transcript show: n; cr. n := n-1] 1 to: 10 do: [:n| Transcript show: n; cr ] (1 to: 10) do: [:n| Transcript show: n; cr ]

© O. Nierstrasz P2 — A bit of Smalltalk Creating objects  Class methods  Factory methods OrderedCollection new Array with: 1 with: 2 1/2 a Point a Fraction

© O. Nierstrasz P2 — A bit of Smalltalk Creating classes  Send a message to a class (!) Number subclass: #Complex instanceVariableNames: 'real imaginary' classVariableNames: '' poolDictionaries: '' category: 'ComplexNumbers'

© O. Nierstrasz P2 — A bit of Smalltalk Roadmap  Some history  Smalltalk syntax & object model  The Smalltalk environment

© O. Nierstrasz P2 — A bit of Smalltalk Three-button mouse Select Operate Window

© O. Nierstrasz P2 — A bit of Smalltalk World Menu and Open Menu

© O. Nierstrasz P2 — A bit of Smalltalk “Hello World”

© O. Nierstrasz P2 — A bit of Smalltalk Do it, Print it, … You can evaluate any expression anywhere in Smalltalk.

© O. Nierstrasz P2 — A bit of Smalltalk The Smalltalk Browser

© O. Nierstrasz P2 — A bit of Smalltalk The Debugger

© O. Nierstrasz P2 — A bit of Smalltalk The Inspector

© O. Nierstrasz P2 — A bit of Smalltalk Other Tools  File List —Browse, import, open files  Change Sorter —Name, organize all source code changes  Method Finder, Message Name tool —Find methods by name, behaviour  SUnit —Manage & run unit tests

© O. Nierstrasz P2 — A bit of Smalltalk Demo Changing a running system …

© O. Nierstrasz P2 — A bit of Smalltalk What you should know!  What are the key differences between Smalltalk, C++ and Java?  What is at the root of the Smalltalk class hierarchy?  What kinds of messages can one send to objects?  What is a cascade?  Why does 1+2/3 = 1 in Smalltalk?  How are control structures realized?  How is a new class created?  What are categories for?  What are Factory methods? When are they useful?

© O. Nierstrasz P2 — A bit of Smalltalk Can you answer these questions?  Which is faster, a program written in Smalltalk, C++ or Java?  Which is faster to develop & debug, a program written in Smalltalk, C++ or Java?  How are Booleans implemented?  Is a comment an Object? How would you check this?  What is the equivalent of a static method in Smalltalk?  How do you make methods private in Smalltalk?  What is the difference between = and ==?  If classes are objects too, what classes are they instances of?

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