Tutorial: Squeak — An Open-Source Smalltalk Environment Oscar Nierstrasz scg.unibe.ch TOOLS Europe 2008 Zürich, July 1, 2008.

Tutorial: Squeak — An Open-Source Smalltalk Environment Oscar Nierstrasz scg.unibe.ch TOOLS Europe 2008 Zürich, July 1, 2008

© O. Nierstrasz 2 Roadmap  What is Smalltalk?  Getting started  Syntax in a nutshell  Test-driven development  Version control  Standard classes — Object, Number, Boolean …  Common idioms  Debugging practices  Metaclasses in 7 points  Seaside demo

© O. Nierstrasz 6 Smalltalk-80 and Squeak Everything is an object Everything is there, all the time First windowing system with mouse First graphical IDE Squeak: a modern, portable, fast, open- source Smalltalk

© O. Nierstrasz 8 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

Everything is an object

Everything happens by sending messages

© O. Nierstrasz 20 Precedence 2 raisedTo: 1 + 3 factorial 1 + 2 * 3 1 + (2 * 3) 128 9(!) 7 First unary, then binary, then keyword: Use parentheses to force order: 2 raisedTo: (1 + (3 factorial)) Same as:

© O. Nierstrasz 21 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] (2@3) <= (5@6) true Method nameArgumentComment Return Binary message Keyword messageInstance variable Block

© O. Nierstrasz 26 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(3628800 120)

© O. Nierstrasz 29 Creating classes  Send a message to a class (!) Number subclass: #Complex instanceVariableNames: 'real imaginary' classVariableNames: '' poolDictionaries: '' category: 'ComplexNumbers'

© O. Nierstrasz 33 Money TestCase subclass: #MoneyTest instanceVariableNames: 'chf2 chf8 chf10' classVariableNames: '' poolDictionaries: '' category: 'Money'  We will implement the Money example in Smalltalk —First, we develop a test case for a single currency NB: This is just a message sent to the TestCase class object (!)

© O. Nierstrasz 34 SetUp  We will need setters for the private Money state MoneyTest>>setUp chf2 := Money new currency: 'CHF'; amount: 2. chf8 := Money new currency: 'CHF'; amount: 8. chf10 := Money new currency: 'CHF'; amount: 10.

© O. Nierstrasz 36 Money  We define Money as a subclass of Object, with getters and setters Object subclass: #Money instanceVariableNames: 'currency amount' classVariableNames: '' poolDictionaries: '' category: 'Money' Money>>currency: aString currency := aString. Money>>currency ^ currency

© O. Nierstrasz 39 Constructors MoneyTest>>testEquals self assert: chf2 = chf2. self assert: chf2 = (Money currency: 'CHF' amount: 2). self assert: chf2 != chf8. We need a constructor on the class side of Money

© O. Nierstrasz 43 Change Sets 'From Squeak3.9alpha of 4 July 2005 [latest update: #7006] on 7 March 2006 at 12:52:26 pm'! Object subclass: #Money instanceVariableNames: 'currency amount' classVariableNames: '' poolDictionaries: '' category: 'Money'! !Money commentStamp: 'on 3/7/2006 12:45' prior: 0! SUnit demo! TestCase subclass: #MoneyTest instanceVariableNames: 'chf2 chf8 chf10' classVariableNames: '' poolDictionaries: '' category: 'Money'! !MoneyTest commentStamp: 'on 3/7/2006 12:44' prior: 0! SUnit demo! !Money methodsFor: 'arithmetic' stamp: 'on 3/7/2006 12:52'! + aMoney ^ Money currency: self currency amount: self amount + aMoney amount! ! …

© O. Nierstrasz 44 Extensions  Creating new instances of Money is still cumbersome —We would like to be able to write instead code like this: TestMoney>>setUp chf2 := 2 chf. chf8 := 8 chf. chf10 := 10 chf.

© O. Nierstrasz 48 Categories, Projects and Packages  A system category MyProject (and possibly MyProject-*) contains the classes of your application  A Monticello package MyProject contains the categories MyProject and MyProject-*  A SqueakSource project MyProject stores everything in the Monticello package MyProject

© O. Nierstrasz 49 Loading a project  To load a project from SqueakSource.com —Find the project in SqueakSource —Open the Monticello Browser —Add an HTTP Repository – Accept the code fragment from the SqueakSource page —Open the repository and load the latest version

© O. Nierstrasz 51 Creating a project  To create a project on SqueakSource.com —Define your categories MyProject or MyProject-* —Create a SqueakSource project MyProject —Define a Monticello package MyProject —Add an HTTP Repository for MyProject – Accept the code fragment from the SqueakSource page —Save the the package

© O. Nierstrasz 54 Identity vs. Equality  = tests Object value —Should normally be overridden – Default implementation is == ! —You should override hash too!  == tests Object identity —Should never be overridden 'foo','bar' = 'foobar' 'foo','bar' == 'foobar' true false

© O. Nierstrasz 55 Printing  Override printOn: to give your objects a sensible textual representation Fraction>>printOn: aStream aStream nextPut: $(. numerator printOn: aStream. aStream nextPut: $/. denominator printOn: aStream. aStream nextPut: $).

© O. Nierstrasz 56 Object methods to support the programmer error: aString Signal an error doesNotUnderstand: aMessage Handle unimplemented message halt, halt: aString Invoke the debugger subclassResponsibility The sending method is abstract shouldNotImplement Disable an inherited method deprecated: anExplanationString Warn that the sending method is deprecated.

© O. Nierstrasz 58 Abstract methods in Smalltalk Number>>+ aNumber "Answer the sum of the receiver and aNumber." self subclassResponsibility Object>>subclassResponsibility "This message sets up a framework for the behavior of the class' subclasses. Announce that the subclass should have implemented this message." self error: 'My subclass should have overridden ', thisContext sender selector printString

© O. Nierstrasz 60 Strings  To introduce a single quote inside a string, just double it. #mac asString 12 printString String with: $A 'can''t' at: 4 'hello', ' ', 'world' 'mac' '12' 'A' $' 'hello world'

© O. Nierstrasz 63 Symbols vs. Strings  Symbols are used as method selectors and unique keys for dictionaries —Symbols are read-only objects, strings are mutable —A symbol is unique, strings are not NB: Comparing strings is slower than comparing symbols by a factor of 5 to 10. However, converting a string to a symbol is more than 100 times more expensive. 'cal', 'vin' == 'calvin'. ('cal', 'vin') asSymbol == #calvin. false true

© O. Nierstrasz 64 Blocks  Syntax: —Returns last expression of the block [ :arg1 :arg2 | |temp1 temp2| expression. expression ] [2 + 3 + 4 + 5] value [:x | x + 3 + 4 + 5 ] value: 2 [:x :y | x + y + 4 + 5] value: 2 value: 3 [:x :y :z | x + y + z + 5] value: 2 value: 3 value: 4 [:x :y :z :w | x + y + z + w] value: 2 value: 3 value: 4 value: 5

© O. Nierstrasz 66 IfTrue: IfFalse: Integer>>factorial "Answer the factorial of the receiver." self = 0 ifTrue: [^ 1]. self > 0 ifTrue: [^ self * (self - 1) factorial]. self error: 'Not valid for negative integers'

© O. Nierstrasz 67 nil A reference to the UndefinedObject true Singleton instance of the class True false Singleton instance of the class False self Reference to this object Method lookup starts from object’s class super Reference to this object (!) Method lookup starts from the superclass thisContext Reification of execution context Six Pseudo-Variables  The following pseudo-variables are hard-wired into the Smalltalk compiler.

© O. Nierstrasz 69 Various kinds of Loops |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 ] 10 timesRepeat: [ Transcript show: 'hi'; cr ] In each case, what is the target object?

© O. Nierstrasz 71 Common messages #(1 2 3 4) includes: 5 #(1 2 3 4) size #(1 2 3 4) isEmpty #(1 2 3 4) contains: [:some | some < 0 ] #(1 2 3 4) do: [:each | Transcript show: each ] #(1 2 3 4) with: #(5 6 7 8) do: [:x : y | Transcript show: x+y; cr] #(1 2 3 4) select: [:each | each odd ] #(1 2 3 4) reject: [:each | each odd ] #(1 2 3 4) detect: [:each | each odd ] #(1 2 3 4) collect: [:each | each even ] #(1 2 3 4) inject: 0 into: [:sum :each | sum + each] false 4 #(1 3) #(2 4) 1 {false.true.false.true} 10

© O. Nierstrasz 72 Iteration — the hard road and the easy road |aCol result| aCol := #( 2 -3 4 -35 4 -11). result := aCol species new: aCol size. 1 to: aCol size do: [ :each | result at: each put: (aCol at: each) abs]. result #(2 3 4 35 4 11) #( 2 -3 4 -35 4 -11) collect: [:each | each abs ] #(2 3 4 35 4 11) How to get absolute values of a collection of integers? NB: The second solution also works for indexable collections and sets.

© O. Nierstrasz 73 Concatenation How do you put two collections together?  Send “,” to the first with the second as argument (1 to: 3), (4 to: 6) a Dictionary(#a->1 #b->2 ) (Dictionary newFrom: { #a -> 1}), (Dictionary newFrom: { #b -> 2}) #(1 2 3 4 5 6)

© O. Nierstrasz 74 Sorted Collection How do you sort a collection?  Convert it to a Sorted Collection. 'hello' asSortedCollection a SortedCollection($o $l $l $h $e) 'hello' asSortedCollection: [:a :b | b<=a ] a SortedCollection($e $h $l $l $o)

© O. Nierstrasz 76 Converting  Send asSet, asBag, asSortedCollection etc. to convert between kinds of collections  Send keys, values to extract collections from dictionaries  Use various factory methods to build new kinds of collections from old Dictionary newFrom: {1->#a. 2->#b. 3->#c}

© O. Nierstrasz 79 Scripting a use case SnakesAndLadders class>>example "self example playToEnd" ^ (self new) add: FirstSquare new; add: (LadderSquare forward: 4); add: BoardSquare new; add: (LadderSquare forward: 2); add: BoardSquare new; add: (SnakeSquare back: 6); add: BoardSquare new; join: (GamePlayer named: 'Jack'); join: (GamePlayer named: 'Jill'); yourself  Construct the board  Add some players  Play the game

© O. Nierstrasz 81 Lots of Little Methods  Once and only once —“In a program written with good style, everything is said once and only once.”  Lots of little pieces —“Good code invariably has small methods and small objects. Only by factoring the system into many small pieces of state and function can you hope to satisfy the ‘once and only once’ rule.” – Kent Beck, Smalltalk Best Practice Patterns

© O. Nierstrasz 82 Composed Method How do you divide a program into methods?  Divide your program into methods that perform one identifiable task. —Keep all of the operations in a method at the same level of abstraction. —This will naturally result in programs with many small methods, each a few lines long.

© O. Nierstrasz 84 How to initialize objects? In Smalltalk, —methods are public, and —instance variables are private So, how can a class (an object) initialize the instance variables of its instances (other objects)?

© O. Nierstrasz 85 Explicit Initialization How do you initialize instance variables to their default values?  Implement a method initialize that sets all the values explicitly. — Override the class message new to invoke it on new instances SnakesAndLadders>>initialize super initialize. die := Die new. squares := OrderedCollection new. players := OrderedCollection new. turn := 1. over := false.

© O. Nierstrasz 86  In Squeak 3.9, the method new calls initialize by default.  NB: You can override new, but you should never override basicNew!  Every metaclass ultimately inherits from Behaviour – More on this later … Behavior>>new ^ self basicNew initialize Who calls initialize?

© O. Nierstrasz 87 Constructor Method How do you represent instance creation?  Provide methods in the class side “instance creation” protocol that create well-formed instances. Pass all required parameters to them. LadderSquare class>>forward: number ^ self new setForward: number SnakeSquare class>>back: number ^ self new setBack: number

© O. Nierstrasz 88 Constructor Parameter Method How do you set instance variables from the parameters to a Constructor Method?  Code a single method that sets all the variables. Preface its name with “set”, then the names of the variables. SnakeSquare>>setBack: aNumber back := aNumber. BoardSquare>>setPosition: aNumber board: aBoard position := aNumber. board := aBoard LadderSquare>>setForward: aNumber forward := aNumber.

© O. Nierstrasz 89 Debug Printing Method How do you code the default printing method? —There are two audiences: – you (wanting a lot of information) – your clients (wanting only external properties)  Override printOn: to provide information about object’s structure to the programmer —Put printing methods in the “printing” protocol

© O. Nierstrasz 91 Implementing printOn: SnakesAndLadders>>printOn: aStream squares do: [:each | each printOn: aStream] BoardSquare>>printOn: aStream aStream nextPutAll: '[', position printString, self contents, ']' LadderSquare>>printOn: aStream super printOn: aStream. aStream nextPutAll: forward asString, '+>' SnakeSquare>>printOn: aStream aStream nextPutAll: '<-', back asString. super printOn: aStream GamePlayer>>printOn: aStream aStream nextPutAll: name

© O. Nierstrasz 94 Query Method How do you represent testing a property of an object?  Provide a method that returns a Boolean. —Name it by prefacing the property name with a form of “be” — is, was, will etc.

© O. Nierstrasz 95 Some query methods SnakesAndLadders>>isNotOver ^ self isOver not BoardSquare>>isFirstSquare ^ position = 1 BoardSquare>>isLastSquare ^ position = board lastPosition BoardSquare>>isOccupied ^ player notNil FirstSquare>>isOccupied ^ players size > 0

© O. Nierstrasz 98 Debug printing  Basic printing —You can use the Transcript to display progress  Optional printing —Use a global or a class to control printing information Transcript cr; show: ‘The total= ’, self total printString. Debug ifTrue: [Transcript show: self total printString] Debug > 4 ifTrue: [Transcript show: self total printString] Debug print: [Transcript show: self total printString]

© O. Nierstrasz 99 Tests are your friends!  Resist the temptation to write debugging print methods —Write a test instead!  Resist the temptation to evaluate ad hoc expressions in a Workspace —Write a test instead! —Tests are reusable – You will have to spend the effort debugging anyway – Amortize the investment by coding your debugging effort as tests

© O. Nierstrasz 100 The Inspector is your friend!  You can inspect anything —Inspect any expression —View the printString state —Interact with any object —Inspect instance variables —Navigate through the system

© O. Nierstrasz 101 Use the Inspector to make ad hoc changes  You can use the Inspector as an ad hoc interface to modify the state of a running system —Use this sparingly! If we change the name of a GamePlayer, this will be reflected in the running system.

© O. Nierstrasz 102 Breakpoints  Send the message self halt to start the debugger at an arbitrary location SnakesAndLadders>>playOneMove | result | self assert: self invariant. self halt. ^ self isOver …

© O. Nierstrasz 105 SystemNavigation default browseMethodsWithSourceString: 'super' A bit slow, and contains many false negatives The Browser is your friend! How can we browse all methods that send to super? —Browse “browse” —Object>>browse —Object>>systemNavigation —SystemNavigation —SystemNavigation>>browseMethodsWithSourceString:

© O. Nierstrasz 106 The Message Name Finder is your friend!  We continue browsing: —SystemNavigation>>browseMethodsWith* —SystemNavigation>>browseAllSelect:  Query the Message Name Finder for “super” —Yields CompiledMethod>>sendsToSuper  Better solution: SystemNavigation default browseAllSelect: [:method | method sendsToSuper ] Fast, and accurate!

© O. Nierstrasz 108 Metaclasses in 7 points 1. Every object is an instance of a class 2. Every class eventually inherits from Object 3. Every class is an instance of a metaclass 4. The metaclass hierarchy parallels the class hierarchy 5. Every metaclass inherits from Class and Behavior 6. Every metaclass is an instance of Metaclass 7. The metaclass of Metaclass is an instance of Metaclass Adapted from Goldberg & Robson, Smalltalk-80 — The Language

© O. Nierstrasz 119 Attribution-ShareAlike 3.0 Unported You are free: to Share — to copy, distribute and transmit the work to Remix — to adapt the work Under the following conditions: Attribution. You must attribute the work in the manner specified by the author or licensor (but not in any way that suggests that they endorse you or your use of the work). Share Alike. If you alter, transform, or build upon this work, you may distribute the resulting work only under the same, similar or a compatible license. For any reuse or distribution, you must make clear to others the license terms of this work. The best way to do this is with a link to this web page. Any of the above conditions can be waived if you get permission from the copyright holder. Nothing in this license impairs or restricts the author's moral rights. License  http://creativecommons.org/licenses/by-sa/3.0/

© O. Nierstrasz 120 Variables  Shared variables — initial uppercase  Private variables — initial lowercase variable Shared variable Pool variable Global variable private variable named temporary variableinstance variable indexed | block temporary | | method temporary | Class variable method parameter : block parameter

© O. Nierstrasz 121 Initializing class variables  Class variables should be initialized by an initialize method on the class side Magnitude subclass: #Date instanceVariableNames: 'julianDayNumber ' classVariableNames: 'DaysInMonth FirstDayOfMonth MonthNames SecondsInDay WeekDayNames ' poolDictionaries: '' category: 'Kernel-Magnitudes' Date class>>initialize... WeekDayNames := #(Sunday Monday...). MonthNames := #(January February... ). DaysInMonth := #(31 28 31 30 31 30 31 31 30 31 30 31).

Tutorial: Squeak — An Open-Source Smalltalk Environment Oscar Nierstrasz scg.unibe.ch TOOLS Europe 2008 Zürich, July 1, 2008.

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Tutorial: Squeak — An Open-Source Smalltalk Environment Oscar Nierstrasz scg.unibe.ch TOOLS Europe 2008 Zürich, July 1, 2008.

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