Smalltalk

1
Smalltalk

• Date 2009.03.10
• Presenter Young-Jun Lim

2
Agenda

• Why Smalltalk?
• History outline
• Environment
• Introducing to the language
• Large-Sacle Features of Smalltalk
• Example
• Alan Kay said

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Why Smalltalk?

• When I invented the term object-oriented I did
  not have C in mind. Alan Kay
• Pure object-oriented language
• Learn concepts, not syntax very easy to learn
• GC, no memory management, no pointers
• Look at and manipulate objects in real time
• High productivity by short round-trip times
• Easy to understand
• Python, Ruby, Awk, Perl, S,
• Objective C, Java, C,

4
History outline

• 1968 SIMULA
• Fisrt object-oriented language
• 1971 Dynabook (Alan Kay)
• easy-to-use computer for anyone
• bitmapped display, pointing device, external
  storage
• programming creating simulations
• Smalltalk-71

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History outline (cont)

• 1973 Xerox Alto computer
• running Smalltalk (implemented in BASIC)
• iconic characters, turtle graphics (LOGO)
• classes (no hierarchies), instances, self
• 1974
• performance improvements

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History outline (cont)

• 1976
• Smalltalk bytecode interpreter in Alto microcode
• everything is an object
• class hierarchy, super
• code browser, inspector, debugger
• 1978 NoteTaker
• step towards Dynabook portable computer
• cancelled "No one wants portability."

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History outline (cont)

• Smalltalk-80
• 2 goals hardware and community
• language standard
• massively adopts MVC
• forerunner of today's GUIs

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Environment

• Environment
• virtual machine (usually with JIT)
• image snapshot
• export applications as image, deliver with VM
  binary
• Benefits
• environment is IDE and run-time all the same
• powerful debugging facilities
• on-the-fly implementation modification

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Introducing to the language

• "Pure" object-oriented language
• absolutely everything is an object
• Features
• single inheritance
• dynamically typed
• powerful library and meta-level
• strict class hierarchy (superclass Object)

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Introducing to the language

• Lexical details
• comments double quotes
• characters ltchargt
• strings single quotes
• symbols ltsymgt
• arrays (ltelemgt ltelemgt ...)
• Operators
• assignment
• equality
• identity

"this is a comment" x 'this is a
string' thisIsASymbol ('this' 'is' 'an' 'array'
'with' 7 'elements')
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Expression

• Expression
• Literals (numbers, strings, and keywords)
• Kind of objects
• Keyword is user-defined indentifier
• Variable names (all varialbles are references)
• Private, Shared
• Variable has no type, only refer to object or
  class
• Message expressions
• Block expressions

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Expression

• Keywords are pervasive

C/Java
Smalltalk
Transformation t float a Vector
v t-gtrotate(a,v) // for C t.rotate(a,v) //
for Java
t a v aTransformation angle aVector
t.Rotate (a, v) t rotate by a around v
t rotateBy a around v
This is Smalltalk!
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Expression

• Message expressions
• Two parts the receiver object and the message
  itself
• The message part specifies the method and
  possibly some parameters
• Replies to message are objects
• Messages can be of three forms
• Unary (no parameters)
• e.g., firstAngle sin (sends a message to the sin
  method of the firstAngle object)
• Binary (one parameterm an object)
• e.g., 12 17 (sends the message 17 to the
  object 12 the object parameter is 17 and the
  method is )
• Keyword (use keywords to organize the parameters)
• e.g., firstArray at 1 put 5 (sends the object
  1 and 5 to the atput method of the object
  firstArray)

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Expression

• Message expressions
• Multiple messages to the same object can be
  strung together, separated by semicolons
• Ex) outPen homeupgoto500_at_500down, home
• Same meanings outPen home.
• ourPen up.
• ourPen goto
  500_at_500.
• ourPen down.

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Methods

• General form
• Message_pattern temps statements
• A message pattern is like the formal parameters
  of a subprogram
• For a unary message, it is just the name
• Ex) currentTotal (return next value)
• (oldTotal newValue) ( mean
  return)
• For others, it lists keywords and formal names
• Ex) x xCoord y yCoord
• ourPen up goto xCoord_at_yCoord down.
• -gt outPen x300 y400
• Temps are just names
• Smalltalk is typeless!

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Assignments

• Simplest form sum lt- total
• Object (referenced by variable total) referenced
  by sum
• It is simply a pointer assianment
• Ex) index lt- index 1
• netPay lt- deducts grossPay 350.0
  dependents 4

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Blocks

• A sequence of statements, seperated by periods,
  delimited by brackets
• e.g., index lt- index 1. sum lt- sum index
• A block specifies something, but doesnt do it
• To request the execution of a block, send it the
  unary message, value
• e.g., value
• If a block is assigned to a variable, it is
  evaluated by sending value to that variable
• e.g., addIndex lt- sum lt- sum index
• addIndex value

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Blocks

• Blocks can have parameters, as in
• x y statements
• e.g., xy sum lt- x10. total lt- sumy
• If a block contains a relational expression, it
  returns a Boolean object, true or false

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Conditions

• Conditions
• there are actually no control structures
• all "control structures" are messages
• Messages that can be sent to Boolean objects
• ifTrue, ifFalse
• ifTrueifFalse, ifFalseifTrue
• conditional functionality encapsulated in blocks

if(a lt 0 (b gt c b lt d)) a -c
a negative (b between c and d) ifTrue a
c negated
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Conditions

• How are conditionals implemented?
• class Boolean is abstract
• classes True and False inherit from Boolean
• each has one instance true resp. false
• implementation of ifTrue in class True
• implementation of ifFalse in class True

ifTrue alternativeBlock alternativeBlock
value
ifTrue alternativeBlock nil
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Logical Operations

• Operators implemented in Boolean
• , - eager evaluation (Boolean argument)
• and, or - lazy evaluation (block argument)
• xor, eqv - Boolean argument
• not

This gets executed in any case but nil does not
understand the gt message!
x x nil. x notNil (x gt 0) ifFalse
Smalltalk beep
x x nil. (x notNil and x gt 0 )
ifFalse Smalltalk beep
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Comparing

• Messages implemented in Object
• isNil, notNil
• , - equal, not equal
• , - exactly equal, not exactly equal
• A word about exact equality (, )
• exact equality same object pointer
• do not overload!
• Other examples for comparing messages
• gt, lt, even, odd, ...

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Loops

• Counting loops
• do something 100 times
• Advanced for loops

100 timesRepeat Transcript show 'Hello
world.' cr
1 to 100 do i Transcript show i cr
1 to 100 by 10 do i Transcript show i
cr
100 to 1 by -1 do ...
0.5 to 7.3 by 1.1 do ...
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Loops

• While loops
• blocks can return Boolean values
• messages whileTrue and whileFalse

a a 100 atRandom. a 42 whileFalse
a 100 atRandom
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Classes and Objects

• Classes and objects
• general superclass Object
• there are class and instance variables and
  methods
• Instantiation
• "constructors" are class methods, e.g., new
• expressive "constructor" names
• possible instantiation method for a Person class
• usage

withName name age age address address (self
new name name age age address address)
withName name age age address address (self
new name name age age address address)
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Classes and Objects

• Visibility
• all instance variables are private
• all messages are public
• instances of the same class cannot access members
• Instance variable access
• convention define two messages of the same name
• instance variable age
• getter message age
• setter message age

age age
age newAge age newAge
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Error Handling

• Dynamic typing caveat unknown messages
• in Object message doesNotUnderstand
• invoked implicitly upon illegal message sends
• override to deal with it
• standard behaviour invoke debugger

doesNotUnderstand aMessage Transcript show
'Do not send me ' show aMessage selector
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Error Handling

• Exceptions
• upon a division by zero, return 1
• Signal exceptions
• superclass of all exceptions Exception
• program errors Error
• notifications that can be ignored Notification
• warnings the user should be aware of Warning
• signal e.g., like this

x / y on ZeroDivide do ex ex resume 1

Error signal 'Something really bad happened.'
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Error Handling

• Return values
• this returns 42 if an error occurs
• return value of ondo
• Resuming
• resumable exceptions (Notification, Warning)
• returns 42, but where the Warning was signalled

Error signal on Error do ex someOp.
42
Warning signal on Warning do ex ex
resume 42
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Error Handling

• Exiting handler blocks
• explicit exit two semantically identical ways
• Retrying
• adjust erroneous values
• use a fallback

Error signal on Error do ex 23
Error signal on Error do ex ex exit
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x / y on ZeroDivide do ex y
0.000001. ex retry
doSomethingUnsafely on Error do ex ex
retryUsing doItSafely
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Error Handling

• Cleaning up
• place a lock on an external file
• locks must be released even if exceptions occur
• Conditionally cleaning up
• perform cleaning up only if an exception occurred

doLocked aBlock self lock. aBlock ensure
self unlock
someBlock ifCurtailed cleanup
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Large-Scale Features of Smalltalk

• Type Checking and Polymorphism
• All bindings of messages to methods is dynamic
• The process is to search the object to which the
  message is sent for the method if not found,
  search the superclass, etc.
• Because all variables are typeless, methods are
  all polymorphic
• Inheritance
• All subclasses are subtypes (nothing can be
  hidden)
• All inheritance is implementation inheritance
• No multiple inheritance
• Methods can be redefined, but the two are not
  related

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Alan Kay said
"The best way to predict the future is to invent
it! ".
Alan Kay
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Reference

• ????? ??? ? 4?, Robert W. Sebesta
• Smalltalk Idioms, Aoki Atsushi
• (http//www.sra.co.jp/people/aoki/SmalltalkI
  dioms/index_e.htm)
• http//www.smalltalk.org/ (Smalltalk Community)
• A Little Smalltalk, Timothy Budd
• Smalltalk with Style, Edward J. Klimas
• Advanced OO Design, Informatik
• Simula and Smalltalk, John Mitchell(Stanford Uni)
• http//www.cincom.com/ (VirtualWorks)
• http//www.squeak.org/(Squeak)

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Question Answer

• Q A