Class 38: Student Presentation: Smalltalk

Back to Discussion of Exam 3. On to Student Presentations II.

Held Wednesday, May 5

Summary

• General overview of Smalltalk with Scott
• Smalltalk Lab with Cathy and Weichao

Contents

• Basic Concepts
• Nature of Types and Class Hierarchy
• Messages
• Syntax
• Smalltalk Lab
• Links

Handouts

Notes

• The annual Math/CS Spring picnic is ths Sunday (mother's day; don't forget to call your mothers) at noon. There's a signup list.
  • If you're on board, entry your number.
  • If you're not on board, the department will cover your food.
• There's a cool statistics talk tomorrow at 4:30 in Science 2413: Statistical Lessons from Basketball. Learn whether the ``hot hand'' is myth or statistical reality.
• We are happy to have a visitor--Mr. Rebelksy here with us today, feel free to ask him tough questions.

• Of all object oriented languages, Smalltalk is the most true to the Object Oriented paradigm.
• Smalltalk uses a virtual image, which contains the compiled code of all the classes and objects that are a part of the system.
• Smalltalk code is compiled into byte codes which are interpreted into actions during runtime.

Basic Concepts

The Basic concepts of who Smalltalk works are based on the interaction of five core concepts, some of which should be very familiar to those of you who have used Objetc Oriented languages before:
• Objects - Everything is an object in Smalltalk.
• Classes - An object that defines properties for a specific type of object. Classes are arranged in a hierarchy of inheritance.
• Instances - An instance is an object of a specific class, with properties and actions defined by the class it derives from.
• Messages - A message is an identifier sent to an object that triggers some sort of action in response.
• Methods - A method is an implimentation detail to the action triggered by an object receiving a message.

How these concepts interact form the basis for how Smalltalk works
• An Object responds to a Message according to how the Methods of teh Object's Class indicates

Nature of Types and Class Hierarchy

Everything in Smalltalk is an object. Everything. This includes:
• data structures
• external devices
• files
• blocks of code
• messages from one object to another

What does this mean in terms of typing?
• We're not entirely sure. It's being argued as we speak
• Some say that Smalltalk is essentially typeless, given that the exact nature of data depends on how it is defined in its class
• Some say that the only type is classes, given everything is an object and therefore an instance of a class

Classes are grouped together into a hierarchy of objects with similar effects/properties
• For instance, Numbers are a subclass of the Magnitude class, which consists of objects that can be compared to one another and be ordered, such as Time, Date, Number and Character
• All Magnitude Objects have a set of conditional methods used for comparisons
• In turn, Integers, Floats, Imaginary numbers and Complex numbers are all children of the Number class.

Messages

But how do you get an object to do something?
• You send the object a message
• To send an object a message in Smalltalk, you specify the object then the message with any parameters afterward
• For example:
  

  3 + 4          The Object '3' is sent the message '+' with the parameter '4'
  5 factorial    The Object '5' is sent the message factorial
  

• Messages must correspond to methods availavle to the object. In the first example above, the message '+ 4' is sent to 3, invoking the '+' method on 3

Messages come in three flavors:
• Unary - Unary messages consist of a single identifier with no parameters
  

  5 factorial               "Return factorial of 5"
  theta sin                 "Return sin of theta"
  'hello' outputToPrinter   "Send 'hello' to the printer"
  

• Binary - Binary messages consist of an identifier with a object as a parameter
  

  a + b                     "Send + b to a, i.e. add b to a"
  a < b                     "Send < b to a, i.e. return true if a is less than b"
  a ~= b                    "Send ~= b to a, i.e. return true is a is not equal to b"
  

• Keyword - Keyword messages are a list of identifier/parameter pairs send to an object

  aDate day:dayInt   
         "Send day:dayInt to aDate (assigning dayInt to the day field of aDate)" 
  aInt between:min and:max 
         "Send between:min and and:max to aInt (returning true if aInt is between min and max)"
  

Since all methods return objects, messages can be nested and strung together
• Messages are resolved left to right, with precidence given in order of Unary messages, Binary messages, then Keyword messages.
• Thus, all unary messages are resolved, from left to right, then all binmary messages, left to right then keyword messages, left to right.
• Note tha 'arithmetic operations' do not follow the usual order of operations. Thus '1 + 10 / 10' is not 2 as one might expect, but 11/10 as '1 + 10' is evaluated and returns 11 and then '11 / 10' is evaluated.

Syntax

Reserved Words - Smalltalk has five reserved words, which are pseudo-variables, meaning they can be referenced as variables, but can not be assigned to:
• true - the constant for Boolean true
• false - the constant for Boolean false
• nil - a constant that represents an uninitialized object
• self - a constant in a method that refers to the object on which the method is being invoked
• super - a constant in a method referring to the immediate parent of the object on which the method is being invoked.

Literals - Literals are instances of particular objects that Smalltalk has built-in support for specifying instances of that class.
• Numbers - Numbers begin with a digit or a minus sign. Floats are specified by the inclusion of a decimal or exponent. base can be specified at the beginning of the number followed by an 'r' then the number in that base
  

  	456		integer
  	1.234   	float
  	1.34e4		float
  	2r11011		base 2 integer
  	36rSMALLTALK	base 36 integer
  	16r34.FC	hexidecimal float
  

• Characters - Characters are any ASCII character preceded by a $, such as $A, $a, $$, $ (a space), $(, etc.
• Strings - A sequence of characters surrounded by single quotes, like 'Hello world!'. Single quotes can be in the string if they are doubles, e.g. 'It''s a wonderful life.'
• Symbols - Symbols are identifiers, binary selectors or keywords proceded by a #. All symbols are unique and thus must have a unique sequence of characters to be distinct. Examples: #aSymbol, #+, #at:, etc.
• Arrays - An array is a collection of elements that can be any valid object, in '#(' and ')' and can be nested as an array is just another object. Example: #( $d 34 #('cows' 1 $!))

Block Context - a Block Context is an object that represents a block of code surrounded by square brackets.
• Within a Block Context, temporary variables can be declared by surrounding the variable name by veritcal bars.
• Such temporary variables are local and exist only in the scope of the Block Context

Assignment to a variable is done via the assignment mesaage ':='

Returning an object is done by placing a '^' before an object.

Example:

• 
  	[|Temp|
  	 Temp := 'Weasels rip my flesh.'
  	 ^Temp ]
  
  

• This Block Context declares a local variable 'Temp', assigns a value to it (in this case, a string) and returns the object pointed to by Temp

Conditionals - Conditionals are actually methods of the Boolean Class. A conditional message (e.g. "3 < 4") resolves to either the object true or the object false. if statements are Keyword messages passed to the Boolean object.
• Boolean ifTrue: BlockContext resolves the Block Context if the Boolean object is true, that is the comparison message resolves to true. ifFalse does a similar thing
• You can pass both ifTrue and ifFalse messages to the same boolean object, having the same effect as an if-then-else.
• There is no if-then-elseif-then-else structure or case statement in Smalltalk. This can be simulated by nesting more boolean objects in the Block Context of the first.

Loops - The equivilent of a while loop is available also as message to a Boolean object.
• Boolean whileTrue: Block Context evaluates the Block Context while the Boolean continues to resolve to true. whileFalse works in a similar way
• No for loops exist explicitly in Smalltalk, but the following example shows how one can be implimented:
  

  	|temp|
  	temp := Array new:10
  	1 to: 10 do: [:index | temp at:index put:1]
  

• This example takes an array and sets each value to one

Smalltalk Lab

Links

• ObjectShare, Inc. The company that brings you VisualWorks -- free, non-commercial versions available for downloading here. Interesting section, "Success Stories", about big (and small) companies that actually use programs written in Smalltalk.

smalltalk_group@ac.grin.edu