Pharo Object Model in a Nutshell

Elegance and Simplicity

Only Objects, Messages, ...

• Objects: mouse pointer, booleans, arrays, numbers, strings, windows, scrollbars, canvas, files, trees, compilers, sound, url, socket, fonts, text, collections, stack, shortcut, streams...
• Messages sent to these objects: size, +, at:put:, do:, ...

... and Block Closures

• Messages are what (intent)
• Methods are how to do it
• Closures are kind of anonymous methods
• Closures are called blocks in Pharo

4 timesRepeat: 
	[ Transcript show: 'Hello World' ]

• [ ... ] delimits a block

A Simple and Uniform Model

• Everything is an object, instance of a class
  • Classes and messages are objects too!
• All computations between objects are done via message passing
• We use the term sending a message because:
  • methods are always looked up dynamically
  • only late binding, only virtual calls
• Only ONE method lookup for all objects

Pharo Object Model

• Instance variables are protected:
  • private to the object
  • accessible from subclasses
• Methods are public and virtually bound
• Single inheritance between classes

Messages

Computation between objects is done via message sends

Example of the cross product of two points:

(point1 x * point2 y) - (point1 y * point2 x)

Object Creation: Creating a Point

A new object can be created by sending a message to another object

10@20

A new Point object is created by:

• sending the message @
• to the object 10 (SmallInteger)
• with the argument 20 (SmallInteger)

Object Creation: Creating a String

'Pharo', ' is Cool'
> 'Pharo is Cool'

A new String is created as the concatenation of two strings by:

• by sending the message ,
• to the string 'Pharo'
• with the string ' is Cool' as argument

Object Creation

Sending the messages new and new: to a class

Monster new
> aMonster

Array new: 6
> #(nil nil nil nil nil nil)

Here we get an array of size 6

Object Creation

Sending instance-creation messages to a class

Tomagoshi withHunger: 10

This executes a class method

Less is More :)

• No constructors
• No static methods
• No type declarations
• No interfaces
• No package/private/protected modifiers
• No parametrized types
• No boxing/unboxing
• still really powerful :)

Summary

• Everything is an object
• Computation is done via messages sent to objects
• Methods are late bound (looked up dynamically in the inheritance chain)
• Blocks are kind of anonymous methods
• Instances are created by sending messages to other objects, or classes