WEBVTT

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In this session we'll see
the Pharo object model.

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We'll take an overview to show you
its elegance and simplicity.

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You don't need
to understand everything now

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because we'll go over these notions
over the next few weeks.

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In Pharo
there are only objects and messages.

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There are lots of objects,

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to represent the mouse pointer,
booleans, arrays,

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numbers, strings,
windows, scrollbars and so on...

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Even compilers,

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system objects such as sockets,
fonts, collections and so on.

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All these are objects

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to which we can send messages.

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There are lots of different messages.

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You can send size to a collection
to get its size.

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There are the messages
plus, at:put:, do: and so on.

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Objects and messages.

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The messages indicate
the programmer's intention.

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When I send a message to an object,
I'm asking it to do something.

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It's really a request
made to the object.

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And the object decides
what method to use.

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This is the how. How to achieve
the programmer's intention

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by deciding what method to employ.

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Next we have another concept,
which is that of closure.

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These are lexical closures,
blocks,

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sorts of anonymous methods in Pharo.

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We call them blocks.
This is important vocabulary.

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Blocks are delimited
by square brackets, as you see here.

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So all this is a block.

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So you should really look at this
as an anonymous method.

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In Pharo we have
a very simple and uniform model.

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Everything is an object,
an instance of a class.

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Even classes and messages
are objects too.

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And all computations between objects
are done via message passing.

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We really talk
about sending a message,

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that's the term,
and not about executing a method,

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because when you send a message
to an object,

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there's a particular algorithm
called the method lookup,

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just one algorithm,

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which selects the right method to use.

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The methods are virtually bound.

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They accept late binding.

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I send a message to an object,

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and the method lookup
selects the right method.

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We'll come back to that
in a dedicated session.

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The Pharo object model is as follows.

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Instance variables are protected.

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All objects have instance variables
and they're protected.

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They're private to the object

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or are accessible from subclasses.

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The methods are public
and virtually bound.

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All the methods are public in Pharo.

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And Pharo accepts
single inheritance between classes.

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A class can only have
one superclass.

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Here's an example of code,
the cross-product of two points.

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There's point1 and point2.

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I multiply the x field of point1
by the y field of point2,

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and subtract the multiplication

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of the y field of point1
and the x field of point2.

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It's an example of a computation
you can do in Pharo.

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To create objects
we have special messages.

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Here, for example,

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if I send the message @
to the integer 10

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with the argument 20,

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it'll a create a Point,
an instance of the class Point.

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The name of the message is @,

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the integer
that received the message is 10,

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the one before the name
of the message,

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and the argument, after the name
of the message, is 20.

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Here's another example.

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I want to create a string
of characters like this one here.

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How is this string obtained?

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We've used a first chain here,
Pharo,

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we've sent it the message comma,

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as argument
we have the string is cool...

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Which is here, right?

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The meaning of the message comma
is the concatenation of strings.

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The two strings are glued together
to produce a single string.

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We can create objects directly

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with the message new.

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I send new to the class Monster
and I'll get an instance of the class:

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aMonster.

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I can create an instance
of the class Array,

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by specifying new

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and attributing a parameter,
which I do with a colon,

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and then here the integer 6.

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And I get an array of size 6.

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We can have dedicated messages
to create objects,

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and I can define
my own messages to a class.

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I could create a class Tomagoshi,

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with a method withHunger

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and for this method
I'll give the integer 10.

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This creates an instance
of the class Tomagoshi

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by attributing a parameter.

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So these are specialised messages
for object creation.

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In this lecture we've seen an overview
of the Pharo object model.

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You don't have to understand it all,
we'll come back to these notions.

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But it's important to remember that
there are no constructors in Pharo,

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no static methods,
type declarations, interfaces,

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package/private/protected modifiers...

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All those classic modifiers
you get in object languages.

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There are no parametrized types,
no boxing/unboxing and so on.

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And you'll see
that it's a really powerful language.

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So, in Pharo,
everything is an object.

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Computation is done
via messages sent to objects.

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Methods are late bound.

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There's an algorithm,
the method lookup,

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which selects the right method
when an object receives a message.

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There's the notion of blocks,
which are anonymous methods.

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Very important in Pharo.

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And we can send particular messages
to classes

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to create objects.