WEBVTT
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Hello. During this course, I will mainly
talk about object-oriented programming,
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and we will learn about
the essence of dispatch
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and late binding
in objects-oriented languages.
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So this course uses Pharo as an example.
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It's good that Pharo has been
well implemented.
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In the next class, we'll talk
about its big potential generalization.
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Let's start.
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Let's look at Booleans.
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Booleans in Pharo
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are the most basic stuff indeed.
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There are eager Boolean operators
like &, |, not,
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lazy operators like or:, and:,
with argument evaluation when necessary;
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there are also conditionals
which we will see in another course.
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So it's well implemented but there's
nothing amazing or specific.
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Last week, I asked you
to do some exercises.
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There were three questions.
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How to implement Not,
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how to implement Or,
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and the most important:
what is the goal of these exercises?
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I will answer the first two questions;
then in the next course,
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I will answer the last question.
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The exercise was really...
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If you send the message not to false,
it will return true.
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If not to true, it will return false.
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You have objects. How to implement this?
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I'll first give you some hints.
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The solution does not use conditionals.
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Mostly, you find
a solution with a condition.
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But I'm telling you that my solution,
implemented by Pharo,
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does not contain any conditions.
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Nor conditionals.
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So you don't have if in the solution.
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Think about it for a bit
to see if you have an idea.
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Normally, this kind of hint
doesn't really work.
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The second hint:
the solution uses three classes.
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It has the class Boolean
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which is abstract.
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It has the class True
and the class False.
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The Boolean object true
is the singleton instance of True.
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Do you see the difference?
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The instance true
starts with a lower case
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and the class False
starts with a capital F.
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And false is the singleton instance
of False.
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If we visualize it in a diagram,
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we can see true is instance of True,
and false of False.
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In theory, with this hint,
the solution should be obvious to you.
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I am not sure you can see it.
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I will let you think more about it.
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The solution is in the end...
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I won't tell you right now...
Let's think about it.
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How do you express a choice
in object-oriented languages?
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A choice is expressed
by defining classes
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with compatible interfaces,
i.e. methods,
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and by sending a message to an instance.
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That's my example.
When I type x open,
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I choose the right method
associated with x.
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It means if it's a file, it will open
a file; if a window, a window;
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if a tool, a tool.
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So the method will be selected
based on x's class.
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Then how can we have the solution
with this hint?
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We will implement it like this.
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It means I implement
the method not in the class False;
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in this case, it will return true.
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I implement the method not
in the class True; it will return false.
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In a diagram, it looks like this.
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You can see this solution
doesn't have any explicit conditions.
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I don't use any if in this case.
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How does it work?
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It works like this.
When I send the message not,
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where do I search for the method not?
In the receiver class.
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true is instance of True.
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So this method here will be executed,
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and the result will be false.
It works.
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Now I send a message
to the instance false.
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Where do I look? In the class False.
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This not is executed,
and it returns true.
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I have implemented my Booleans,
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the Boolean negation with two methods
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without the use of conditionals.
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We can also look at the implementation
of the superclass.
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The Boolean class is abstract.
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It has two subclasses
which implement the necessary operators.
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In Pharo,
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you express not
as an abstract method on Boolean
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by using self subclassResponsibility.
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I just wanted to tell you about Pharo
to be comprehensive.
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By now, you must have understood
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how to express the behavior of Or.
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I'll give you time to express this.
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The idea here is to define a method
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which will use an extra argument.
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Depending on its value,
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it'll be followed by the right result.
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You often think
using a condition is enough:
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No. That's the thing.
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Once again, you don't need
conditionals to implement Or.
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I will give you 10 seconds
to think about it.
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You were supposed to have prepared this.
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I will define Or
in the abstract Boolean class
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as an abstract method.
Very good.
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Then in the class False:
it's written here.
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The receiver belongs to False.
What do I return?
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When it's true, it returns true: when
false, false; when anything, anything.
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So it means I return the argument.
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So here, the implementation of Or
in the class False
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is about returning the argument.
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That's exactly what we did.
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Likewise,
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if we look at the class True,
it's explained.
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It's explained here.
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When I send or to the receiver true,
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I return the receiver.
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So I return true.
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As you can see here again,
I don't use conditionals,
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I just used message sends.
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So in a cleaner way,
how is it written in Pharo?
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We know that true
is the receiver of the message;
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so instead of writing true,
we can write self.
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If you read the definition,
you can see self and say:
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"It makes sense.
Since the receiver is true,
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"it's exactly the same."
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Once again, let's visualize it.
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When I send the message or
with something
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to the object true,
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I look for this definition of or here,
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and it will return self, hence true.
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When I send the message or
with something,
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I look into the class False
which is false.
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So I end up with this implementation,
and I return alpha.
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This is the Boolean table
I was trying to implement.
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What you have to remember is:
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the solution we have implemented
does not use any conditionals
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or any conditional instructions
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such as an explicit loop;
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it lets the receiver decide.
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It means that I tell the Boolean object
which receives the message
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to find the right solution.
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I am not here to dictate
what has to be decided.
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This principle is found elsewhere too.
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It's a fundamental principle of OOP.
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It's the heuristics I mentioned
in the beginning of this lesson:
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don't ask, tell.
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It means I don't want to express
any conditionals;
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I just want to give an order.
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That's one of the important keys of OOP.
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The other is about
letting the receiver decide.
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I give the receiver an order:
it must encapsulate its knowledge
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and make the right decisions.