WEBVTT

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Hello.

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Today we will review
the implementation of ifTrue:ifFalse:

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If you've been attentive
and I'm sure you have,

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you surely noticed
that when I suggested examples for Not,

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I said you could implement
Not, Or and ifTrue:ifFalse:

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But in the solution,
I never mentioned ifTrue:ifFalse:

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We'll solve this problem now.

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In effect, in Pharo,
conditions are messages.

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Here's an example.

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We have a weather class:
"Weather isRaining".

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This means that if it rains...

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If it's true, I'll take my umbrella.

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If it's false, I'll take my sunglasses.

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With ifTrue:ifFalse:

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You see the two colons?
This means they're keyword messages.

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The message ifTrue:ifFalse:
is a keyword message

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and it's sent to a boolean instance.

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In reality, it is heavily optimized
by the compiler and not really sent,

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but conceptually speaking it is.

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So if you want to redefine
the method ifTrue:

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ifFalse:

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You can do this in Pharo.

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I'll explain the technique now.

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The ifTrue:ifFalse:
method is so important

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and we need it to go so fast,

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that the function
is not actually invoked.

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The compiler makes jumps inline instead.

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But for the purpose of this class,
we'll see how it functions.

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I want you
to propose an implementation.

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You know about blocks.

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You know what we did with
True and False,

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and with Or and Not.

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You can do it with ifTrue:ifFalse:

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In sum, if my receiver is false,
I return 5.

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And if my receiver is true,
I return 3.

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Once again, we have objects,
messages and blocks.

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It's a bit repetitive,
but that's the way it is.

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You remember that brackets...

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freeze the execution

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of the expression they contain,

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and that "value"
kicks the execution of a frozen code.

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With this, and Not and Or,

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you should be able
to implement your conditions.

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Likewise, you should
be able to implement ifTrue:ifFalse:

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I'll give you a few seconds to think.

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How is it implemented?

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In exactly the same manner
as Or and Not.

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In the True class,
I have a method called

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ifTrue: ifFalse: and two arguments.

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One block for True
and one block for False.

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What do I do?

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When I'm in the True block,
I execute True.

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And when I'm in the False class,

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I have the same method
with the same two arguments,

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but since I'm in the False class,
I execute the False block.

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I have the booleans' implementation.

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How does it work?

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Like this.

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When I send a message to my boolean,

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which is in fact True,

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I send ifTrue: do something True
or do something False.

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Then I look up ifTrue:ifFalse:
in the True class,

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It's right here.

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It says to execute the True block,
so I do.

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Now I send the message
"B is a boolean: False."

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I send the message and look here.
I find ifTrue:ifFalse:

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It says to execute the False block,
and that's what I do.

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You cannot test this by putting
a breakpoint in ifTrue:ifFalse:

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It's optimized
and the system won't accept it.

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But what I do suggest you try,
is implement in the same manner:

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ifFalse: or ifTrue:

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ifTrue: a Block 1,

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and ifFalse: A Block 2.
Then implement them.

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Use a breakpoint and experiment.

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So what have we shown
with this example?

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We saw that sending a message
selects the right method.

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Here we decided
to let the receiver decide its behavior.

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And we saw that brackets
freeze computation,

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and that "value"
forces the execution of a frozen code.