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In this course, we'll study
Booleans and conditions in Pharo.

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There are two Booleans objects:
true and false.

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true is the only instance
of the class True.

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false is the only instance
of the class False.

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We'll send messages to these objects
to set Boolean operations

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or alternative structures
such as ifTrue:IfFalse:, etc.

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We'll study this.

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In the first example of this slide,

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you send the message &,
a binary operator,

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to the object false.

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You pass it a parameter,
which is all this.

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It's an expression between parentheses,
(1 error:'crazy').

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This expression triggers an error.

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You can see the evaluation of this
whole expression triggers an error.

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Why? Because the operand
has also been evaluated

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in this Boolean expression.

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The operand has been evaluated,
hence the mistake.

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If you want to get the lazy version
of the Boolean operators,

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you must use the method and:
of the class Boolean.

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This method takes a block
as a parameter.

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Blocks start with [ and end with ].

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They contain expressions,
but the definition of a block

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doesn't trigger
the execution of its expressions.

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For now, these expressions
are not evaluated.

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It is only if the first operand
is true or false

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that the block passed as a parameter
is evaluated.

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In the second example,
you have false and: a block.

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It returns false without evaluating
the block passed as a parameter.

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It is a typical lazy operator
but built using blocks.

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In Pharo, conditionals
(if, false, while, etc.)

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are always messages
sent to Booleans or blocks.

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Let's take another example:

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the message ifTrue:ifFalse:, the typical
alternative of any programming language.

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But in Pharo, it's actually a message:

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I can send the message ifTrue:ifFalse
to a Boolean object.

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This expression returns a Boolean.

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I pass two blocks as parameters
of this message:

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one block is evaluated
if the receiver Boolean is true,

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the other if the receiver is false.

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Careful: the message ifTrue:ifFalse:

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is heavily used by Pharo:
it's at the heart of the system.

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It's so important
that it's been heavily optimized

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to be efficient without slowing down
the entire system.

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Let's talk again
about Boolean implementation in Pharo:

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it's extremely elegant.

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the objects true and false are both
instances of the classes True and False

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which inherit the class Boolean.

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Each of these classes
define a set of methods

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to return the expected behavior.

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We'll give you more information
about this elegant implementation

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in a further lesson.

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Back to the message ifTrue:ifFalse:.

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There are different versions
of these messages.

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There's a version with only ifTrue:
with a block as a parameter

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to return only the first part
of the alternative.

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The message ifTrue:ifFalse
takes two blocks

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to return an alternative
when the Boolean is true or false.

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There are several examples here.

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This only sends the message ifTrue:

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with only one block to execute
if the expression is true.

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In this example,
there's one block to execute:

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this one if the receiver Boolean
is true,

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that one if it is false.

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There are two different messages.

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It's the same with ifFalse:

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when it comes to the classes
Boolean, True, and False.

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There are other types of messages
available to express conditions.

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For instance,
there's ifEmpty: ifNotempty:

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which I can send to a collection.

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With ifEmpty:, I pass a block.

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The block is evaluated only if the
receiver collection is or isn't empty.

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ifNotEmpty: is the opposite condition.

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If the collection isn't empty,
then you execute the block.

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But there's an extra problem:

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this block takes an argument
as a parameter

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which is the non-empty collection.

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The collection might be the result
of a complex computation.

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You can query once more
for the collection.

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If the collection isn't empty,
it is passed as the block's parameter.

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It can be used
in the expressions of the block.

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In this session,

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we saw Booleans
are normal objects in Pharo

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and instances of the classes True
and False which inherit Booleans.

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You can send them messages.

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These messages define
all the constructions of alternatives

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or of possible conditions in Pharo.

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In other languages, it would rather be
structures in the compiler.