WEBVTT

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Hi everyone, in this video
we'll talk about the Anti-IF Campaign.

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In particular,
we'll explain the reasons why

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the Return Nil and Null Check
functions are not great.

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Here's an example of a code,
which is not an object.

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Its method parameter is an animal.

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Its actions vary
according to the animal.

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For example, we can ask a dog
to wag its tail or a duck to quack.

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For a cat, we request other actions.

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So, why is it problematic
to use If statements?

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Particularly when they're used
to check the type of receiver.

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For example, if we want to add
a new animal here,

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we would need to check
the entire project code

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to search for If statements
that may apply.

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We would have to modify
numerous codes throughout the project.

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Additionally, adding cases to methods
makes them cumbersome,

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and they become lost
in too many details.

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Adding animals
makes this method very long,

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and each animal type
can have many details.

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Even for a simple feature,
such as not all dogs having tails,

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we would have to create
separate cases for each option.

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The code would become
complicated and harder to understand.

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So, to replace these actions,
we send messages.

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This is a point we keep reiterating.

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The key point to remember
is the sending of messages.

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We can replace
the previous code with this one.

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It applies the showHappiness method
in each relevant class.

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Each class will decide how each animal
will show its "happiness."

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For each animal, all we need to do
is send the message

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showHappiness to the animal,

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and one of its methods will be executed.

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We see here that Pharo
is pursuing the If function.

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Pharo decides which method to apply
to a particular type of animal.

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This is executed automatically.

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There's no need for us
to specify Ifs for object types.

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It only makes codes
less coherent and dynamic.

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Now we'll discuss
the specific case of Nil.

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If a method returns nil,

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you'll oblige your clients
to use If statements.

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Whereas, using If is rarely recommended.

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Here we use an example of a code

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based on a parameter and an inferencer.

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The type of code is not important.

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Here we see that in some cases,
nil is returned.

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This means that when we use this code,

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we need to test
the message rulesForFact.

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Did rulesForFact return nil?

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We act differently
depending on the reply.

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We see that in this case,

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since we're using a plural term,

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the method will probably
return a collection.

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An effective solution for avoiding nil

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in this situation
is to return an empty collection.

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This works in many cases.

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Returning an empty collection
instead of nil simplifies the code.

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Because clients can simply
iterate the collection,

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and if it's empty, nothing will happen.

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For exceptional cases,

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such as when you have a file stream

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that has not been opened for writing
and shows an error,

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instead of returning nil, we inform
the system by raising an exception.

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In Pharo,
we call this filing an exception.

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We create an instance
of the Exception class or subclass

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and send the message, or signal.

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This avoids obliging
the client of the method nextPutAll

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to test if it is nil,
when a problem has likely occurred.

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Either the client handles the exception

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or it's handled
by the client of the client, and so on.

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We can focus on one specific level
to capture the exception.

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It avoids overuse of Ifs.

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Another case where we find checks
for the nil value

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is in instance variables
that are not initialized.

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If a code says that if the variable
is still nil, it must react a certain way,

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it's better to initialize the variable
straightaway,

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with a value that works for all cases.

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So, here,

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for "members,"
which contains a collection,

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we initialize an empty collection
instead of using nil.

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Once again, this often works well.

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If you want to give a value
to a variable,

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and if it's costly
to calculate its value,

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you can wait until the last moment
to calculate it.

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It may never be calculated,
so it saves execution time.

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In such cases,
we use lazy initialization.

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This is used when a value is required.

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If the value is still nil,
we assign it a value.

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If it's no longer nil,
we return its value immediately.

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Here we have an If associated with nil,
but we have only one.

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All other users of the variable
utilize the descent method,

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and have not tested if it is nil.

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Sometimes we come across cases

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in which it's necessary to check
whether or not we need to respond.

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As we see in this example.

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Here we have a ToolPalette.

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If a tool is selected, we can respond,

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but if none is selected,
we prefer not to act.

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Look at the selectedTool function.

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If it returns nil, no tools are selected,
so no action is required.

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If selectedTool returns something,

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we will ask it to perform an action.

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A good way of replacing this

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is to use the NullObject pattern.

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Instead of having two cases,
one with tools and one without,

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we have one case
in which one of our tools does nothing.

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This tool will be selected by default.

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We create a tool that does nothing
when asked to perform actions.

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Instead of not selecting anything,
we enable a tool that does nothing.

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To find out more about NullObject,
see these references.

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In conclusion,

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messages are more effective than Ifs.

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You will utilize Ifs in certain cases,

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but you can often avoid using them
and send messages instead.

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Avoid returning nil because
it obliges you to insert If checks

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to find out whether or not
the value is nil.

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Initialize variables either on creation
or using lazy initialization.

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Create objects representing default
behavior or an absence of behavior.

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This applies not only to Pharo
but to all object languages.

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It's important to remember these points
whichever language you use.