WEBVTT

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Hello everyone.

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In this class, I'll illustrate Pharo

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by comparing it
to another OOP language, Java.

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In Pharo, you only manipulate objects.

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You do things
thanks to messages only

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and by assigning values to variables.

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The following slides will show a few
equivalences between Java and Pharo

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to understand their differences
and similitudes.

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Over there,

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Java is used to create a new collection

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which contains strings.

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This collection is affected
to a variable called strings.

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In Pharo, it's the same.

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The class which manages collections

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is called OrderedCollection,
not ArrayList,

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but the behavior is the same.

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You have to be aware of several things:

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first, in Pharo,
new is a message sent to a class.

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The message new is sent
to the class OrderedCollection

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to create a new instance
of the collection.

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In Java, new comes first.

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This syntactic collection is specific

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to object instantiation in Java,

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whereas in Pharo
it's standard message sending.

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There's no static typing in Pharo.

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Many things which are needed in Java
are not used in Pharo.

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You don't need to specify
that the collection contains strings.

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And strings isn't a collection.

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There's another equivalence when
it comes to creating a child process.

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We'd like to execute this bit of code

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in another process.

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In Java, you must create

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a new instance of the class Thread

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by giving it Runnable() as a parameter.

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You write new Runnable().

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Runnable() being an abstract class,
you have to define the missing method.

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This method is called run().

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It contains the code
you want to execute in a child process:

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this.doSomething().

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The method doSomething
belongs to the same class

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as that where the code is located.

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In Pharo, the syntax is much simpler.

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What is equivalent to all that part
is this part.

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In the end,

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this start helps to launch
the child process.

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You can do this thanks to fork,

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a message which gets sent to the block.

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Since Java 8,
the latest version of Java,

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the syntax of Java has improved much.

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As you can see,
code in Java is much clearer.

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The equivalent of this block

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is the lexical closure located here.

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The rest is the same
as in the previous slide.

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In Pharo,
there are three types of messages:

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unary messages,

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which are messages without arguments
applied to a receiver,

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such as the method factorial
we already studied

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or as the method cr

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which displays an empty line
in the console.

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Binary messages
are applied to a receiver

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and take a value as an argument.

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For instance, operators

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such as + which adds three and four.

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-> is used to create a pair
with a key and a value.

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5->10 creates the pair 5 10.

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The third type of messages
is keyword-based.

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They can take one, two, three,
or more parameters.

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They are separated by a colon.

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The keyword message here is show.
It gets sent to the object Transcript

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and takes the string in the console
as a parameter.

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In the second example,
the message is between: and:.

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It's one message
separated in two keywords.

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It takes two arguments
and is applied to one receiver.

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The message between: and:
is applied to the object 2

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and takes 0 and 5 as arguments.

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To compare with Java,

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especially when it comes
to keyword-based messages,

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as they are very different
compared to Java:

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In Java, you have one receiver,
one period,

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the name of the message
sent to the object,

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and the arguments between parentheses
separated by commas.

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In Pharo, you express the same thing

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by separating the keywords
from the name of the method

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and by putting the arguments
in the middle.

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For instance:

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I send the message send to a postman

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in order to send mail to a recipient.

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This is the Java syntax.

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I'll decompose it gradually
to end up with the Pharo syntax.

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First step:

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extract the parts that belong
to the Java syntax:

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the period, the parentheses,
the comma, and the semicolon.

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Let's remove them.

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You end up with:
postman send mail recipient.

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If you turn this into a sentence
to make reading easier,

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you get: postman send mail to recipient.
It's easier to read.

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To get the right Pharo syntax,
you add colons.

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There's one message, the message
send: to: applied to the postman.

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It takes two arguments,
the mail and the recipient.

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Conditionals
such as if and else,

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and loops such as do, for, etc.

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are keywords in Java.
In Pharo, they aren't.

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They aren't specific words defined
by the language and the compiler.

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They're just methods and messages
sent to some types of objects.

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In Pharo, for instance, to execute if,

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you send the message ifTrue: if False:.

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It's a message you send to one Boolean.

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It takes two blocks as parameters:
one is executed if the Boolean is true,

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one if the Boolean is false.

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Likewise, loops are just methods
sent to some types of objects.

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In the first example,

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the object 4 is a number.

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I can send the message timesRepeat:
with a block as a parameter.

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The underlying method executes the block

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as many times
as is indicated by the receiver.

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4timesRepeat:
executes something four times.

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Likewise, the message to: do:
is sent to a number.

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It takes a number and a block
as parameters.

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The value i
which is the parameter of the block

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takes every value between 0 and 100.

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Likewise,

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the message to: by: do:

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selects the values comprised
between 0 and 100

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which are multiples of 3:
0, 3, 6, 9, etc.

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Finally, the message do:
sent to a collection

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is the same as a foreach loop in Java.

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It executes the block
for each element of the collection.

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The variable :each

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first takes the first value
of the collection,

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its first element.

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The block is executed using
this value of :each.

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Then :each takes
the second element of the collection.

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The block is executed using this value
until the end of the collection.

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To sum up,
there are three types of messages:

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unary messages, binary messages,
and keywords.

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The order of precedence:

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first execute
what's between parentheses,

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then unary messages,
then binary messages, then keywords.

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Conditionals and loops

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are messages sent
to some types of objects,

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unlike as in other languages
such as Java,

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where specific keywords of the language

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have specific instructions
regarding the compiler.

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In Pharo, they're messages
sent to specific objects.