WEBVTT

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Hello. This week,
we'll get to the heart of the matter,

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about syntax especially.

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This class will be mostly about dealing
with the messages of Pharo's syntax,

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which is most of Pharo.

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In Pharo, there are three main things:

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objects, messages,
and closures called blocks.

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There'll be a course about blocks.

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You can cover almost
the entire syntax of Pharo with this.

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Pharo's syntax stems
from that of Smalltalk

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which was designed
to teach children how to program.

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It's about writing programs
which are like short sentences

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a child might write.

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It's also about minimizing
the number of parentheses.

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We'll see how it works.

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For instance,
let's take a tile on openstreetmap.

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I want to open it in a graphic window.

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This is an example of the syntax.

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I use parentheses
because I want to get a png file first.

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I convert the png file
into a Pharo graphic object

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before opening it in a window: do it
with Pharo and you'll get the result.

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It'll probably be water
as there are many water tiles.

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In Pharo's syntax,
there are three types of messages

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to minimize the number of parentheses.

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There are unary, binary,
and keyword messages.

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Let's see about them.

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With unary messages, you have
one receiver then the method's name.

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With binary messages, one receiver,
the selector, then an argument.

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With keyword messages, one receiver,
then one argument or arguments.

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You can distinguish them
thanks to the colon here.

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We'll study this.

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What is the use
of these three types of messages?

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You can differentiate them.

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It changes the way
programs are executed.

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You always execute
what's between parentheses first;

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then unary messages;
then binary; then keywords.

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When at the same level,
you use the same order,

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from left to right.

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Let's see about unary messages:

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if I ask small integer 1 what
its class is, it'll return SmallInteger.

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This will be your homework
for next week:

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if I send the message not to the Boolean
false, I get the Boolean true.

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If I query the class Date
for today's date,

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it'll return 24 May 2009, for instance,
which is a Date Object.

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If I send the message pi
to the class Float,

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it'll return a representation
of the object pi.

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I don't know whether you noticed,

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but I sent messages to objects

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as well as messages to classes
in the exact same way.

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I didn't say: "It's a class.
Careful, you have to do something."

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I send the message class to the small
integer, it returns the class.

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I send the message today
to the class Date, it returns a date.

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Nothing fancy.

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In Pharo, classes are also objects.

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Messages sent to classes are processed
like messages to instances.

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Which makes complete sense.

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I have an example for you.

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This exercise isn't related to syntax.
It's just to have fun.

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I thought it would be fun to determine
what are all the methods

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implemented in the class Point.
It's a unary message. I can say:

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"class Point, return all your methods
and the message names you understand."

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It'll return a long list
with all the messages.

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See how easily you can build tools?

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If I put this in a list,

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I'll get a list
with all the messages the class returns.

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That's how the browser you'll use works.

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Let's refine this:
I'd like to query the system

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for all the unary selectors
of the class Point.

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I write it like this:
"Class Point, list all your selectors."

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It'll return an array, a collection.

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I write that I want to select...

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This is a keyword message.

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I want to select among the selectors
those which are unary.

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There's a query called #isUnary.

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It's a message about method names
which selects such things.

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This is a small footnote in the syntax.

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You'll learn about it in the course
about iterators, select: being one.

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It's very elegant:
I can query the system in one line.

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Back to the subject at hand.

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There are binary messages:
they are composed of anObject,

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aBinarySelector, anArgument.

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Binary selectors are a combination
of these incomprehensible things:

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%, |...

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What can't be understood is binary.
What looks like math is binary.

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For instance, I send the message +
to the object 1

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with the object 2 as argument.
It returns 3, which is great.

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> is a binary message.

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@ is also a binary message
(it creates points).

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String concatenation with a comma
is a binary message.

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I write: 'Black chocolate','is good'

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I get the string 'Black chocolate
is good'. Which is true.

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About keyword messages...

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People often find them frustrating

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when they're used
to a Java-like or C-like syntax,

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with a period, parentheses,
and a comma in the middle.

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But we do it like this: in the middle.

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What you have to see in this example
is that the message is:

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kayword1keyword2.

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The message is full.
It's composed of two parts.

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It's time to test yourself.
I'll give you a couple seconds.

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What are these messages?
What types do they belong to?

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The first has no argument:
it must be unary.

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The second too.

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There's a colon: it must be a keyword.

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There's something with a comma
I don't understand: it's binary.

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It's weird: it's binary.
There's no argument: it's unary.

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There's a colon: it's a keyword.

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It might seem difficult, but...

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This is the solution.

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For example,

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let's imagine I want to change
the x value of a point.

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This is a point. It's a binary message.

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I write setX: with an argument.

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It returns a point
in which 2 has been changed.

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It doesn't look so hard, does it?

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Now with the message at:put:.
I have an array.

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It contains strings:
'Calvin' 'hates' 'Suzie'.

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I want to change the second element.

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I write at: 2 put: 'loves'.

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This is 2. So 'loves' comes here.

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About arrays, you can see

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the method is called at:put:.

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This method changes the value
of a collection at a given index.

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As you can see, collections
start at 1 in Pharo, not at 0.

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I want the second element,
so I count: 1, 2.

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You don't have to subtract anything.

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I'll give another example
of keyword messages:

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when I ask whether 12,
which is an integer,

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is between 10 and 20,

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it's the message between: and:.

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In the syntax, it would be the same as:
12.between

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And(10,20).

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I think I've focused on this enough.

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There are three types of messages:

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unary, binary, and keywords.

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That's all.