WEBVTT

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In this session, I'll introduce blocks.

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They're sort of anonymous methods
also called lexical closures.

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They're everywhere in Pharo.

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They're the base of loops,
conditionals, iterators,

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and even GUIs or dedicated languages.

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They're at the heart of the system.

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They've only been introduced
in the latest version of Java.

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To define a block, use square brackets.

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This an example:

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[expressions...]

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We'll talk about block definition.

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Another example:

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this expression, 1/0,

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returns an error when executed.

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Let's encapsulate it in a block

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by defining a block that contains
the expression 1/0.

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If I evaluate this block's definition,
nothing happens: no error.

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It returns the block

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as a block definition
doesn't execute its body.

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You can define as many blocks
as you wish:

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the expressions in its body are frozen.

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To evaluate a block
and the expressions in its body,

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you must do it explicitly
through the message value.

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For instance, this is a block:
[2 + 6]

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You send it the message value.

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The expressions in the body
are evaluated.

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It returns the result, 8.

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However, if the block
contains erroneous expressions,

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the error is returned
when the block is evaluated,

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as in the example 2.

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[1/0]: I send the message value
to the block.

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It returns an error.

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Blocks can also take arguments

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as in this argument.
It's just like methods.

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A block is always opened by [

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and closed by ].

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All the arguments are prefixed
by a colon.

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Then there's |, a pipe.

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It separates the argument declaration
from the block's body,

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the expressions it contains.

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The argument in this example is x.

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x + 2 is the only expression
in the block.

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If I send the message value
to the block,

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you have to use a specific message,
value: with a parameter.

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At the same time, you pass the value 5
which replaces the argument x

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when the expressions of the block
are evaluated.

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If I send the message value: 5
to this block,

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it returns 7

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since the value of x is 5
when the block is evaluated.

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This is like my previous example,

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but with several expressions
in the block.

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I have two here: x+33 and x+2.

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When I send value: 5

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to evaluate the expressions
in the block,

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the evaluation returns

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only the value of the last expression
in the block.

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It only returns the result of x+2,

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7.

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Blocks are normal objects in Pharo.

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They can be stored
in temporary variables.

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They can be sent messages
like normal objects.

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This is the example I have for you here.

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You can store the definition
of this block, x+2,

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in a variable called add2.

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Then you send messages
to evaluate the block several times.

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I send the message value: once

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by telling it to evaluate itself
with the value 5: it returns 7.

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Then the message value: 33.

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It returns 35.

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You can also define blocks
with several arguments.

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For instance, x and y.
This block has two arguments.

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But how would you evaluate this block

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since you need to pass two values
to trigger its evaluation,

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5 and 7 which would replace x and y?

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The answer is that you use
the message value: value:,

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a method of the class block.

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5 replaces x and 7 replaces y.

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It returns 12.

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Like methods, blocks
can also define temporary variables.

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This example is an actual example:

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it comes from the class Collection.

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The block starts here.
It ends there.

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It takes one argument called :index.

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It takes one temporary variable
defined here between two |.

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It's called args.

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This temporary variable args

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exists only during the evaluation
of the expressions in the block.

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Blocks are defined in methods.

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In a block, you can also use a return.

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This is an example
from the class Integer.

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The method is called factorial.

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There are several blocks in the method.

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These blocks contain returns, or ^.

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The return helps
to exit the method factorial.

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For instance, if I send the message
factorial to the integer 0,

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it returns the answer 1.

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I get this answer thanks to this ^,
the first in the block,

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which helps
to exit the method factorial.

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A return in a block helps
to exit the method

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which defines the block.

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A piece of advice
for when you use blocks:

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blocks are extremely powerful elements.

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You must use them carefully.

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Don't go beyond two to three arguments
for one block.

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If you have more,
it becomes incomprehensible.

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It's better to define a class
with instance variables and methods:

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it'll be much more legible.

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In the course, we studied blocks,
their syntax,

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the fact they were some sort
of anonymous methods,

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that is, lexical closures;

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they can be stored in variables
as they're like any other object.

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In other lessons, we'll see that blocks
are the basis of many constructions

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such as loops, iterations, etc.