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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>Constants & Scope</title>
<meta content="" name="description" />
<meta content="" name="author" />
<link href="css/reveal.css" rel="stylesheet" />
<link href="css/simple.css" rel="stylesheet" />
<link href="css/googlecode.css" rel="stylesheet" />
</head>
<body>
<div class="reveal">
<div class="slides"><section class="center" data-background="#000">
<h1 class="white">
Constants & Scope
</h1>
</section>
<section class="center">
<img class="inline" src="img/openfest.png" />
</section>
<section class="center" data-background="#000">
<h2 class="white">
Questions
</h2>
</section>
<section class="center">
<p>
What does Object#is_a? do?
</p>
</section>
<section class="center">
<p>
What does Object#kind_of? do?
</p>
</section>
<section class="center">
<p>
What does Object#instance_of? do?
</p>
</section>
<section class="center">
<p>
What is a method alias?
</p>
</section>
<section class="center">
<p>
What is the difference between alias and alias_method?
</p>
</section>
<section class="center">
<p>
What does Object#send does?
</p>
</section>
<section class="center">
<p>
What does Object#public_send does?
</p>
</section>
<section class="center">
<p>
What does Kernel#raise do?
</p>
</section>
<section class="center">
<p>
What does Kernel#fail do?
</p>
</section>
<section class="center">
<pre><code class="ruby" contenteditable="true">begin
{}.fetch(42)
rescue IndexError
:index_error
rescue KeyError
:key_error
end </code></pre>
</section>
<section class="center">
<pre><code class="ruby" contenteditable="true">begin
[].fetch(42)
rescue StandardError
:standard_error
rescue IndexError
:index_error
end </code></pre>
</section>
<section class="center">
<pre><code class="ruby" contenteditable="true">begin
raise Exception
rescue
:what_kind_of_error?
end </code></pre>
</section>
<section class="center">
<pre><code class="ruby" contenteditable="true">begin
[].fetch(42)
rescue
:what_kind_of_error?
end </code></pre>
</section>
<section class="center">
<p>
What does Object#=== does?
</p>
</section>
<section class="center">
<p>
What do we use Kernel#try and Kernel#catch for?
</p>
</section>
<section class="center">
<pre><code class="ruby" contenteditable="true">class something
private def shh() "IT'S A SECRET!" end
end </code></pre>
</section>
<section class="center">
<img class="inline" src="img/troll.png" />
</section>
<section class="center" data-background="#000">
<h2 class="white">
Constants
</h2>
</section>
<section>
<h2>
Constants
</h2>
<p>
Constants start with a capital letter
</p>
<pre><code class="ruby" contenteditable="true">ANSWER = 42 # This is a constant!
Europe = Continent.new # So is this! </code></pre>
</section>
<section>
<h2>
Constants
</h2>
<p>
Constants are not strictly enforced
</p>
<pre><code class="ruby" contenteditable="true">>> Foo = 42
=> 42
>> Foo = 43
(irb):40: warning: already initialized constant Foo
(irb):39: warning: previous definition of Foo was here
=> 43 </code></pre>
</section>
<section>
<h2>
Constants
</h2>
<p>
Modules are mostly used as constants
</p>
<pre><code class="ruby" contenteditable="true">module Awesome
# The best module ever, cause you can query:
#
# object.is_a? Awesome
#
# Mind = blown, bro!
end </code></pre>
</section>
<section>
<h2>
Namespace
</h2>
<p>
Modules are used as namespaces
</p>
<pre><code class="ruby" contenteditable="true">module Foo
INSIDE = :foo
module Bar
INSIDE = :bar
module Quux
INSIDE = :quux
end
end
end </code></pre>
</section>
<section class="center">
<pre><code class="ruby" contenteditable="true">Foo::INISDE #=> :foo
Foo::Bar::INISDE #=> :bar
Foo::Bar::Quux::INSIDE #=> :quux </code></pre>
</section>
<section>
<h2>
Namespace
</h2>
<p>
Access constants with implicit prefix
</p>
<pre><code class="ruby" contenteditable="true">module Foo
INSIDE = :foo
module Bar
INSIDE = :bar
module Quux
INSIDE = :quux
end
end
end </code></pre>
</section>
<section>
<h2>
Namespace
</h2>
<p>
Access constants with explicit prefix
</p>
<pre><code class="ruby" contenteditable="true">module Foo
module Bar
module Quux
p INSIDE #=> :quux
p Bar::INSIDE #=> :bar
p Foo::INSIDE #=> :foo
end
end
end </code></pre>
</section>
<section>
<h2>
Namespace
</h2>
<p>
You can access top-level constants with ::
</p>
<pre><code class="ruby" contenteditable="true">module Foo
module Bar
module Quux
p ::Foo::Bar::Quux::INSIDE #=> :quux
p ::Foo::Bar::INSIDE #=> :bar
p ::Foo::INSIDE #=> :foo
end
end
end </code></pre>
</section>
<section>
<h2>
Namespace
</h2>
<p>
Constants form a tree
</p>
<pre><code class="ruby" contenteditable="true">module Foo
module Bar
module Quux
p Object::Foo::Bar::Quux::INSIDE #=> :quux
p Object::Foo::Bar::INSIDE #=> :bar
p Object::Foo::INSIDE #=> :foo
end
end
end </code></pre>
</section>
<section>
<h2>
Namespace
</h2>
<p>
The root of it is Object
</p>
<pre><code class="ruby" contenteditable="true">module Foo
module Bar
module Quux
p Object::Foo::Bar::Quux::INSIDE #=> :quux
p Object::Foo::Bar::INSIDE #=> :bar
p Object::Foo::INSIDE #=> :foo
end
end
end </code></pre>
</section>
<section>
<h2>
Namespace
</h2>
<p>
The root of it is Object
</p>
<pre><code class="ruby" contenteditable="true">module Foo
module Bar
module Quux
module Object
INSIDE = :object
end
p Object::INSIDE #=> :foo
p Object::Foo::Bar::Quux::INSIDE #=> NameError
p Object::Foo::Bar::INSIDE #=> NameError
p Object::Foo::INSIDE #=> NameError
end
end
end </code></pre>
</section>
<section>
<h2>
Namespace
</h2>
<p>
The root of it is ::Object
</p>
<pre><code class="ruby" contenteditable="true">module Foo
module Bar
module Quux
module Object
INSIDE = :object
end
p Object::INSIDE #=> :foo
p ::Object::Foo::Bar::Quux::INSIDE #=> :quux
p ::Object::Foo::Bar::INSIDE #=> :bar
p ::Object::Foo::Bar::INSIDE #=> NameError
end
end
end </code></pre>
</section>
<section>
<h2>
Namespace
</h2>
<p>
Only module constants can be nodes
</p>
<pre><code class="ruby" contenteditable="true">module Foo
module Bar
module Quux
module Object
INSIDE = :object
end
p Object::INSIDE #=> :foo
p ::Object::Foo::Bar::Quux::INSIDE #=> NameError
p ::Object::Foo::Bar::INSIDE #=> NameError
p ::Object::Foo::Bar::INSIDE #=> NameError
end
end
end </code></pre>
</section>
<section>
<h2>
Namespace
</h2>
<p>
Non modules can only be leafs
</p>
<pre><code class="ruby" contenteditable="true">A = 42
A::C # TypeError: 42 is not a class/module
Foo::Egg # NameError: uninitialized constant Foo::Egg </code></pre>
</section>
<section>
<h2>
Module.nesting
</h2>
<p>
The list of Modules nested at the point of call
</p>
<pre><code class="ruby" contenteditable="true">module Foo::Bar::Quux
p Module.nesting #=> [Foo::Bar::Quux]
end </code></pre>
</section>
<section>
<h2>
Module.nesting
</h2>
<p>
The list of Modules nested at the point of call
</p>
<pre><code class="ruby" contenteditable="true">module Foo::Bar::Quux
p Bar::INSIDE #=> NameError: uninitialized constant Foo::Bar::Quux::Bar
end </code></pre>
</section>
<section class="center" data-background="#000">
<p class="white">
Module.nesting is where Ruby looks to find a constant
</p>
</section>
<section class="center" data-background="#000">
<p class="white">
Think of it as something lexical
</p>
</section>
<section class="center" data-background="#000">
<p class="white">
If you don't lexically nest, you won't get parent constants
</p>
</section>
<section class="center">
<pre><code class="ruby" contenteditable="true">module Foo
module Bar
module Quux
p Module.nesting #=> [Foo::Bar::Quux, Foo::Bar, Foo]
end
end
end </code></pre>
</section>
<section class="center">
<pre><code class="ruby" contenteditable="true">module Foo::Bar::Quux
p Module.nesting #=> [Foo::Bar::Quux]
end </code></pre>
</section>
<section class="center" data-background="#000">
<h2 class="white">
Scope
</h2>
</section>
<section>
<h2>
Scope
</h2>
<ul>
<li>
Local Variables Scope
</li>
<li>
Global Variables Scope
</li>
<li>
Class Variables Scope
</li>
<li>
Instance Variables Scope
</li>
<li>
Constant Scope
</li>
</ul>
</section>
<section>
<h2>
Local Variables Scope
</h2>
<p>
Local to the code construct in which they are declared
</p>
<pre><code class="ruby" contenteditable="true">x = 42
def change_x
x = 43
end
change_x
x #=> 42 </code></pre>
</section>
<section>
<h2>
Local Variables Scope
</h2>
<p>
Local to the code construct in which they are declared
</p>
<pre><code class="ruby" contenteditable="true">x = 42
def change_x
x = 43
end
change_x
x #=> 42 </code></pre>
</section>
<section>
<h2>
Local Variables Scope
</h2>
<p>
Local to the code construct in which they are declared
</p>
<pre><code class="ruby" contenteditable="true">x = 42
class Something
puts x #=> NameError: undefined local variable or method `x' for Something:Class
end </code></pre>
</section>
<section>
<h2>
Local Variables Scope
</h2>
<p>
Local to the code construct in which they are declared
</p>
<pre><code class="ruby" contenteditable="true">x = 42
module Something
puts x #=> NameError: undefined local variable or method `x' for Something:Class
end </code></pre>
</section>
<section>
<h2>
Global Variables Scope
</h2>
<p>
Global variables in Ruby are accessible from anywhere in the Ruby program
</p>
<pre><code class="ruby" contenteditable="true">$stdout #=> #<IO:<STDOUT>>
class Printer
def print(*messages)
messages.each { |message| $stdout.write(message) }
end
end </code></pre>
</section>
<section>
<h2>
Global Variables Scope
</h2>
<p>
No matter where they were declared
</p>
<pre><code class="ruby" contenteditable="true">$stdout #=> #<IO:<STDOUT>>
class Printer
def print(*messages)
messages.each { |message| $stdout.write(message) }
end
end </code></pre>
</section>
<section>
<h2>
Class Variables Scope
</h2>
<p>
A class variable is a variable that is shared amongst all instances of a class
</p>
</section>
<section>
<h2>
Class Variables Scope
</h2>
<p>
Only one variable value exists for all objects instantiated from this class
</p>
</section>
<section>
<h2>
Class Variables Scope
</h2>
<p>
If class or module changes the value, it will change for all
</p>
<pre><code class="ruby" contenteditable="true">class Base
@@store = { base: 'Base was here!' }
end
class Der < Base
@@store[:der] = 'Der was here!'
end
class Base
p @@store # {:base=>"Base was here!", :der=>"Der was here!"}
end </code></pre>
</section>
<section class="center" data-background="#000">
<p class="white">
I don't like class variables
</p>
</section>
<section class="center" data-background="#000">
<p class="white">
They have different semantics than the instance variables
</p>
</section>
<section class="center">
<pre><code class="ruby" contenteditable="true">class Bro
p @@homies #=> NameError: uninitialized class variable @@homies in Bro
end </code></pre>
</section>
<section class="center">
<img class="inline" src="img/troll.png" />
</section>
<section>
<h2>
Instance Variables Scope
</h2>
<p>
Specific to an object
</p>
<pre><code class="ruby" contenteditable="true">class Bro
attr_reader :homies
def set_homies(*homies)
@homies = homies
end
end </code></pre>
</section>
<section>
<h2>
Instance Variables Scope
</h2>
<p>
Specific to an object
</p>
<pre><code class="ruby" contenteditable="true">genadi = Bro.new
genadi.set_homies 'Stefan', 'Dimitar', 'Radoslav'
genadi.homies #=> ["Stefan", "Dimitar", "Radoslav"]
stefan = Bro.new
stefan.set_homies 'Yves', 'Dimitar', 'Bozhidar'
stefan.homies #=> ["Yves", "Dimitar", "Bozhidar"] </code></pre>
</section>
<section>
<h2>
Constant Scope
</h2>
<p>
<a href="#/24"><img class="inline" src="img/troll.png" /></a>
</p>
</section>
<section>
<h2>
Binding
</h2>
<p>
Encapsulate the execution context at some particular place in the code and retain this context for future use
</p>
</section>
<section>
<h2>
Binding
</h2>
<p>
The variables, methods, value of self, and possibly an iterator block that can be accessed in this context are all retained
</p>
</section>
<section>
<h2>
Binding
</h2>
<p>
Created using Kernel#binding, and are made available to the callback of Kernel#set_trace_func
</p>
</section>
<section>
<h2>
Kernel#binding
</h2>
<p>
Created using Kernel#binding, and are made available to the callback of Kernel#set_trace_func
</p>
</section>
<section class="center">
<pre><code class="ruby" contenteditable="true">class Secret
def initialize(n)
@secret = n
end
def peek
binding
end
private
def shh
"IT'S A SECRET"
end
end </code></pre>
</section>
<section class="center">
<pre><code class="ruby" contenteditable="true">s1 = Secret.new(99)
b1 = s1.peek
s2 = Secret.new(-3)
b2 = s2.peek
eval("@secret", b1) #=> 99
eval("@secret", b2) #=> -3
eval("@secret") #=> nil </code></pre>
</section>
<section class="center">
<pre><code class="ruby" contenteditable="true">s1 = Secret.new(99)
b1 = s1.peek
s2 = Secret.new(-3)
b2 = s2.peek
eval("shh", b1) #=> "IT'S A SECRET"
eval("shh", b2) #=> "IT'S A SECRET"
eval("shh") #=> NameError: undefined local variable or method `shh' for main:Object` </code></pre>
</section>
</div>
</div>
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