The Well-Grounded Nuby

The Well-Grounded
Nuby
Advice for Ruby newcomers*

David A. Black
Senior Developer
Cyrus Innovation, Inc.

* and their mentors!

Copyright © 2010, Ruby Power and Light, LLC

Friday, September 3, 2010

About me
• Senior Developer at Cyrus Innovation, Inc.
• Rubyist-at-large since 2000 (Pickaxe baby)
• Owner/director of Ruby Power and Light, LLC
• Author of Ruby for Rails (Manning, 2006) and The Well-
Grounded Rubyist (Manning, 2009)
• Director of Ruby Central, Inc. (RubyConf, RubyForge,
etc.)
• Twitter: @david_a_black, @compleatrubyist
• Ruby learning and history: http://www.ruby-versions.net



Goals for today
• Learn some major common points of
confusion, and how to navigate them
• Improve your ability to understand Ruby
code
• Give yourself some tools for reasoning
through code as you write it



How we'll proceed
• Seven major points, with code demos
• chosen because they are common
stumbling blocks, even among
experienced Ruby programmers
• Examples of how the seven points can help
you understand Ruby
• Questions (throughout)


I.
Every expression
evaluates to an object



• Everything is not an object
• if statements aren't
• argument lists aren't
• keywords aren't
• But everything evaluates to an object
• including if statements, method deﬁnitions, class
deﬁnitions



irb is your friend
>> if 3 > 5
A failed if statement
>> "Not reached!"
returns nil >> end
=> nil

>> puts "Hello!"
puts returns nil Hello!
=> nil

Empty class deﬁnition >> class C
evaluates to nil >> end
=> nil

Class deﬁnition >> class C
>> "Some value in a class definition"
evaluates to the
>> end
last expression inside => "Some value in a class definition"
it



II.
It's all about sending
messages to objects



Messages everywhere...
• Post-dot messages are... messages
• as in "abc".upcase
• Inﬁx and unary operators are (mostly)
messages
• Square brackets are a message
• case statements send messages



Operators are (mostly)
messages (methods)
This... ...is really this!

3+2 3.+(2)
5*3 5.*(3)
a[2] a.[](2)
(Except you can't actually call the
+x x.+@ +@ method this way.)



Roll your own + logic!
class Person
attr_accessor :name
class Couple
def initialize(one, two)
@pair = [one, two]
end

def to_s
@pair[0].name + " and " + @pair[1].name
end
end

def +(other) Adding two people is deﬁned
Couple.new(self, other)
end
as returning a Person::Couple
end object with those two people.

john = Person.new
john.name = "John"
marcia = Person.new
marcia.name = "Marcia"

couple = john + marcia
puts couple

John and Marcia



Even case statements send
messages to objects!
This... ...is really this... ...which is really this!...

case some_string if "abc" === some_string if "abc".===(some_string)
when "abc" # ... # ...
# ... elsif /def/ === some_string elsif /def/.===(some_string)
when /def/ # ... # ...
# ... end end
end

Case statements use the === method,
aka the case equality operator, to
determine a match



Roll your own ===!
Strictly speaking, you
class Person
attr_accessor :name, :ssn could just deﬁne ==
def ===(other) here, since === is the
self.ssn == other.ssn same as == by default.
end
end But deﬁning === itself
means that you can
david = Person.new
separate "plain" equality
david.ssn = "123-45-6789"
from case equality if you
joe = Person.new wish to.
joe.ssn = "987-65-4321"

mystery = david

case mystery
when joe # i.e., if joe === mystery
puts "It's Joe!"
when david # i.e., if david === mystery
puts "It's David!"
end

It's David!



III.
Objects resolve
messages into methods



The intelligence of
objects
• Objects do not "have" methods
• they have the intelligence to ﬁnd methods when
required to do so
• Methods are stored in classes and modules
• Every object has a lookup-path consisting of classes
and modules
• singleton class
• modules mixed into singleton class
• class
• modules mixed into class
• superclass, etc., all the way up



class Person
attr_accessor :name
end

david = Person.new
david.name = "David"



class Person
attr_accessor :name
end

david = Person.new

module Vocal
def talk
"Hi, my name is #{name}."
end
end

class Person
include Vocal
end

puts david.talk Hi, my name is David.



class Person
attr_accessor :name
end

david = Person.new

module Vocal
def talk
end
end

class Person
include Vocal
end

puts david.talk

module Loud
def talk
super.upcase.chomp('.') + "!!!"
end
end

class Person
include Loud
end

puts david.talk HI, MY NAME IS DAVID!!!



class Person
attr_accessor :name
end

david = Person.new

module Vocal
def talk
end
end

class Person
include Vocal
end

puts david.talk

module Loud
def talk
super.upcase.chomp('.') + "!!!"
end
end

class Person
include Loud
end

puts david.talk

module Quiet
def talk
"(((" + super.delete('!').downcase + ".)))"
end
end

david.extend(Quiet) (((hi, my name is david.)))
puts david.talk



IV.
Classes and modules
are objects



"The answer to 75% of all questions about Ruby is: Because classes are objects!"

-- Me



Classes and modules as
objects
• You can send them messages

• They can be put in arrays, etc.

• They can be assigned to local variables

• They have state (instance variables), just like other objects

• They have some special behaviors (nesting, instantiation,
inclusion in lookup path, etc.)

• but in many respects, they are just objects!



Seen in the wild...

def do_something(class_string)
THINGS.each do |thing|
str = "#{Object.const_get(class_string)}"
str << ".#{thing}_method"
eval str
end
end

do_something(an_object.class.name)



Don't work so hard!

def do_something(klass)
THINGS.each do |thing|
klass.send("#{thing}_method")
end
end

do_something(an_object.class)



Classes are objects
class Person
end
You can put a class in a
my_class = Person
local variable!
david = my_class.new

You can't use it with the # class my_class # ERROR!
class keyword. However...
my_class.class_eval do
def speak
puts "Hi!"
end
end

david.speak # Hi!



V.
There's always a self



self-centeredness
• self is the default receiver of messages
• self owns instance variables
• and, by definition, any instance variable you see
belongs to self
• The value of self changes:
• in class definition (becomes the class)
• in method definition (to object that will call
method)
• in instance_eval (becomes the receiver)
• in class_eval (becomes the class/module)



self
class Person
puts "self in the class definition body: #{self}"

def speak
puts "Hi! self in the instance method: #{self}"
end
end

puts "self at the top level: #{self}"

david = Person.new
david.speak

self in the class definition body: Person
self at the top level: main
Hi! self in the instance method: #<Person:0x1281bc>



self
str = "I am a string!"
puts "self is: #{self}"
str.instance_eval do
puts "self is now: #{self}"
puts upcase
end

self is: main
self is now: I am a string!
I AM A STRING!



self
my_class = Class.new
my_class.class_eval do
puts "self in class_eval is: #{self}"
end

self in class_eval is: #<Class:0x12857c>



VI.
Variables contain
references to objects



str = "Hi"
str2 = str
str2 << " there!"
puts str

Hi there!



array = [1,2,3]
array2 = array
array2[3] = 4
p array

[1, 2, 3, 4]



one = "one"
array = [one, "Two", "Three"]
one.capitalize!
puts one
p array

One
["One", "Two", "Three"]



str = "Hi"
str2 = str
+= creates a new str2 += " there"
string object! puts str

Hi



def add_bang(string)
string << "!"
end

str = "Hi"
add_bang(str)
puts str

Hi!



VII.
true and false are objects;
true and false are states



Truth and falsehood
• Every object has a boolean value
• The objects false and nil have a boolean value of
false.
• Every other object has a boolean value of true.
• true is true
• 0 is true

• "" is true
• [] is true



The truth test
For any object obj, think of:

if obj
puts "obj has a boolean value of true"
end

And remember... every expression evaluates to an object, including class deﬁnitions!

if class A
end
puts "True!"
end

# No output(because the class definition evaluates to nil)

if class A
0
end
puts "True!"
end

# Output: True!(because the class definition evaluates to 0,
# which has a boolean value of true)



Summary
I. Every expression evaluates to an object
II. It's all about sending messages to objects
III. Objects resolve messages into methods
IV. Classes and modules are objects
V. There's always a self
VI. Variables contain references to objects
VII. true and false are objects; true and false are states



Questions so far?



What do these evaluate to,
what do they output,
and why?



10 if false

10 if true

puts 10 if true

if 10 > 3
puts "True!"
else
puts "Not true!"
end

if 10 > 3
"True!"
end



class Talker
@x = 1
def talk
puts "x is #{@x}"
end
end

t = Talker.new
t.talk



class Talker
def talk
@x = 1
puts "x is #{@x}"
end
end

t = Talker.new
t.talk



class C
def talk
puts "Hi!"
end
end



class C
"Hi!"
end



class C
self
end



obj = Object.new
class << obj
self
end



class Person
attr_accessor :name
end

d = Person.new
d.name = "David"
puts d.name
d.instance_eval { puts name }



class Person
attr_accessor :name
end

d = Person.new
d.name = "David"
puts d.name
d.instance_eval { puts @name }



class Person
attr_accessor :name
end

d = Person.new
d.name = "David"
@name = "Matz"
puts d.name
d.instance_eval { puts @name }



That's all!

Enjoy your Ruby literacy!

David A. Black
Senior Developer
Cyrus Innovation, Inc.

dblack@rubypal.com



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