• classes are open
• class is an object
• self
• defining methods
• Eigenclasses

Example

def test(x)
  x += 5
  yield(x)
end

puts test(5) {|x| x * x } # => 100

Example

def a_method(a, b)
  a + yield(a, b)
end

a_method(1,2) {|x,y| (x+y) * 3} #=>10

block_given?

def a_method
  return yield if block_given?
  'no block'
end

a_method # => "no block"
a_method { "here's a block!" } # => "here's a block!"

API DSL

use_api do
  conn.get("/forms")
  conn.get("/form-submissions")
end

API DSL

def use_api
  conn = connect_to_api
  yield
  disconect_from_api
end

use_api do
  conn.get("/forms")
  conn.get("/form-submissions")
end

def my_method
  x = "Goodbye"
  yield("cruel")
end

x = "Hello"
my_method {|y| "#{x}, #{y} world" } # => "Hello, cruel world"

v1 = 1
class MyClass
  v2 = 2
  local_variables # => [:v2]

  def my_method
    v3 = 3
    local_variables # => [:v3]
  end

  local_variables # => [:v2]
end

obj = MyClass.new
obj.my_method # => [:v3]
obj.my_method # => [:v3]
local_variables # => [:v1, :obj]

Scope gates

• Class definitions
• Module definitions
• Methods

v1 = 1
class MyClass
  v2 = 2
  local_variables # => [:v2]

  def my_method
    v3 = 3
    local_variables # => [:v3]
  end

  local_variables # => [:v2]
end

obj = MyClass.new
obj.my_method # => [:v3]
obj.my_method # => [:v3]
local_variables # => [:v1, :obj]

Flattering scope

my_var = "Success"

class MyClass
  my_var # I want access!
end

Flattering scope

$my_var = "Success"

class MyClass
  $my_var # I want access!
end

my_var = "Success"

MyClass = Class.new do
  # Now we can print my_var here...
  puts "#{my_var} in the class definition!"

  def my_method
    # ...but how can we print it here?
  end
end

my_var = "Success"

MyClass = Class.new do
  # Now we can print my_var here...
  puts "#{my_var} in the class definition!"

  define :my_method do
    puts "#{my_var} in the method!"
  end
end

MyClass.new.my_method
Success in the class definition!
Success in the method!

def define_methods
  shared = 0

  Kernel.send :define_method, :counter do
    shared
  end

  Kernel.send :define_method, :inc do |x|
    shared += x
  end
end

define_methods
counter       # => 0
inc(4)
counter       # => 4

Why not @@ ?!

class MyClass
  @@v = "I am great"
  def self.show
    puts @@v
  end
end

class MyClass2 < MyClass
  @@v = "I am awasome"
end

puts MyClass.show

class MyClass
  @@v = "I am great"
  def self.show
    puts @@v
  end
end

class MyClass2 < MyClass
  @@v = "I am awasome"
end

puts MyClass.show # => I am awesome

@@v = 1
class MyClass
  @@v = 2
end
puts @@v #=>2

def my_method
  x = "Goodbye"
  yield("cruel")
end

x = "Hello"
my_method {|y| "#{x}, #{y} world" } # => "Hello, cruel world"

nc = Proc.new {|x| x + 1 }
# more code...
inc.call(2) # => 3

dec = lambda {|x| x - 1 }
# more code...
dec.call(2) # => 1

p = Proc.new {|a, b| [a, b]}
p.call(1, 2, 3)   # => [1, 2]
p.call(1)         # => [1, nil]

def math(a, b)
  yield(a, b)
end

def teach_math(a, b, &operation)
  puts operation.call(a, b)
  puts "Let's do the math:"
  puts math(a, b, &operation)
end

teach_math(2, 3) {|x, y| x * y}
# => 6
# => Let's do the math: 6

HighLine

name = hl.ask("Name?", lambda {|s| s.capitalize })
puts "Hello, #{name}"
⇒ Name?
⇐ bill
⇒ Hello, Bill!

zip = ask("Zip?  ") { |q| q.validate = /\A\d{5}(?:-?\d{4})?\Z/ }

HighLine

def ask( question, answer_type = String, &details )

class Person
  include CheckedAttributes
  attr_checked :age { |v| v >= 18 }
end

me = Person.new
me.age = 39
me.age = 12
# OK
# Exception!

eval

array = [10, 20]
element = 30
eval("array << element") # => [10, 20, 30]

kernel method

def add_checked_attribute(clazz, attribute)
  eval "
    class #{clazz}
    def #{attribute}=(value)
      raise 'Invalid attribute' unless value
      @#{attribute} = value
    end
    def #{attribute}()
      @#{attribute}
    end
  end
"
end

add_checked_attribute(Person, :age)

class Person
  def my_attr=(value)
    raise 'Invalid attribute' unless value
    @my_attr = value
  end
  def my_attr()
    @my_attr
  end
end

kernel method

def add_checked_attribute(clazz, attribute)
  eval "
    class #{clazz}
    def #{attribute}=(value)
      raise 'Invalid attribute' unless value
      @#{attribute} = value
    end
    def #{attribute}()
      @#{attribute}
    end
  end
"
end

def add_checked_attribute(clazz, attribute)
  clazz.class_eval do
    define_method "#{attribute}=" do |value|
      raise 'Invalid attribute' unless value
      instance_variable_set("@#{attribute}", value)
    end
    define_method attribute do
      instance_variable_get "@#{attribute}"
    end
  end
end

def add_checked_attribute(clazz, attribute, &validation)
  clazz.class_eval do
    define_method "#{attribute}=" do |value|
      raise 'Invalid attribute' unless validation.call(value)
      instance_variable_set("@#{attribute}", value)
    end
    define_method attribute do
      instance_variable_get "@#{attribute}"
    end
  end
end

add_checked_attribute(Person, :age, { |v| v >= 18 })

class Person
  include CheckedAttributes
  attr_checked :age { |v| v >= 18 }
end

class Class
  def attr_checked(attribute, &validation)
    def add_checked_attribute(clazz, attribute, &validation)
      clazz.class_eval do
        define_method "#{attribute}=" do |value|
          raise 'Invalid attribute' unless validation.call(value)
          instance_variable_set("@#{attribute}", value)
        end
        define_method attribute do
          instance_variable_get "@#{attribute}"
        end
      end
    end
  end
end

class Person
  include CheckedAttributes
  attr_checked :age { |v| v >= 18 }
end

included

module M
  def self.included(othermod)
    puts "M was mixed into #{othermod}"
  end
end

class C
  include M # => M was mixed into C
end

module CheckedAttributes
  def self.included(base)
    base.extend ClassMethods
  end
  module ClassMethods
    def attr_checked(attribute, &validation)
      def add_checked_attribute(clazz, attribute, &validation)
        clazz.class_eval do
          define_method "#{attribute}=" do |value|
            raise 'Invalid attribute' unless validation.call(value)
            instance_variable_set("@#{attribute}", value)
          end
          define_method attribute do
            instance_variable_get "@#{attribute}"
          end
        end
      end
    end
  end
end

class Person
  include CheckedAttributes
  attr_checked :age { |v| v >= 18 }
end