e x p r e s s i c a

Hey Everyone,
Vinsol is proudly taking charge to spread Rubyism in delhi and to grow the Ruby & Rails communities here in New Delhi, India. We are organizing delhi.rb meetups around once every month, the meetup is all about ruby and rails as well. The meetup was on 19th July 2007 was our second meetup, first was on 22nd June 2007.

Manik presenting SOLR

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Me presenting Some Advance Ruby Skills

More photos here.

It was really a nice experience attending the meetup, sharing the ruby/rails thoughts and upcoming features. It helps keeping yourself up-to-date with the latest trends in this technology domain at least in Ruby and Rails(what else m talking except ruby :D). So, there were two presentations in the meetup — first Manik presented Full text search implementation for Rails using SOLR(it was really an interesting presentation, i got SOLR learning for free, thanks Manik :)), second I presented Some Advance Ruby Skills which i am going to share in this post too. Though in the first meetup I presented Caching on RubyOnRails but i haven’t posted here…

Some Advance Ruby Skills

1.) Everything is object

A popular phrase about Ruby, “Everything is Object”. At the root of the ruby it is Object. Everything we define in ruby is object. Even the classes we define are actually object. A class defined with class ClassName; end is actually an object of the class Class.
The Object keeps the record of whatever class or module we define. We can justify it as
[source:ruby]
class Klass
end
Object.constants.include?(”Klass”) # => true
[/source]

2.) module_eval

Use module_eval to define instance and class methods of a class at runtime, when you are outside the class.
example 1
defining an instance method
[source:ruby]
class C
end

C.module_eval do
define_method :wish do
p “hello instance method”
end
end

c = C.new
c.wish # => hello instance method
[/source]
example 2
defining a class method
[source:ruby]
class C
end

C.module_eval do
class << self
define_method :wish do
p "hello class method"
end
end
end

C.wish # => hello class method
[/source]
example 3
another form of using module_eval
when method body is available as a String object
[source:ruby]
class D
class << self
def method_body
ret =<<-EOS
def wish
p "hello, supplied as String object"
end
EOS
end
end

class C
end

c = C.new

c.class.module_eval(D.method_body)

c.wish # => hello, supplied as String object
end
[/source]

3.) alias_method

It is NOT method call delegation but insertion of customized functionalities on a specific method call.
[source:ruby]
class C

def wish
p “hello”
end

end

c = C.new
c.wish # hello

class D

class << self
def keep_some_record
p "I am keeping some records"
end
end

end

# aliasing the wish method

c.class.module_eval do

alias_method :wish_orig, :wish

define_method :wish do
D.keep_some_record
wish_orig
end

end

c.wish # I am keeping some records; hello
[/source]

4.) The Anonymous class

I just presented same a la this post

5.) send

Calling a method when method name is stored as a string object in a variable i.e. you can not see which method to call.
example 1
when method name is simply stored as a String object
[source:ruby]
class C
def wish
p “hello DELHI.rb”
end
end
a = “wish”
c = C.new
c.send(a)
[/source]
example 2
making set method at runtime
[source:ruby]
class C
attr_accessor :name
end

c = C.new

a = “name”

c.send(a + “=”, “SUR MAX”)

p c.send(a)
[/source]
example 3
this is interesting, when attribute name itself is send
[source:ruby]
class C
attr_accessor :send
end

c = C.new

a = “send”

c.__send__(a + “=”, “SUR MAX”)

p c.__send__(a) # => Sur Max
[/source]
well, don’t say “what if attribute name is __send__”

6.) The Method class

Methods of the class are objects of the Method class when retrieved with the method method and can be called with the method call.
example 1
anything we define with def-end is an object of the class Method
[source:ruby]
class C
def wish
p “hello”
end
end

c = C.new

m1 = c.method(”wish”)

p m1.class # => Method

m1.call # => hello
[/source]
example 2
method can hold the object’s reference and associated instance variables
[source:ruby]
class C
attr_accessor :name

def initialize(name)
self.name = name.to_s
end

def wish
p “hello ” + name.to_s
end
end

c = C.new(”Sur Max”)

m1 = c.method(”wish”)
m1.call # => hello Sur Max
[/source]
example 3
we are able to let this method object flow throughout the application code and let it available anywhere in the code.
[source:ruby]
class C
attr_accessor :name

def initialize(name)
self.name = name.to_s
end

def wish
p “hello ” + name.to_s
end

def self.supply_wish
c = new(”Sur Max”)
return c.method(”wish”)
end

end

C.supply_wish.call # => hello Sur Max
[/source]

7.) what is “self”

I just presented a la this post

8.) Single Method Delegation - using Forwardable

Allows you to delegate named method calls to other objects.
[source:ruby]
require ‘forwardable’

class C
extend Forwardable

attr_accessor :h

def initialize
@h = {}
end

def_delegator(:@h, :[], :show)
def_delegator(:@h, :[]=, :add)

end

end

c = C.new

c.add(1, “asdf”)

p c.show(1)

p c.h
[/source]
Notice the beauty of ruby here… The methods [], []= of a hash object are usually called as
[source:ruby]
h = {}
h["key"] # this will return the corresponding value
h["key"] = “value” # this will set the “value” corresponding to the “key”
[/source]
BUT in the above delegation code we are calling them as(delegating the method call on them as)
[source:ruby]
h = {}
h.[](”key”) # this will return the corresponding value
h.[]=(”key”, “value”) # this will set the “value” corresponding to the “key”
[/source]

9.) Full class Delegation - using Delegator

Extending an object(instance of Class) with the capabilities of another.
[source:ruby]
require ‘delegate’

class Words < DelegateClass(Array)

def initialize(list = "one two three four")
super(list.split)
end

end

w = Words.new

p w # => ["one", "two", "three", "four"]

p w.length # => 4

[/source]

10.) SimpleDelegator

Write memory optimized code with SimpleDelegator…
[source:ruby]
require ‘delegate’

a = SimpleDelegator.new([10, 20])

old_id = a.__id__

b = a

a.__setobj__(”a new object”) # this is not possible otherwise with the method “replace” which can replace only object of same class on same memory location

new_id = a.__id__

p a # => “a new object”
p b # => “a new object”

p new_id == old_id # => true
[/source]

sur advance_ruby, classes, delegation, delhi.rb, duck typing, metaprogramming, rails, ror, ruby, rubyonrails

Doing meta-programming in ruby gives all new ways of getting closer to ruby. There are a lots of Ruby Idioms that attracted a lots of programmers to fall in love with Ruby-Lang. class << self; self; end is a ruby idiom which gives the Big Picture of coding-less and producing-more by providing the way to add an anonymous class to any class which is an instance of class Class. And singleton class can be implemented to any class which will be equally good as implementing it on any object and the reason is that every class that we define with class is again an instance of a class Class. And the beauty is…
[source:ruby]
“string”.class # => String
“string”.class.class # => Class
“string”.class.class.class # => Class
“string”.class.class.superclass # => Module
“string”.class.class.superclass.class # => Class
“string”.class.class.superclass.superclass # => Object
“string”.class.class.superclass.superclass.class # => Class
“string”.class.class.superclass.superclass.superclass # => nil
[/source]

class << self; self; end
Lets try what exactly this idiom returns
[source:ruby]
class A
puts self
puts(class << self; self; end)
end
# => A
# => # <Class:A>
[/source]

As it is cleared from the output that inside the class A, self is reflecting the class A itself whereas class << self; self; end is <Class:A>, so what is the exact difference ?
Lets exemplify it…
[source:ruby]
class A

self.module_eval do
define_method :wish do
puts “hello instance method”
end
end

(class << self; self; end).module_eval do
define_method :wish do
puts “hello class method”
end
end

end

A.wish # => hello class method
A.new.wish # => hello instance method

class B

self.module_eval do
define_method :wish do
puts “hello instance method”
end
end

end

B.new.wish # => hello instance method
B.wish # => undefined method `wish’ for B:Class (NoMethodError)

[/source]

I suppose that it is clear now, in case if it is not than look at this… The following code will do exactly same as above but the class method wish will be available to all classes or we can say to all instances of class Class

[source:ruby]

class Class

self.module_eval do
define_method :wish do
puts “hello — this is class method for instance of class Class”
end
end

end

class A

self.module_eval do
define_method :wish do
puts “hello instance method”
end
end

end

A.wish # => hello — this is class method for instance of class Class
A.new.wish # => hello instance method

class B

self.module_eval do
define_method :wish do
puts “hello instance method”
end
end

end

B.wish # => hello — this is class method for instance of class Class
B.new.wish # => hello instance method

[/source]

sur classes, metaprogramming, ruby