Closures in Ruby

gistfile1.rb

# Closures can be done in Ruby using both lambdas and procs.


# Lambdas with closures:
# A lambda can be declared so that it is created at runtime but evaluated only when it is called.

class Person
  def initialize(age)
    @age = age
  end

  def birthday
    lambda { @age += 1 }
  end

  def print_age
    lambda { puts "Age: #{@age}" }
  end
end

# The above class has two methods that return lambda expressions. When #birthday is called,
# for example, a lambda is returned-- the lambda#call method must be called to execute the code
# contained within:

rebecca = Person.new(0)

birthday_closure = rebecca.birthday
age_closure = rebecca.print_age

# Again, birthday_closure and age_closure are simply lambdas.
# In order to increment and print the Person's new age, the following code must be executed:

rebecca.print_age.call              # prints "Age: 0"
rebecca.birthday_closure.call       # increments the age
rebecca.print_age.call              # prints "Age: 1"

# This follows the properties outlined in the original Gist (gist.github.com/4196824):
# first, that the lambda allows the function to be passed around without executing (as we saw in calling
# rebecca.birthday); second, that the values persisted outside of the scope of the object-- the instance
# of Person, rebecca, was able to keep the @age variable and access and increment it from outside of the
# Person class.


# Procs with closures:
# Procs also allow for passing blocks of code around to be executed later. An example of this can be found
# in the Rails source code*:

:stderr => Proc.new { |message, callstack|
             $stderr.puts(message)
             $stderr.puts callstack.join("\n  ") if debug
           }

# In the example above, Proc.new will create an error message if the debug variable declared within the class,
attr_accessor :debug 
# , is assigned "true". The Proc can be called with the message and callstack assigned while the program
# is executing. Furthermore, the state of :debug can be accessed from outside of the scope of the
# class-- from wherever the deprecated message ought to be displayed.


# * https://github.com/rails/rails/blob/master/activesupport/lib/active_support/deprecation/behaviors.rb