Tag: Proc

Ruby Concepts 💠: Blocks, Constants, Meta-Programming, Enum

Here we will look into the detailed explanation of some Ruby concepts with practical examples, and real-world scenarios:

1. Handling Many Constants in a Ruby Class

Problem:
A class with numerous constants becomes cluttered and harder to maintain.

Solutions & Examples:

  1. Nested Module for Grouping:
   class HTTPClient
     module StatusCodes
       OK = 200
       NOT_FOUND = 404
       SERVER_ERROR = 500
     end

     def handle_response(code)
       case code
       when StatusCodes::OK then "Success"
       when StatusCodes::NOT_FOUND then "Page missing"
       end
     end
   end

Why: Encapsulating constants in a module improves readability and avoids namespace collisions.

  1. Dynamic Constants with const_set:
   class DaysOfWeek
     %w[MON TUE WED THU FRI SAT SUN].each_with_index do |day, index|
       const_set(day, index + 1)
     end
   end
   puts DaysOfWeek::MON # => 1

Use Case: Generate constants programmatically (e.g., days, months).

  1. External Configuration (YAML):
   # config/constants.yml
   error_codes:
     NOT_FOUND: 404
     SERVER_ERROR: 500

   class App
     CONSTANTS = YAML.load_file('config/constants.yml')
     def self.error_message(code)
       CONSTANTS['error_codes'].key(code)
     end
   end

Why: Centralize configuration for easy updates.

2. Meta-Programming: Dynamic Methods & Classes

Examples:

  1. define_method for Repetitive Methods:
   class User
     ATTRIBUTES = %w[name email age]

     ATTRIBUTES.each do |attr|
       define_method(attr) { instance_variable_get("@#{attr}") }
       define_method("#{attr}=") { |value| instance_variable_set("@#{attr}", value) }
     end
   end

   user = User.new
   user.name = "Alice"
   puts user.name # => "Alice"

Use Case: Auto-generate getters/setters for multiple attributes.

  1. Dynamic Classes with Class.new:
   Animal = Class.new do
     def speak
       puts "Animal noise!"
     end
   end

   dog = Animal.new
   dog.speak # => "Animal noise!"

Use Case: Generate classes at runtime (e.g., for plugins).

  1. class_eval for Modifying Existing Classes:
   String.class_eval do
     def shout
       upcase + "!"
     end
   end

   puts "hello".shout # => "HELLO!"

Why: Add/redefine methods in existing classes dynamically.

3. Why Classes Are Objects in Ruby

Explanation:

  • Every class is an instance of Class.
  String.class # => Class
  • Classes inherit from Module and ultimately Object, allowing them to have methods and variables:
  class Dog
    @count = 0 # Class instance variable
    def self.increment_count
      @count += 1
    end
  end
  • Real-World Impact: You can pass classes as arguments, modify them at runtime, and use them like any other object.

4. super Keyword: Detailed Usage

Examples:

  1. Implicit Argument Passing:
   class Vehicle
     def start_engine
       "Engine on"
     end
   end

   class Car < Vehicle
     def start_engine
       super + " (Vroom!)"
     end
   end

   puts Car.new.start_engine # => "Engine on (Vroom!)"
  1. Explicit super() for No Arguments:
   class Parent
     def greet
       "Hello"
     end
   end

   class Child < Parent
     def greet
       super() + " World!" # Explicitly call Parent#greet with no args
     end
   end

Pitfall: Forgetting () when overriding a method with parameters.

5. Blocks in Ruby Methods: Scenarios

A simple ruby method that accepts a block and executing via yield:

irb* def abhi_block
irb*   yield
irb*   yield
irb> end
=> :abhi_block
irb* abhi_block do.             # multi-line block
irb*   puts "*"*7
irb> end
*******
*******
irb> abhi_block { puts "*"*7 }. # single-line block
*******
*******
=> nil
irb* def abhi_block
irb*   yield 3
irb*   yield 7
irb*   yield 9
irb> end
=> :abhi_block
irb> abhi_block { |x| puts x }. # pass argument to block
3
7
9
=> nil

Note: We can call yield any number times that we want.

Proc

Procs are similar to blocks, however, they differ in that they may be saved to a variable to be used again and again. In Ruby, a proc can be called directly using the .call method.

To create Proc, we call new on the Proc class and follow it with the block of code

my_proc = Proc.new { |x| x*x*9 }
=> #<Proc:0x000000011f64ed38 (irb):34>

my_proc.call(6)
=> 324

> my_proc.call      # try to call without an argument
(irb):34:in 'block in <top (required)>': undefined method '*' for nil (NoMethodError)
lambda
> my_lambda = lambda { |x| x/3/5 }
=> #<Proc:0x000000011fe6fd28 (irb):44 (lambda)>

> my_lambda.call(233)
=> 15

> my_lambda = lambda.new { |x| x/3/5 } # wrong
in 'Kernel#lambda': tried to create Proc object without a block (ArgumentError)

> my_lambda = lambda                   # wrong
(irb):45:in 'Kernel#lambda': tried to create Proc object without a block (ArgumentError)

> my_lambda.call     # try to call without an argument
(irb):46:in 'block in <top (required)>': wrong number of arguments (given 0, expected 1) (ArgumentError)

Difference 1: lambda gets an ArgumentError if we call without an argument and Proc doesn’t.

Difference 2: lambda returns to its calling method rather than returning itself like Proc from its parent method.

irb* def proc_method
irb*   my_proc = Proc.new { return "Proc returns" }
irb*   my_proc.call
irb*   "Retun by proc_method"  # neaver reaches here
irb> end
=> :proc_method

irb> p proc_method
"Proc returns"
=> "Proc returns"

irb* def lambda_method
irb*   my_lambda = lambda { return 'Lambda returns' }
irb*   my_lambda.call
irb*   "Method returns"
irb> end
=> :lambda_method
irb(main):079> p lambda_method
"Method returns"
=> "Method returns"

Use Cases & Examples:

  1. Resource Management (File Handling):
   def open_file(path)
     file = File.open(path, 'w')
     yield(file) if block_given?
   ensure
     file.close
   end

   open_file('log.txt') { |f| f.write("Data") }

Why: Ensures the file is closed even if an error occurs.

  1. Custom Iterators:
   class MyArray
     def initialize(items)
       @items = items
     end

     def custom_each
       @items.each { |item| yield(item) }
     end
   end

   MyArray.new([1,2,3]).custom_each { |n| puts n * 2 }
  1. Timing Execution:
   def benchmark
     start = Time.now
     yield
     puts "Time taken: #{Time.now - start}s"
   end

   benchmark { sleep(2) } # => "Time taken: 2.0s"
Procs And Lambdas in Ruby

proc = Proc.new { puts "I am the proc block" }
lambda = lambda { puts "I am the lambda block"}

proc_test.call # => I am the proc block
lambda_test.call # => I am the lambda block

6. Enums in Ruby

Approaches:

  1. Symbols/Constants:
   class TrafficLight
     STATES = %i[red yellow green].freeze

     def initialize
       @state = STATES.first
     end

     def next_state
       @state = STATES[(STATES.index(@state) + 1) % STATES.size]
     end
   end
  1. Rails ActiveRecord Enum:
   class User < ActiveRecord::Base
     enum role: { admin: 0, user: 1, guest: 2 }
   end

   user = User.new(role: :admin)
   user.admin? # => true

Why: Generates helper methods like admin? and user.admin!.

7. Including Enumerable

Why Needed:

  • Enumerable methods (map, select, etc.) rely on each being defined.
  • Example Without Enumerable:
  class MyCollection
    def initialize(items)
      @items = items
    end

    def each(&block)
      @items.each(&block)
    end
  end

  # Without Enumerable:
  collection = MyCollection.new([1,2,3])
  collection.map { |n| n * 2 } # Error: Undefined method `map`
  • With Enumerable:
  class MyCollection
    include Enumerable
    # ... same as above
  end

  collection.map { |n| n * 2 } # => [2,4,6]

8. Class Variables (@@)

Example & Risks:

class Parent
  @@count = 0

  def self.count
    @@count
  end

  def increment
    @@count += 1
  end
end

class Child < Parent; end

Parent.new.increment
Child.new.increment
puts Parent.count # => 2 (Shared across Parent and Child)

Why Avoid: Subclasses unintentionally modify the same variable.
Alternative (Class Instance Variables):

class Parent
  @count = 0

  def self.count
    @count
  end

  def self.increment
    @count += 1
  end
end

9. Global Variables ($)

Example & Issues:

$logger = Logger.new($stdout)

def log_error(message)
  $logger.error(message) # Accessible everywhere
end

# Problem: Tight coupling; changing $logger affects all code.

When to Use: Rarely, for truly global resources like $stdout or $LOAD_PATH.
Alternative: Dependency injection or singleton classes.

class AppConfig
  attr_reader :logger

  def initialize(logger:)
    @logger = logger
  end

  def info(msg)
    @logger.info(msg)
  end
end

config = AppConfig.new(Logger.new($stdout))
info = config.info("Safe")

Summary:

  • Constants: Organize with modules or external files.
  • Meta-Programming: Use define_method/Class.new for dynamic code.
  • Classes as Objects: Enable OOP flexibility.
  • super: Call parent methods with/without arguments.
  • Blocks: Abstract setup/teardown or custom logic.
  • Enums: Simulate via symbols or Rails helpers.
  • Enumerable: Include it and define each.
  • Class/Global Variables: Rarely used due to side effects.

Enjoy Ruby! 🚀