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Inheritance & Mixins

Inheritance and mixins are powerful features of Ruby's object-oriented programming. T-Ruby extends these concepts with type safety, allowing you to build complex class hierarchies and share functionality through modules while maintaining strong type guarantees.

Basic Inheritance

Classes can inherit from parent classes, gaining access to their methods and instance variables:

basic_inheritance.trb

class Animal
  attr_accessor :name: String
  attr_accessor :age: Integer

  def initialize(name: String, age: Integer)
    @name = name
    @age = age
  end

  def speak(): String
    "Some sound"
  end

  def info(): String
    "#{@name} is #{@age} years old"
  end
end

class Dog < Animal
  attr_accessor :breed: String

  def initialize(name: String, age: Integer, breed: String)
    super(name, age)
    @breed = breed
  end

  def speak(): String
    "Woof!"
  end

  def fetch(item: String): String
    "#{@name} fetched the #{item}"
  end
end

class Cat < Animal
  def speak(): String
    "Meow!"
  end

  def scratch(): void
    puts "#{@name} is scratching"
  end
end

# Usage
dog = Dog.new("Buddy", 5, "Golden Retriever")
puts dog.speak()      # "Woof!"
puts dog.info()       # "Buddy is 5 years old"
puts dog.fetch("ball") # "Buddy fetched the ball"

cat = Cat.new("Whiskers", 3)
puts cat.speak()      # "Meow!"
cat.scratch()

Method Overriding

Child classes can override parent methods with the same signature:

override.trb

class Shape
  def initialize()
    @sides: Integer = 0
  end

  def area(): Float
    0.0
  end

  def perimeter(): Float
    0.0
  end

  def describe(): String
    "A shape with #{@sides} sides"
  end
end

class Rectangle < Shape
  attr_accessor :width: Float
  attr_accessor :height: Float

  def initialize(width: Float, height: Float)
    super()
    @width = width
    @height = height
    @sides = 4
  end

  def area(): Float
    @width * @height
  end

  def perimeter(): Float
    2 * (@width + @height)
  end

  def describe(): String
    "Rectangle: #{@width} x #{@height}"
  end
end

class Circle < Shape
  attr_accessor :radius: Float

  def initialize(radius: Float)
    super()
    @radius = radius
    @sides = 0  # Infinite sides technically
  end

  def area(): Float
    3.14159 * @radius * @radius
  end

  def perimeter(): Float
    2 * 3.14159 * @radius
  end

  def describe(): String
    "Circle with radius #{@radius}"
  end
end

# Usage
rect = Rectangle.new(10.0, 5.0)
puts rect.area()       # 50.0
puts rect.perimeter()  # 30.0
puts rect.describe()   # "Rectangle: 10.0 x 5.0"

circle = Circle.new(5.0)
puts circle.area()     # 78.53975

Super Keyword

Use super to call parent class methods:

super.trb

class Employee
  attr_accessor :name: String
  attr_accessor :salary: Float

  def initialize(name: String, salary: Float)
    @name = name
    @salary = salary
  end

  def annual_bonus(): Float
    @salary * 0.1
  end

  def total_compensation(): Float
    @salary + annual_bonus()
  end
end

class Manager < Employee
  attr_accessor :team_size: Integer

  def initialize(name: String, salary: Float, team_size: Integer)
    super(name, salary)
    @team_size = team_size
  end

  # Override: Managers get better bonuses
  def annual_bonus(): Float
    base_bonus = super()
    base_bonus + (@team_size * 1000.0)
  end

  def team_bonus(): Float
    @team_size * 500.0
  end

  # Override to include team bonus
  def total_compensation(): Float
    super() + team_bonus()
  end
end

# Usage
employee = Employee.new("Alice", 50000.0)
puts employee.total_compensation()  # 55000.0

manager = Manager.new("Bob", 80000.0, 5)
puts manager.annual_bonus()         # 8000.0 + 5000.0 = 13000.0
puts manager.total_compensation()   # 80000.0 + 13000.0 + 2500.0 = 95500.0

Modules and Mixins

Modules allow you to share functionality across multiple classes:

modules.trb

module Timestampable
  def created_at(): Time?
    @created_at
  end

  def updated_at(): Time?
    @updated_at
  end

  def touch(): void
    @updated_at = Time.now
  end

  def set_created(): void
    @created_at = Time.now
    @updated_at = Time.now
  end
end

module Searchable
  def search(query: String): Boolean
    searchable_fields().any? { |field| field.include?(query) }
  end

  def searchable_fields(): Array<String>
    []
  end
end

class BlogPost
  include Timestampable
  include Searchable

  attr_accessor :title: String
  attr_accessor :content: String
  attr_accessor :author: String

  def initialize(title: String, content: String, author: String)
    @title = title
    @content = content
    @author = author
    @created_at: Time? = nil
    @updated_at: Time? = nil
    set_created()
  end

  def searchable_fields(): Array<String>
    [@title, @content, @author]
  end

  def update_content(new_content: String): void
    @content = new_content
    touch()
  end
end

class Product
  include Timestampable
  include Searchable

  attr_accessor :name: String
  attr_accessor :description: String
  attr_accessor :sku: String

  def initialize(name: String, description: String, sku: String)
    @name = name
    @description = description
    @sku = sku
    @created_at: Time? = nil
    @updated_at: Time? = nil
    set_created()
  end

  def searchable_fields(): Array<String>
    [@name, @description, @sku]
  end
end

# Usage
post = BlogPost.new("Hello World", "This is my first post", "Alice")
puts post.created_at()
post.update_content("Updated content")
puts post.updated_at()
puts post.search("first")  # true

product = Product.new("Laptop", "Gaming laptop", "SKU-001")
puts product.search("Gaming")  # true

Module Methods

Modules can define methods that work with instance variables:

module_methods.trb

module Validatable
  def valid?(): Boolean
    errors().empty?
  end

  def errors(): Array<String>
    @errors ||= []
  end

  def add_error(message: String): void
    @errors ||= []
    @errors.push(message)
  end

  def clear_errors(): void
    @errors = []
  end
end

class User
  include Validatable

  attr_accessor :email: String
  attr_accessor :age: Integer

  def initialize(email: String, age: Integer)
    @email = email
    @age = age
    @errors: Array<String> = []
  end

  def validate(): Boolean
    clear_errors()

    if !@email.include?("@")
      add_error("Email must contain @")
    end

    if @age < 18
      add_error("Must be 18 or older")
    end

    valid?()
  end
end

# Usage
user = User.new("invalid-email", 15)
if !user.validate()
  user.errors().each { |e| puts e }
  # "Email must contain @"
  # "Must be 18 or older"
end

Multiple Mixins

Classes can include multiple modules:

multiple_mixins.trb

module Comparable
  def ==(other: self): Boolean
    compare_fields() == other.compare_fields()
  end

  def !=(other: self): Boolean
    !self.==(other)
  end

  def compare_fields(): Array<String | Integer>
    []
  end
end

module Serializable
  def to_hash(): Hash<String, String | Integer | Boolean>
    {}
  end

  def to_json(): String
    to_hash().to_json
  end
end

module Cloneable
  def clone(): self
    self.class.new(*clone_params())
  end

  def clone_params(): Array<String | Integer>
    []
  end
end

class Person
  include Comparable
  include Serializable
  include Cloneable

  attr_accessor :name: String
  attr_accessor :age: Integer
  attr_accessor :email: String

  def initialize(name: String, age: Integer, email: String)
    @name = name
    @age = age
    @email = email
  end

  def compare_fields(): Array<String | Integer>
    [@name, @age, @email]
  end

  def to_hash(): Hash<String, String | Integer | Boolean>
    { "name" => @name, "age" => @age, "email" => @email }
  end

  def clone_params(): Array<String | Integer>
    [@name, @age, @email]
  end
end

# Usage
person1 = Person.new("Alice", 30, "alice@example.com")
person2 = Person.new("Alice", 30, "alice@example.com")
person3 = Person.new("Bob", 25, "bob@example.com")

puts person1 == person2  # true
puts person1 == person3  # false

clone = person1.clone()
puts clone.name  # "Alice"

Type Safety with Inheritance

T-Ruby ensures type safety across inheritance hierarchies:

type_safety.trb

class Vehicle
  attr_accessor :make: String
  attr_accessor :model: String

  def initialize(make: String, model: String)
    @make = make
    @model = model
  end

  def start(): void
    puts "Vehicle starting"
  end
end

class Car < Vehicle
  attr_accessor :num_doors: Integer

  def initialize(make: String, model: String, num_doors: Integer)
    super(make, model)
    @num_doors = num_doors
  end

  def start(): void
    puts "Car engine starting"
  end
end

class Motorcycle < Vehicle
  attr_accessor :has_sidecar: Boolean

  def initialize(make: String, model: String, has_sidecar: Boolean)
    super(make, model)
    @has_sidecar = has_sidecar
  end

  def start(): void
    puts "Motorcycle roaring to life"
  end
end

def start_vehicle(vehicle: Vehicle): void
  vehicle.start()
end

# Usage - all work because Car and Motorcycle inherit from Vehicle
car = Car.new("Toyota", "Camry", 4)
motorcycle = Motorcycle.new("Harley", "Street 750", false)

start_vehicle(car)         # "Car engine starting"
start_vehicle(motorcycle)  # "Motorcycle roaring to life"

# Type checking works
vehicles: Array<Vehicle> = [car, motorcycle]
vehicles.each { |v| v.start() }

Practical Example: Content Management System

A complete example showing inheritance and mixins:

cms.trb

module Publishable
  def publish(): void
    @published = true
    @published_at = Time.now
  end

  def unpublish(): void
    @published = false
    @published_at = nil
  end

  def is_published?(): Boolean
    @published || false
  end

  def published_at(): Time?
    @published_at
  end
end

module Taggable
  def tags(): Array<String>
    @tags ||= []
  end

  def add_tag(tag: String): void
    @tags ||= []
    @tags.push(tag) unless @tags.include?(tag)
  end

  def remove_tag(tag: String): void
    @tags ||= []
    @tags.delete(tag)
  end

  def has_tag?(tag: String): Boolean
    tags().include?(tag)
  end
end

module Commentable
  def comments(): Array<Comment>
    @comments ||= []
  end

  def add_comment(comment: Comment): void
    @comments ||= []
    @comments.push(comment)
  end

  def comment_count(): Integer
    comments().length
  end
end

class Content
  attr_accessor :title: String
  attr_accessor :author: String
  attr_reader :created_at: Time

  def initialize(title: String, author: String)
    @title = title
    @author = author
    @created_at = Time.now
  end

  def summary(): String
    @title
  end
end

class Article < Content
  include Publishable
  include Taggable
  include Commentable

  attr_accessor :body: String
  attr_accessor :excerpt: String

  def initialize(title: String, author: String, body: String, excerpt: String)
    super(title, author)
    @body = body
    @excerpt = excerpt
    @published: Boolean = false
    @published_at: Time? = nil
    @tags: Array<String> = []
    @comments: Array<Comment> = []
  end

  def summary(): String
    @excerpt
  end

  def word_count(): Integer
    @body.split.length
  end
end

class Page < Content
  include Publishable

  attr_accessor :slug: String
  attr_accessor :content: String

  def initialize(title: String, author: String, slug: String, content: String)
    super(title, author)
    @slug = slug
    @content = content
    @published: Boolean = false
    @published_at: Time? = nil
  end

  def url(): String
    "/pages/#{@slug}"
  end
end

class Comment
  attr_reader :author: String
  attr_reader :body: String
  attr_reader :created_at: Time

  def initialize(author: String, body: String)
    @author = author
    @body = body
    @created_at = Time.now
  end
end

# Usage
article = Article.new(
  "Introduction to T-Ruby",
  "Alice",
  "T-Ruby is a typed superset of Ruby...",
  "Learn about T-Ruby's type system"
)

article.add_tag("ruby")
article.add_tag("types")
article.add_tag("programming")
article.publish()

comment = Comment.new("Bob", "Great article!")
article.add_comment(comment)

puts article.is_published?()    # true
puts article.has_tag?("ruby")   # true
puts article.comment_count()    # 1
puts article.word_count()       # 5

page = Page.new("About", "System", "about", "This is the about page")
page.publish()
puts page.url()  # "/pages/about"

Best Practices

1. Keep inheritance hierarchies shallow: Prefer composition over deep inheritance chains.

2. Use modules for shared behavior: When multiple unrelated classes need the same functionality, use modules.

3. Type all method parameters: Even in modules, type all parameters and return values.

4. Initialize module instance variables: When modules use instance variables, initialize them in the including class.

5. Use super appropriately: When overriding methods, consider whether you need to call the parent implementation.

6. Document module requirements: If a module expects certain methods or instance variables, document them clearly.

Common Patterns

Template Method Pattern

template_method.trb

class DataImporter
  def import(file_path: String): void
    data = read_file(file_path)
    parsed = parse_data(data)
    validate_data(parsed)
    save_data(parsed)
  end

  def read_file(file_path: String): String
    # Common implementation
    File.read(file_path)
  end

  def validate_data(data: Array<Hash<String, String>>): void
    # Common validation
    raise "Empty data" if data.empty?
  end

  def parse_data(data: String): Array<Hash<String, String>>
    raise "Must implement parse_data"
  end

  def save_data(data: Array<Hash<String, String>>): void
    raise "Must implement save_data"
  end
end

class CSVImporter < DataImporter
  def parse_data(data: String): Array<Hash<String, String>>
    # Parse CSV
    []
  end

  def save_data(data: Array<Hash<String, String>>): void
    # Save to database
  end
end

Decorator Pattern with Modules

decorator.trb

module Logging
  def log(message: String): void
    puts "[#{Time.now}] #{message}"
  end

  def with_logging(&block: Proc<[], void>): void
    log("Starting operation")
    block.call
    log("Operation completed")
  end
end

module Caching
  def cache(): Hash<String, String>
    @cache ||= {}
  end

  def cached(key: String, &block: Proc<[], String>): String
    if cache().key?(key)
      cache()[key]
    else
      result = block.call
      cache()[key] = result
      result
    end
  end
end

class DataService
  include Logging
  include Caching

  def fetch_data(id: String): String
    cached(id) do
      with_logging do
        # Expensive operation
        "Data for #{id}"
      end
    end
  end
end

Summary

Inheritance and mixins in T-Ruby provide powerful code reuse mechanisms with type safety:

• Inheritance creates "is-a" relationships with type guarantees
• Modules share functionality across unrelated classes
• Method overriding maintains type signatures from parent classes
• Super calls parent implementations while preserving types
• Multiple mixins combine orthogonal concerns

Use inheritance for hierarchical relationships and modules for cross-cutting concerns. Always maintain type safety throughout your class hierarchies.