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Ruby, despite its elegant design and focus on developer happiness, still has common anti-patterns that can lead to bugs, performance issues, and maintenance problems. Here are the most important anti-patterns to avoid when writing Ruby code.
# Anti-pattern: Using eval
def calculate(formula)
  eval(formula) # Security risk!
end

# Usage
calculate("2 + 2") # Returns 4
calculate("system('rm -rf /')") # Disaster!

# Better approach: Use safer alternatives
require 'dentaku'

def calculate(formula)
  calculator = Dentaku::Calculator.new
  calculator.evaluate(formula)
end
Using eval can lead to serious security vulnerabilities. Use safer alternatives like parsing libraries or DSLs.
# Anti-pattern: Not using blocks
file = File.open('file.txt', 'r')
content = file.read
file.close # What if an exception occurs before this line?

# Better approach: Use blocks
content = File.open('file.txt', 'r') do |file|
  file.read
end # File is automatically closed
Ruby’s block syntax ensures resources are properly cleaned up, even if exceptions occur.
# Anti-pattern: Using class variables
class User
  @@count = 0
  
  def initialize
    @@count += 1
  end
  
  def self.count
    @@count
  end
end

class Admin < User
  # Inherits and modifies the same @@count variable
end

# Better approach: Use class instance variables
class User
  def initialize
    self.class.count += 1
  end
  
  def self.count
    @count ||= 0
  end
  
  def self.count=(value)
    @count = value
  end
end
Class variables (@@var) are shared across the entire class hierarchy, which can lead to unexpected behavior. Use class instance variables (@var in class methods) instead.
# Anti-pattern: Monkey patching core classes
class String
  def to_boolean
    self == 'true'
  end
end

# Better approach: Use refinements (Ruby 2.0+)
module StringExtensions
  refine String do
    def to_boolean
      self == 'true'
    end
  end
end

# Usage
using StringExtensions

# Or create utility methods
module StringUtils
  def self.to_boolean(str)
    str == 'true'
  end
end
Monkey patching core classes can lead to conflicts with other libraries and unexpected behavior. Use refinements or utility modules instead.
# Anti-pattern: method_missing without respond_to_missing?
class DynamicFinder
  def method_missing(method_name, *args)
    if method_name.to_s.start_with?('find_by_')
      # Handle dynamic finder
    else
      super
    end
  end
end

# Better approach: Implement respond_to_missing?
class DynamicFinder
  def method_missing(method_name, *args)
    if method_name.to_s.start_with?('find_by_')
      # Handle dynamic finder
    else
      super
    end
  end
  
  def respond_to_missing?(method_name, include_private = false)
    method_name.to_s.start_with?('find_by_') || super
  end
end
When using method_missing, always implement respond_to_missing? to ensure methods like respond_to? and method work correctly.
# Anti-pattern: Manual iteration
def find_adults(people)
  adults = []
  people.each do |person|
    adults << person if person.age >= 18
  end
  adults
end

# Better approach: Use Enumerable methods
def find_adults(people)
  people.select { |person| person.age >= 18 }
end

# More examples
people.map { |person| person.name } # Get all names
people.any? { |person| person.age < 18 } # Check if any minors
people.all? { |person| person.age >= 18 } # Check if all adults
people.reduce(0) { |sum, person| sum + person.age } # Sum ages
Ruby’s Enumerable module provides powerful methods for working with collections. Use them instead of manual iteration.
# Anti-pattern: Using global variables
$db_connection = Database.connect

def fetch_user(id)
  $db_connection.query("SELECT * FROM users WHERE id = #{id}")
end

# Better approach: Dependency injection
class UserRepository
  def initialize(db_connection)
    @db_connection = db_connection
  end
  
  def fetch_user(id)
    @db_connection.query("SELECT * FROM users WHERE id = #{id}")
  end
end
Global variables make code hard to test and reason about. Use dependency injection or configuration objects instead.
# Anti-pattern: String concatenation
def greeting(name)
  "Hello, " + name + "! Welcome to " + SITE_NAME + "."
end

# Better approach: String interpolation
def greeting(name)
  "Hello, #{name}! Welcome to #{SITE_NAME}."
end
String interpolation is more readable and often more efficient than string concatenation.
# Anti-pattern: Options hash
def create_user(name, email, options = {})
  age = options[:age] || 18
  admin = options[:admin] || false
  # Create user
end

# Usage
create_user("John", "john@example.com", { age: 25, admin: true })

# Better approach: Keyword arguments
def create_user(name, email, age: 18, admin: false)
  # Create user
end

# Usage
create_user("John", "john@example.com", age: 25, admin: true)
Keyword arguments are more explicit and provide better error messages than options hashes.
# Anti-pattern: Raising generic exceptions
def process_data(data)
  raise "Invalid data" if data.nil?
  # Process data
end

# Better approach: Use specific exceptions
class InvalidDataError < StandardError; end

def process_data(data)
  raise InvalidDataError, "Data cannot be nil" if data.nil?
  # Process data
rescue JSON::ParserError => e
  raise InvalidDataError, "Could not parse JSON: #{e.message}"
end
Create a proper exception hierarchy and use specific exception types for better error handling.
# Anti-pattern: Manual assignment
def process_coordinates(coords)
  x = coords[0]
  y = coords[1]
  z = coords[2]
  # Process coordinates
end

# Better approach: Destructuring assignment
def process_coordinates(coords)
  x, y, z = coords
  # Process coordinates
end

# Works with methods returning multiple values
def get_dimensions
  [width, height, depth]
end

width, height, depth = get_dimensions
Use destructuring assignment to extract values from arrays and other enumerable objects.
# Anti-pattern: Nested if/elsif
def describe_status(status)
  if status == :pending
    "Pending approval"
  elsif status == :approved
    "Approved"
  elsif status == :rejected
    "Rejected"
  else
    "Unknown status"
  end
end

# Better approach: Case expression
def describe_status(status)
  case status
  when :pending then "Pending approval"
  when :approved then "Approved"
  when :rejected then "Rejected"
  else "Unknown status"
  end
end
Use case expressions for cleaner, more readable code when comparing a value against multiple options.
# Anti-pattern: Nested nil checks
def get_city(user)
  if user && user.address && user.address.city
    user.address.city.name
  else
    "Unknown"
  end
end

# Better approach: Safe navigation operator (Ruby 2.3+)
def get_city(user)
  user&.address&.city&.name || "Unknown"
end
Use the safe navigation operator (&.) to simplify nil checks in method chains.
# Anti-pattern: Mutable constants
ALLOWED_ROLES = ['admin', 'editor', 'viewer']

# Later in the code
ALLOWED_ROLES << 'guest' # Modifies the constant!

# Better approach: Freeze constants
ALLOWED_ROLES = ['admin', 'editor', 'viewer'].freeze

# Even better: Freeze nested objects too
ALLOWED_ROLES = ['admin'.freeze, 'editor'.freeze, 'viewer'.freeze].freeze
Use freeze to prevent accidental modification of constants and other objects that should be immutable.
# Anti-pattern: Manual array handling
def sum(numbers)
  total = 0
  numbers.each { |n| total += n }
  total
end

# Usage
sum([1, 2, 3, 4, 5])

# Better approach: Splat operator
def sum(*numbers)
  numbers.sum
end

# Usage
sum(1, 2, 3, 4, 5)

# Double splat for keyword arguments
def configure(base_url:, **options)
  # Use base_url and options
end

# Usage
configure(base_url: "https://example.com", timeout: 30, retries: 3)
Use splat operators (* and **) for more flexible method arguments.
# Anti-pattern: Verbose blocks
names = people.map { |person| person.name }
valid_users = users.select { |user| user.valid? }

# Better approach: Symbol to proc
names = people.map(&:name)
valid_users = users.select(&:valid?)
Use the symbol to proc shorthand (&:method_name) for cleaner code when calling a single method in a block.
# Anti-pattern: Temporary variables
def create_user(attributes)
  user = User.new
  user.name = attributes[:name]
  user.email = attributes[:email]
  user.save
  user
end

# Better approach: Use tap
def create_user(attributes)
  User.new.tap do |user|
    user.name = attributes[:name]
    user.email = attributes[:email]
    user.save
  end
end
Use tap to perform operations on an object and return the object itself, avoiding temporary variables.
# Anti-pattern: Deep inheritance
class Vehicle; end
class LandVehicle < Vehicle; end
class Car < LandVehicle; end
class SportsCar < Car; end

# Better approach: Composition with modules
module Engine
  def start_engine
    # Start engine
  end
end

module Drivable
  def drive
    # Drive logic
  end
end

class Car
  include Engine
  include Drivable
  
  # Car-specific methods
end

class Boat
  include Engine
  
  # Boat-specific methods
end
Prefer composition over inheritance by using modules to share behavior across classes.
# Anti-pattern: Manual value objects
class Point
  attr_reader :x, :y
  
  def initialize(x, y)
    @x = x
    @y = y
  end
  
  def ==(other)
    x == other.x && y == other.y
  end
end

# Better approach: Use Struct
Point = Struct.new(:x, :y) do
  def distance_from_origin
    Math.sqrt(x**2 + y**2)
  end
end
Use Struct for simple value objects to reduce boilerplate code.
# Anti-pattern: Eager evaluation
def process_large_file(file_path)
  lines = File.readlines(file_path) # Reads entire file into memory
  lines.each do |line|
    process_line(line)
  end
end

# Better approach: Lazy evaluation with Enumerator
def process_large_file(file_path)
  File.open(file_path) do |file|
    file.each_line do |line|
      process_line(line)
    end
  end
end

# Or using lazy enumerator
def find_first_match(file_path, pattern)
  File.open(file_path).lazy.each_line
    .map(&:chomp)
    .find { |line| line.match?(pattern) }
end
Use enumerators and lazy evaluation for more memory-efficient processing of large collections.
# Anti-pattern: Repeated expensive calculations
def fibonacci(n)
  return n if n <= 1
  fibonacci(n - 1) + fibonacci(n - 2)
end

# Better approach: Memoization
def fibonacci(n)
  @fibonacci ||= {}
  @fibonacci[n] ||= begin
    return n if n <= 1
    fibonacci(n - 1) + fibonacci(n - 2)
  end
end
Use memoization to cache the results of expensive calculations.
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