Kernel#raise

raise([exception [, msg [, backtrace]]]) -> nil

raise is a Kernel method that triggers an exception, interrupting the normal program flow. When an exception is raised, Ruby searches up the call stack for a rescue block that can handle it. If no handler is found, the program terminates and displays the exception message along with a backtrace. This mechanism is fundamental to Ruby’s error handling and is used for everything from validating input to handling unexpected runtime conditions.

Syntax

raise
raise message
raise ExceptionClass
raise ExceptionClass, message
raise ExceptionClass, message, backtrace

Parameters

Parameter	Description
exception	Optional. An exception class (e.g., RuntimeError, ArgumentError) or an instance of an exception. Defaults to RuntimeError when omitted.
message	Optional. A string describing the error. Defaults to “x” for RuntimeError or the exception’s default message.
backtrace	Optional. A custom backtrace array (typically from caller).

Examples

Basic Usage

raise "Something went wrong"
# => (raises RuntimeError with message "Something went wrong")

Raising a Specific Exception Type

raise ArgumentError, "Invalid argument provided"
# => (raises ArgumentError: Invalid argument provided)

Using raise in a Method

def divide(a, b)
  raise ZeroDivisionError, "Cannot divide by zero" if b == 0
  a / b
end

divide(10, 0)
# => raises: ZeroDivisionError: Cannot divide by zero

Conditional Error Handling

def withdraw(amount)
  balance = 100
  if amount > balance
    raise "Insufficient funds"
  end
  balance - amount
end

withdraw(150)
# => raises: RuntimeError: Insufficient funds

Common Patterns

Validation Pattern

def set_age(age)
  raise ArgumentError, "Age must be a number" unless age.is_a?(Numeric)
  raise ArgumentError, "Age cannot be negative" if age < 0
  @age = age
end

Re-raising Exceptions

begin
  # Some operation that might fail
  File.read("nonexistent.txt")
rescue => e
  puts "Error occurred: #{e.message}"
  raise  # Re-raises the same exception
end

Custom Exception Classes

class ValidationError < StandardError; end

def validate!(input)
  raise ValidationError, "Input cannot be empty" if input.nil? || input.empty?
end

Errors (Common Exception Types)

Exception	Description
RuntimeError	Generic error raised when no specific class is specified (default)
ArgumentError	Raised when method arguments are invalid
TypeError	Raised when an object is of the wrong type
NoMethodError	Raised when calling a method that doesn’t exist
StandardError	Base class for most user-defined exceptions
ZeroDivisionError	Raised when attempting to divide by zero

See Also

• Kernel#abort
• Kernel#exit
• Kernel#exit!