How to Use raise in Ruby: Syntax and Examples

In Ruby, use raise to trigger an exception and stop normal program flow. You can raise a generic error with raise or specify an error type and message like raise RuntimeError, 'message'.

Syntax

The raise keyword is used to trigger an exception. You can use it in three main ways:

• raise - raises a generic RuntimeError with no message.
• raise "message" - raises a RuntimeError with a custom message.
• raise ExceptionClass, "message" - raises a specific exception type with a message.

ruby

raise
raise "Something went wrong"
raise ArgumentError, "Invalid argument"

Example

This example shows how to use raise to stop execution when a condition is not met, and how to rescue the error to handle it gracefully.

ruby

def check_age(age)
  raise ArgumentError, "Age must be 18 or older" if age < 18
  "Access granted"
end

begin
  puts check_age(20)  # This will print "Access granted"
  puts check_age(16)  # This will raise an error
rescue ArgumentError => e
  puts "Error: #{e.message}"
end

Output

Access granted
Error: Age must be 18 or older

Common Pitfalls

Common mistakes when using raise include:

• Raising without a message, which can make debugging harder.
• Raising the wrong exception type for the error situation.
• Not rescuing exceptions when needed, causing the program to crash unexpectedly.

Always provide meaningful messages and use appropriate exception classes.

ruby

def divide(a, b)
  # Wrong: no message
  # raise if b == 0

  # Better: raise with message and exception type
  raise ZeroDivisionError, "Cannot divide by zero" if b == 0
  a / b
end

Quick Reference

Usage	Description
raise	Raises a RuntimeError with no message
raise "message"	Raises a RuntimeError with a custom message
raise ExceptionClass, "message"	Raises a specific exception type with a message
raise e	Re-raises the current exception object e

Key Takeaways

Use raise to trigger exceptions and stop normal program flow in Ruby.

Provide clear messages and use specific exception classes for better error handling.

Rescue exceptions when you want to handle errors gracefully and prevent crashes.

Avoid raising exceptions without messages to make debugging easier.

Use raise inside methods to enforce conditions and signal problems.