Can you rescue from specific errors with messages in ruby?

Question

I'm trying to understand how errors propogate between classes in Ruby. I have this so far:

class User
  def charge
    puts "charging order soon"
    raise RuntimeError.new("This is a runtime error")
  rescue ArgumentError
    puts "should never gets here"
  end
end

class Runner
  def run
    begin
      User.new.charge
    rescue RuntimeError => e
      puts e.message
    end
  end
end

Runner.new.run

When I run this, I get this which seems right:

$ ruby errors.rb
charging order soon
This is a runtime error

Inside runner, can I rescue from the RuntimeError with a specific message? If I have multiple RuntimeErrors being raised around my application, is there any way for the Runner's rescue clause to be raised only for RuntimeErrors with specific messages?

Answer 1

See https://stackoverflow.com/a/23771227/2981429

If you call raise inside a rescue block, the last raised exception will be re-raised.

In your exception block, you can check the message and choose to re-raise or not:

begin
  User.new.charge
rescue RuntimeError => e
  case e.message
  when "This is a runtime error"
    # put your handler code here
  else
    raise # re-raise the last exception
  end
end

However if it's your goal to solely rescue errors that you yourself raise manually, then it's probably easier to define a custom error class instead:

class MyError < StandardError; end

Then instead of raise RuntimeError.new("message") use raise MyError.new("message"), and rescue it normally:

begin
  User.new.charge
rescue MyError => e
  # handler
end

This way you don't have to worry about your rescues interfering with the built-in exceptions.