Introduction to Ruby custom errors

Introduction to Ruby custom Errors

Generic Ruby errors could be helpful and informative but why not make them even more informative and helpful by tweaking them a little bit. Ever wanted to make your own custom Ruby error ? Now you got a chance. We would need two files:

app.rb and errors.rb for simplicity let’s store them in one folder. Let’s start with errors.rb file:

module Exceptions
  class FooError < ScriptError
  end
end

class BarError < NameError
end

What is going here ? We made a class which inherits from bulid in Ruby class ScriptError and we wrap it in module Exceptions. Second one simillar but without module around it.

Now it’s time for app.rb

require_relative 'errors.rb'

class App
  def self.raiser(input)
    if input == "foo"
      raise Exceptions::FooError
    elsif input == "bar"
      raise BarError, "This is bar!"
    else
      p "Nothing to raise"
    end
  end
end

In line one we are requiring errors.rb file to have access to its classes. Next we define a simple class App with simple if statement which output is based on provided later on input data. Thing is what is happening when certain inputs are provided. If "foo" then we raise an error called FooError which is inside Exceptions module. When it is "bar" then we raise, a BarError with additional message "This is bar!" to give us even more info and maybe hint how to deal with this kind of exception later on. Now how to make it work? At the bottom of app.rb file let’s call an App class with raiser method. For example like this:

call = App.raiser("foo")

After saving our work and runing our program with ruby app.rb we should get:

app.rb:6:in ‘raiser’: Exceptions::FooError (Exceptions::FooError) from app.rb:15

But after changing from "foo" to "bar" in call variable output of app.rb will be:

app.rb:8:in ‘raiser’: This is bar! (BarError) from app.rb:16

We are getting a little more information with it than plain generic standarError and in the future this little info could make our work with debugging a lot easier and faster. Of course this is dead simple example you could make your errors more sophisticated. For more reference hit: ruby-doc.org/Exception

Cheers