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Error Handling in Ruby: Part I

In this article, we’re going to explore the following topics:

• what is an error in Ruby?
• how to throw & catch an error?
• errors & magic variables

What’s an error in Ruby?

As Ruby is a fully object-oriented language, an error is an instance of a class that includes the Exception class in its ancestor chain

irb> RuntimeError.ancestors.include? Exception
 => true
irb> NoMethodError.ancestors.include? Exception
 => true

That’s why an error is commonly called an “exception” in Ruby.

NB: feel free to have a look to the Ruby Object Model: Part I article if you’re unfamiliar with the ancestor chain in Ruby.

As an error is a class, Ruby provides a set of methods to deal with an occurring error.

These methods are defined in the Exception class and used by all of its children.

For example, the Exception#message method returns the message associated with the error when the Exception#backtrace method returns the error backtrace.

Feel free to browse the official documentation for further information.

How to throw & catch an error?

Kernel#raise is in charge of raising errors in Ruby

irb> raise 'an error occurred'
RuntimeError: an error occurred
irb> raise NoMethodError, 'an error occurred'
NoMethodError: an error occurred

In the first call to Kernel#raise, we can see that the method raises a RuntimeError — as no error class is explicitly specified as an argument.

On the other hand, the second call to Kernel#raise the raises a NoMethodError — as we defined it as the first argument.

These exceptions can be caught by a rescue clause within a begin...end block

produces

NoMethodError: an error occurred

Here, the rescue clause caught a NoMethodError exception raised by the call to Kernel#raise within the begin...end block.

begin...end is used to compartmentalize blocks of code depending on the possible errors that can occur for each of them

produces

ZeroDivisionError: divided by 0
NoMethodError: undefined method `odd?' for "my string":String

Note that in the first begin...end block, the3 / 0 operation internally raised ZeroDivisionError via the Integer#/ method.

Feel free to read the Numbers and Class Hierarchy in Ruby article if you are unfamiliar with the Integer class and its ancestor chain.

It’s the same for the second begin...end block where the BasicObject#method_missing Ruby hook method raised NoMethodError.

Feel free to read the Metaprogramming: Ruby Hook Methods article if you are unfamiliar with the hook methods in Ruby.

Errors & magic variables

Ruby provides 2 magic variables that are set when an exception is raised:

• $! which contains the raised exception
• $@ which contains the exception backtrace

produces

NoMethodError: undefined method `odd?' for "my string":String
/workspace.rb:87:in `eval'
/workspace.rb:87:in `evaluate'
...
/bin/irb:11:in `<main>'

These variables are cleared — set to nil — when the rescue block ends.

Feel free to have a look to the Part II.

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