Rescue's Elegant Trick for Knowing Which Exceptions to Catch

You probably know how to ask Ruby to rescue specific exceptions. But how does Ruby know if a particular exception meets your criteria? In this article, we'll walk through Ruby's simple exception matching mechanism and see how we can use it to our advantage.

If you've worked with Ruby's exceptions before, you know you can specify which exceptions get rescued and which are not:

begin
  raise ArgumentError
rescue ArgumentError
  # Rescues the `ArgumentError`
end

...and you probably know that when you rescue a "parent" you rescue all of its "children" as well.

begin 
  raise ArgumentError
rescue StandardError
  # Rescues `ArgumentError`, because it inherits from 
  # `StandardError`
end

When I say "parent" and "child" I'm simply referring to class inheritance. Somewhere deep in the Ruby source code there is something equivalent to this:

class ArgumentError < StandardError
   ...
end

An interesting trick

Here's my question: how does Ruby know if any given exception inherits from the class you specified?

The most obvious approach would be to use the is_a? or kind_of? method. We could imagine it looking like this:

if the_exception.is_a?(StandardError)
   # do the rescue
end

But that's not what happens. Instead, Ruby uses the more interesting === operator.

if StandardError === the_exception
   # do the rescue
end

If you've never used a === b, it usually answers the question "does a inherently belong to the group defined by b"? Here are some examples:

(1..10) === 5             # true
('a'..'f') === "z"        # false

String === "hello"        # true
String === 1              # false

/[0-9]{3}/ === "hello123" # true
/[0-9]{3}/ === "hello"    # false

Because === is just an ordinary ruby method like ==, we can define it ourself:

class RedThings
   def self.===(thing)
     thing.color == :red
   end
end

So, what do we know? We know that rescue uses === to determine which exceptions get rescued. And we know that we can define our own === method. That means we can create a class that decides on-the-fly which exceptions are rescued:

class SevereMatcher
  def self.===(exception)
    exception.message =~ /severe/    
  end
end

begin
  raise RuntimeError, "Something severe happened"
rescue SevereMatcher
  # rescues all exceptions with the word "severe" in
  # the message, regardless of class.
end

Once you know this trick, the only limit is your imagination.

Conclusion

I'll admit: you may not ever need to create a dynamic exception matcher. But this is a really interesting example of how a seemingly-trivial implementation detail like using === instead of kind_of? makes Ruby much more flexible and interesting.

author photo

Starr Horne

Starr Horne is a Rubyist and Chief Javascripter at Honeybadger.io. When he's not neck-deep in other people's bugs, he enjoys making furniture with traditional hand-tools, reading history and brewing beer in his garage in Seattle.

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