Question

We can put a class or module after a rescue statement, but in the code below, I see a method following rescue, which does not fit into this pattern. How is it working and how is it producing the output it has been designed to show?

def errors_with_message(pattern)
  # Generate an anonymous "matcher module" with a custom threequals
  m = Module.new
  (class << m; self; end).instance_eval do
    define_method(:===) do |e|
      pattern === e.message
    end
  end
  m
end
puts "About to raise"
begin
  raise "Timeout while reading from socket"
rescue errors_with_message(/socket/)
  puts "Ignoring socket error"
end
puts "Continuing..."

Output

About to raise
Ignoring socket error
Continuing...

Answer 1

Rescue requires a class or a module, true. So, that method creates an anonymous module with special behaviour. You see, when rescue searches for a handler, it applies === operator to exception classes/modules you provided, passing as an argument the actual exception.

begin
  # do something
rescue MyCustomError
  # process your error
rescue StandardError
  # process standard error
end

So, if StandardError (or one of its descendants) was raised, the first handler will be skipped and second handler will be matched.

Now, the module from errors_with_message is special. It redefines threequals operator to match on exception message. So, if an error was raised and its message contains word “socket”, this handler will match. Cool trick, huh?