Question

following snippet is from rails code

  def rescue_from(*klasses, &block)
    options = klasses.extract_options!

    unless options.has_key?(:with)
      if block_given?
        options[:with] = block
      else
        raise ArgumentError, "Need a handler. Supply an options hash that has a :with key as the last argument."
      end
    end

    klasses.each do |klass|
      key = if klass.is_a?(Class) && klass <= Exception
        klass.name
      elsif klass.is_a?(String)
        klass
      else
        raise ArgumentError, "#{klass} is neither an Exception nor a String"
      end

      # put the new handler at the end because the list is read in reverse
      self.rescue_handlers += [[key, options[:with]]]
    end
  end
end

Notice the operator <=

what is that?

Answer 1

See http://ruby-doc.org/core/classes/Module.html#M001669 for documentation on all the comparison operators exposed by Modules (and therefore Classes).

In this specific case:
“Returns true if mod is a subclass of other or is the same as other. Returns nil if there‘s no relationship between the two. (Think of the relationship in terms of the class definition: “class A < B” implies “A < B”).”