The Budding Rubyist

I recently wrote a case statement (case/when) in which the branching condition was based on a passed-in class name. Simplified, it looked something like this:

class CommandExecutor

  def execute(command)
    case command
    when StopClientCommand.name
      print "Stopping the Client..."
    when StartClientCommand.name
      print "Starting the Client..."
    when DisableClientCommand.name
      print "Disabling the Client..."
    else
      puts "Can't handle command: #{command}"
      return
    end

    puts "DONE."
  end

end

class StopClientCommand; end
class StartClientCommand; end
class DisableClientCommand; end

executor = CommandExecutor.new
executor.execute 'StopClientCommand'
executor.execute 'StartClientCommand'
executor.execute 'DisableClientCommand'

This code totally worked…

Stopping the Client...DONE.
Starting the Client...DONE.
Disabling the Client...DONE.

…but it gnawed at me that I was passing a string representation of the class name. It just felt dirty.

I pinged a colleague for input, and his reaction was immediate: “Oh, don’t pass a string; just pass and check for the actual class. It’s the more idiomatic way of doing class-checking.”

Ah, good to know. So…I made a couple of tweaks:

class CommandExecutor

  def execute(command)
    case command
    when StopClientCommand    # <-- check against a class, not a class name
      print "Stopping the Client..."
    when StartClientCommand
      print "Starting the Client..."
    when DisableClientCommand
      print "Disabling the Client..."
    else
      puts "Can’t handle command: #{command}"
      return
    end

    puts "DONE."
  end

end

class StopClientCommand; end
class StartClientCommand; end
class DisableClientCommand; end

executor = CommandExecutor.new
executor.execute StopClientCommand    # <-- pass a class, not a string
executor.execute StartClientCommand
executor.execute DisableClientCommand

And ran it…

Can't handle command: StopClientCommand
Can't handle command: StartClientCommand
Can't handle command: DisableClientCommand

Um. What the heck? Maybe I fat-fingered something in there? I threw in a little ghetto-debugging for a sanity check and got the same results:

Passed-in command   : StopClientCommand
Can't handle command: StopClientCommand
Passed-in command   : StartClientCommand
Can't handle command: StartClientCommand
Passed-in command   : DisableClientCommand
Can't handle command: DisableClientCommand

The reason?

Case statements are evaluated using the Object#=== (Case Equality) method which, according to the documentation, is “typically overridden by descendents [sic] to provide meaningful semantics in case statements.”

Any chance that the Class class overrides this method? You betcha. Its case-equality behavior is to evaluate whether the case condition represents an instance of that class, NOT if it is a matching class definition.

With that in mind, I made one last change:

class CommandExecutor

  def execute(command)
    case command.new  # <-- check against an instance of the class
    when StopClientCommand
      print "Stopping the Client..."
    when StartClientCommand
      print "Starting the Client..."
    when DisableClientCommand
      print "Disabling the Client..."
    else
      puts "Can't handle command: #{command}"
      return
    end

    puts "DONE."
  end

end

And voila:

Stopping the Client...DONE.
Starting the Client...DONE.
Disabling the Client...DONE.

We finally have the correct output, and we’re using idiomatic Ruby to get there.