AresMUSH
A New Breed of MUSH Server

Command Errors

You may already have stumbled upon this, but the code we used in the previous exercise had a problem. If you ran the tinker command witohut specifying any arguments, you’d get an error.

Whenever the code suffers an unexpected failure, you’ll get a red “Oops! The code has lost its mind…” message explaining what the problem is.

Try It! - What Errors Look Like

Let’s see what this looks like. Change the tinker code back to our previous example:

def handle
  piggies = integer_arg(cmd.args)
  if (piggies < 5)
    client.emit_ooc "#{piggies} is a small number of piggies."
  else
    client.emit_ooc "#{piggies} is a lot of piggies!"
  end
end

Now run the tinker command by itself without giving it any arguments. You should get a bunch of red error text including this message:

Error: "undefined method `<' for nil:NilClass"

Try It! - Looking in the Error Log

There’s minimal information about the error printed to the screen, but a lot more is captured in the game’s log file. The log file records most commands (minus some for privacy), errors, and other important game events.

  1. In the web portal, go to Admin -> Logs. (You can open this in a new tab to avoid losing your tinker window).
  2. Select the most recent (highest-numbered) log file.
  3. Search for “ERROR” and near the top you should see an entry like this:
2018-10-23 20:08:50 ERROR - Error in tinker: client=73 error=undefined method `<' for nil:NilClass backtrace=["/home/ares/aresmush/plugins/tinker/commands/tinker_cmd.rb:14:in `handle'", "/home/ares/aresmush/engine/aresmush/plugin/command_handler.rb:25:in `on_command'",
... other junk ...
 "/home/ares/aresmush/engine/aresmush/commands/dispatcher.rb:137:in `with_error_handling'", "/home/ares/aresmush/engine/aresmush/commands/dispatcher.rb:66:in `on_command'"] 

This shows you the exact link of code that had the problem - tinker_cmd.rb line 14 - and what’s called a “stack trace” showing all the method calls leading up to it. We can use this information to find out that our problem is the line of code: if (piggies < 5).

Try It! - Debugging

So now we know where the error is happening, but we still don’t know why. We’ll need to do some debugging. The ‘5’ looks okay, but maybe there’s something wrong with our piggies variable. Let’s add a log statement to our code to figure out what’s going on.

def handle
  piggies = integer_arg(cmd.args)
  Global.logger.debug "Number of piggies was: #{piggies}."
  if (piggies < 5)
    client.emit_ooc "#{piggies} is a small number of piggies."
  else
    client.emit_ooc "#{piggies} is a lot of piggies!"
  end
end

If we log in the log file again (you’ll need to reload the page if you left it open in a different tab), we’ll see a line like this:

2018-10-23 20:15:09 DEBUG - Number of piggies was: . 

We have no number piggies specified! That makes sense, because we didn’t actually type a number in our tinker command. Because we left off the number of piggies, integer_arg(cmd.args) just returns nil. In Ruby, you aren’t allowed to do a comparison between nil and a number - hence the error.

Try It! - Adding An Error Check

So now we know where and why the error is happening. Let’s add an error check to make sure the player specified a number of piggies. Give it a try:

def handle
  piggies = integer_arg(cmd.args)
  if (!piggies)
    client.emit_failure "You didn't say how many piggies you had."
    return
  end
  
  if (piggies < 5)
    client.emit_ooc "#{piggies} is a small number of piggies."
  else
    client.emit_ooc "#{piggies} is a lot of piggies!"
  end
end

Now if we fail to specify an argument, we’ll get an error message and the handler will return (aka stop handling the command.)

This article is part of the Code Quickstart tutorial.