From: Joe Van Dyk <joevandyk@...>
Date: 2005-09-02T05:18:52+09:00
Subject: Re: custom exceptions

On 8/30/05, Eric Hodel <drbrain@segment7.net> wrote:
> On 30 Aug 2005, at 10:52, Joe Van Dyk wrote:
> 
> > When should you define your own exceptions?  Any rules of thumb?
> 
> I define custom exceptions when I need to differentiate them from the
> built-in exceptions.
> 
> > I'm writing an application that runs on a computer and listens for
> > requests to start, kill, and view log files for other applications on
> > that computer.  Say I get a request to start an application on that
> > computer and the requested executable doesn't exist.  Should I throw a
> > custom exception then?  Or, say an application dies unexpectedly.
> > What should I use for an exception then?
> 
> Sometimes an exception with a message is enough.  Other times I need
> to know the difference between two RuntimeErrors with different
> messages.
> 
> I create my own exceptions for the second case because rescue
> FooError is easier than reading exception messages.

Should the custom exception be in its own class?  Or in a module?

Say I have the following class

class NodeManager
  def start_application args
  end
end 

There are a few different ways start_application could fail:
  - bad arguments
  - user attempted to start the same application twice (which is an
error for us)
  - not enough resources available

I want to distinguish between those different reasons.  So, would I
want to have something like this:

module JoesExceptions # figure out some name
  class BadArgumentsForApplication < Exception
  end

  class ApplicationStartedTwice < Exception
  end

  class NotEnoughResourcesForApplication < Exception
  end
end

And then, in start_application:

def start_application args
  # if arguments are bad
  raise BadArgumentsForApplication
  # and so on
end