From: Joel VanderWerf <vjoel@...>
Date: 2008-04-03T04:09:29+09:00
Subject: Re: Does Ruby support exception wrapping (exception chaining

Joel VanderWerf wrote:
> Brian Hartin wrote:
>> Joel,
>>
>> This doesn't really provide the main benefit to exception wrapping 
>> mechanism:  the ability to wrap low-level exceptions in an appropriate 
>> higher-level exception and yet not lose the entire stack trace, or the 
>> types and messages of the low-level exceptions.  Exception wrapping 
>> (a.k.a. chaining) in Java (a good model for this feature, I think) 
>> makes the idea of a 'causal Exception' part of the class (via a 
>> 'cause' field), and the stack trace: an Exception's stack trace 
>> includes the causal 'chain' of Exceptions automatically.

Here's an alternative, perhaps with too much magic:


class ServiceNotFoundException < StandardError
   attr_reader :cause
   def initialize cause
     @cause = cause
   end

   DEFAULT_MESSAGE = "The service could not be contacted"

   def self.[](e, msg = DEFAULT_MESSAGE)
     e2 = new(e)
     raise e2, "#{msg}: #{e.message}", e.backtrace
   end
end

begin
   begin
     raise "foo"
   rescue Exception => e
     ServiceNotFoundException[e]
   end
rescue Exception => e_outer
   puts e_outer
   p e_outer.cause
   puts e_outer.backtrace.join("\n  ")
end

__END__

Output:

The service could not be contacted: foo
#<RuntimeError: foo>
wrapped-exceptions.rb:17

-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407