From: Joel VanderWerf <vjoel@...>
Date: 2003-11-29T08:53:40+09:00
Subject: Re: re-raising an exception with the original backtrace

David A. Black wrote:
> Hi --
> 
> On Sat, 29 Nov 2003, Joel VanderWerf wrote:
> 
> 
>>Is there a way to re-raise an exception without replacing its backtrace 
>>with the current one?
>>
>>The reason I want to do this is that I am catching an exception and 
>>passing it to a handler. The handler needs to make a decision and then, 
>>possibly, re-raise the same exception, preferrably with the same 
>>information as it originally contained.
>>
>>But if I do the following, the exception takes on the backtrace of the 
>>handler, which has lost some information. I know I can add the original 
>>backtrace to the exception's message, but that's not the same thing...
>>
>>$ cat exception.rb
>>
>>   class MyError < StandardError; end
>>
>>   def foo
>>     raise MyError
>>   end
>>
>>   def main
>>     foo
>>   rescue Exception => e
>>     handler(e)
>>   end
>>
>>   def handler(e)
>>     ok = false
>>     if ok
>>       return true
>>     else
>>       puts "backtrace:", e.backtrace, "---"
>>       raise e
> 
> 
> I think if you just do 'raise' there, instead of 'raise e', it will
> propagate the original exception and backtrace.

Ooh, that's nice. I thought 'raise' would just re-raise the _last_ 
exception, but it's actually sensitive to the dynamic context. The 
example below would fail in the former case, but since it succeeds, I 
guess raise looks back down the stack and re-raises the first exception 
it finds there?

$ cat exception.rb

   class MyError < StandardError; end

   def foo
     raise MyError
   end

   def main
     foo
   rescue Exception => e
     handler(e)
   end

   class OtherError < StandardError; end
   def do_something_that_might_use_exceptions
     raise OtherError
   rescue Exception => e
     p e
     false
   end

   def handler(e)
     ok = do_something_that_might_use_exceptions
     if ok
       return true
     else
       puts "backtrace:", e.backtrace, "---"
       raise
     end
   end

   main

$ ruby exception.rb
#<OtherError: OtherError>
backtrace:
exception.rb:4:in `foo'
exception.rb:8:in `main'
exception.rb:31
---
exception.rb:4:in `foo': MyError (MyError)
         from exception.rb:8:in `main'
         from exception.rb:31