From: Greg Millam <ruby-talk@...>
Date: 2005-04-29T11:58:35+09:00
Subject: Compressing or modifying exception backtraces

In PageTemplate, a project of mine with Brian Wisti, we provide a 
framework of sorts that has a large number of layers between the call to 
PageTemplate.output, and a point where the user's own code is later called.

If an error exists in the user's code, it looks somewhat like this:

/home/walker/public_html/deafcode/rbsource/user_instance.rb:3:in 
`fullname': undefined local variable or method `last_nam' for 
#<User:0xb7b92e38> (NameError)
   from /usr/lib/ruby/site_ruby/1.8/PageTemplate/parser.rb:86:in `send'
   from /usr/lib/ruby/site_ruby/1.8/PageTemplate/parser.rb:86:in `get'
   from /usr/lib/ruby/site_ruby/1.8/PageTemplate/parser.rb:78:in `each'
   from /usr/lib/ruby/site_ruby/1.8/PageTemplate/parser.rb:78:in `get'
   from /usr/lib/ruby/site_ruby/1.8/PageTemplate/commands.rb:248:in `output'
   from /usr/lib/ruby/site_ruby/1.8/PageTemplate/commands.rb:145:in `output'
   from /usr/lib/ruby/site_ruby/1.8/PageTemplate/commands.rb:145:in `map'
   from /usr/lib/ruby/site_ruby/1.8/PageTemplate/commands.rb:145:in `output'
     ... 13 levels...
   from /usr/lib/ruby/site_ruby/1.8/PageTemplate/commands.rb:145:in `output'
   from /usr/lib/ruby/site_ruby/1.8/PageTemplate/commands.rb:187:in `output'
   from /home/walker/public_html/deafcode/rbsource/site.rb:90:in `output'
from index.cgi:23

About 23 levels of output of PageTemplate alone.

What I'd like is to shorten that to one:

/home/walker/public_html/deafcode/rbsource/user_instance.rb:3:in 
`fullname': undefined local variable or method `last_nam' for 
#<User:0xb7b92e38> (NameError)
   from /usr/lib/ruby/site_ruby/1.8/PageTemplate/commands.rb:187:in `output'
   from /home/walker/public_html/deafcode/rbsource/site.rb:90:in `output'
from index.cgi:23

Is there any safe way to do this?

Alternately, I'd also like to potentially generate output like this:

/home/walker/public_html/deafcode/rbsource/user_instance.rb:3:in 
`fullname': undefined local variable or method `last_nam' for 
#<User:0xb7b92e38> (NameError)
   from Template:user_profile.html in "Value: user.fullname"
   from Template:user_profile.html in "If: user"
   from Template:main_page.html in "Include user_profile.html"
   from Template:main_page.html in "Loop users: user"
   from /usr/lib/ruby/site_ruby/1.8/PageTemplate/commands.rb:187:in `output'
   from /home/walker/public_html/deafcode/rbsource/site.rb:90:in `output'
from index.cgi:23

Considerably more useful for the developer using PT.

Is it possible to modify the backtrace like this?

Thanks,

- Greg