From: Yohanes Santoso <ysantoso-rubytalk@...>
Date: 2007-11-08T06:57:42+09:00
Subject: Re: Rambling: try/catch

Casimir P <pikselNOSPAMMi@welNOSPMAMho.com> writes:

> Just had an vague idea about generic java "try-catch" converted to ruby-
> world I would like some feedback on.
>
> I'm building a "microscopic CMS" system to learn more ruby, and of course 
> it would be good to implement proper error catching system. Try-catch is 
> what java uses.
>
> Now the vague idea was to create one error catching function, and to 
> throw all things that might fail (say loading a file) "through" that 
> function. Passed as a code block? I am not sure, it keeps slipping from 
> me. :)
>
> What do you think?
>
> -- 
> Casimir Pohjanraito 
> Art Portfolio: http://csmr.dreamhosters.com

The idea of passing around a signal handler is not new. 
It allows you to handle low-level condition in a high-level manner.

Here is one, done à la SRFI-34.



def with_exception_handler(k = nil) 
  begin
    yield
  rescue Exception => e
    if k then if k.call(e) == :retry then retry end end
  end
end


limited_patience_handler = 
  lambda {
    # can be called to handle exception three times.
    # after that it loses patience and just reraise things.
    i=[1,2,"last one"]
    lambda{|e| if not i.empty?
                 puts "Got exception ##{i.shift}: #{e.inspect}" 
                 :retry
               else
                 puts "I'm done covering for you. Reraising to my supervisor"
                 raise e
               end}}.call

with_exception_handler(limited_patience_handler) {
  puts "Raising"
  raise "Test exception"
  puts "Never reach here"
}



****Output:

Raising
Got exception #1: #<RuntimeError: Test exception>
Raising
Got exception #2: #<RuntimeError: Test exception>
Raising
Got exception #last one: #<RuntimeError: Test exception>
Raising
I'm done covering for you. Reraising to my supervisor
/tmp/x.rb:24: Test exception (RuntimeError)
	from /tmp/x.rb:3:in `with_exception_handler'
	from /tmp/x.rb:22



YS