From: Brian Candler <B.Candler@...>
Date: 2003-07-05T07:07:14+09:00
Subject: Re: The best way to catch errors

On Sat, Jul 05, 2003 at 05:58:37AM +0900, Nigel Gilbert wrote:
> My ruby program processes web pages, which it collects using the 
> methods in net/http.  Every now and again, it tries to access a web 
> page which is no longer available, is protected with a password, is on 
> a slow server that yields a timeout, and so on.  What I'd like to do, 
> therefore, is protect the page fetching method with a 
> begin...rescue..end construction that will cause the program simply to 
> move on to the next web page that needs to be fetched if there is an 
> access error.  However, I don't want to trap any other kinds of error 
> (e.g. I'd still like to be able to interrupt the program with ^C).

^C does not generate an exception. Try it:

begin
  loop do
    puts "hello"
    sleep 1
  end
rescue Exception => e
  print 
end

# ^C gets you out of this program quite easily

> Now, the obvious way of doing this is to append exception classes for 
> all the different types of web access error to the rescue clause, thus:
> 
> rescue ProtocolError, ProtoSyntaxError, ..., TimeoutError => e
> 
> where ... represents all the other possible errors.  As far as I can 
> see, however, there are two problems with this.  The first is that 
> ProtocolError (etc.) is defined in net/protocol.rb, which is 'required' 
> by 'net/http', which in turn is required by my program.  The 
> consequence is that my program has no knowledge of ProtocolError and 
> the compiler objects that it is an uninitialized constant.

Err...?

If you do 'requre "net/http"' then the constant is available in your
program, end of story. However you need to give the correct constant name,
because it's defined in a different *module*: e.g.

   rescue Net::ProtocolError

> The second 
> problem is that I don't know how to construct a complete list of all 
> the different types of exception that I should include in my rescue 
> clause.  I've already discovered relevant exceptions defined in 
> http.rb, protocol.rb and timeout.rb, but there may be more.  How can I 
> find a list of all the exceptions that could occur?

That's down to the modules. But exceptions are organised in a hierarchy so
usually you can find a sensible parent exception which catches all the
things you want. Notice, for example, that all the exceptions in
net/protocol.rb are subclasses of Net::ProtocolError, so a rescue on just
that will catch all the more specific subclass errors as well. The
higher-level exceptions RuntimeError or StandardError might also be
appropriate for catching a wider range of exceptions.

Regards,

Brian.