[ruby-talk:6015] Re: Exception handling

David Suarez de Lis <[e-mail]> writes:

> Hi all,
> 
> from the User's Manual:
> http://www.math.sci.hokudai.ac.jp/~gotoken/ruby/ruby-uguide/uguide25.html
> 
> ruby> begin
> ruby|   file = open("/tmp/some_file", "w")
> ruby|   # something to do
> ruby| ensure
> ruby|   file.close
> ruby| end
> 
> Let's suppose /tmp/some_file does not exist: then I get, from the ensure part: 
> 
> NameError: undefined method `close' for nil
> 
> Obviously, as open has failed in opening the file, 'file' references 'nil'...
> 
> How do we cope with this kind of things?

The issue really is that there are two levels of exception here, but
there's only one exception handler.

Let's look at it a different way.

We'll start by opening the file.

   file = open(...)

hmmm, we think, that might raise an exception which we'll want to
handle, so let's wrap it:

   begin
     file = open(..)

     file.close
   rescue ...
     do whatever
   end

Note how we can just use 'file.close', as we can assume the file
object is OK.

Now we need to process the contents:

   begin
     file = open(..)

     process..
     process..

     file.close
   rescue ...
     do whatever
   end


hmmm, we say again, the processing might itself raise an exception,
which would bypass the file.close, so we need another wrapper:

   begin
     file = open(..)

     begin
       process..
       process..
     ensure
       file.close
     end
   rescue ...
     do whatever
   end


et voila!


However, you'd probably write this more simply using the block form of 
File.open:

   File.open(...) do |file|
     process...
   end

This does the necessary handling to ensure that the file is closed at
the end of the block's execution.



Regards



Dave