From: Kurt Stephens <ks@...>
Date: 2008-02-11T11:25:55+09:00
Subject: Re: Restartable Exceptions

Brent Roman wrote:
> I think a "restartable" Exception could be implemented by
> creating an Exception with an instance variable that holds
> a Continuation.  The Continuation would assigned when
> the Exception was created such that when .call'd it
> resumed (or restarted) execution at the desired point.
>
> It would be the responibility of the code that initially
> threw the restartable exception to set up the Continuation
> properly.
>   
If we changed raise:

1) take a block for the continuation,
2) return the value of the restartable exception's continuation call

  begin
    File.open_restartable(file, mode) do | fh |
       process_file fh
    end
  rescue RestartableFileOpenError => err
    $stderr.write "Problem opening file: #{err.inspect}: try another? "
    different_file = $stdin.readline
    if different_file
      err.call different_file # does not return.
    end
     raise err
  end

  def File.open_restartable file, mode, &blk
    result = nil

    do
      begin
          result = File.open(file) do | fh |
             yield blk
          end
          file = nil
      rescue Exception => err
        raise RestartableFileOpenError, :error => err, :file => file do 
| new_file |
          file = new_file
        end
      end
   while file

    result
  end

Uncaught restartable errors at top-level could invoke a debugger.
> I believe no new formalisms would be required to implement this.
> However, Continuations are quite expensive.
> Would you want *all* Exceptions to be resumable in this way?
>   
No. Too bad continuations are so expensive in Ruby.  Are they really 
going away in 2.0?

http://kurtstephens.com