Exception vs. Error

[Sean Kelly]

Stewart Gordon wrote:
> 
> You're probably right.  I'm wondering whether it would make sense for 
> Exception to derive from Error.

I don't think it would.  If Errors are not recoverable, then that 
suggests that Exceptions would not be recoverable as well.  The reverse 
problem holds when Exception derives from Error.

 > Java has a class Throwable, from which Exception and Error are derived.

We considered this for Tango, but decided against it because it still 
strictly defines what is and is not a recoverable error based on which 
parent class is chosen.  For common applications this might be fairly 
straightforward, but things are less clear-cut when designing an 
operating system kernel, for example.  As a systems programming 
language, I feel that D can't make the same assumptions that Java 
typically can.

> In D, you can throw an object of any class, but having a common base 
> class for exceptions and errors is still useful as it enables some of 
> the workings to be shared.  At the moment it's really just the exception 
> message and hence toString, but who knows if it'll eventually have stuff 
> like getting a stack trace?
>
> What is the 'next' member of Error for?  Phobos doesn't seem to use it 
> at all.

Tango has these in Exception, and they are intended for something like this:

     void fnA()
     {
         try
         {
             fnB();
         }
         catch( Exception e )
         {
             throw new Exception( "Call failed with: ", e );
         }
     }

     void fnB()
     {
         throw new Exception( "Danger, Will Robinson!" );
     }

In essence, there are times when an enclosing routine can't entirely 
recover from an exception it catches, and it may occasionally be useful 
to re-categorize the exception or to provide additional information, but 
also to retain the original exception for later inspection.  One example 
might be a low-level IO failure that causes in a cascade of failures up 
the call stack.  At some high level the user code might catch and handle 
a LoginFailureException, but that might include a chain of: 
InvalidProtocolException, InvalidSocketStateException, and 
CorruptSocketBufferException.


Sean