Conclusions of the exception discussion

[Jonathan M Davis]

Okay, the "The Right Approach to Exceptions" thread is a huge, confusing mess 
at this point without a clear, definitive conclusion, and we need one. So, I'm 
posting here, in a new thread, what appears to me to be the conclusion that 
that thread comes to and see if we can get some sort of consensus on this.

There are three things that that thread appears to conclude that we should do 
to improve the current situation with exceptions in Phobos and in D in 
general:

1. Phobos' exceptions should be reorganized into a class hierarchy designed 
such that the exceptions can be reusable outside of Phobos. We will no longer 
follow the policy of an exception per modules. A module may declare 0 to many 
exceptions depending on the problems that it would be reporting by throwing 
exceptions, and some modules will reuse the exceptions from other modules 
where appropriate.

The exact set of exceptions that we're going to end up with and how the 
hierarchy will be laid out has yet to be determined. It may or may not be 
similar to what Java and C# have. We will probably look at what they have as 
examples but will do whatever we decide works best for D and Phobos, and that 
may or may not have any relation to what those languages do. This will likely 
start with a minimal number of exceptions and will grow as necessary rather 
than trying to create a lot of exception types up front which we may not ever 
need. Regardless, we may know that we want a hierarchy, but the exact 
hierarchy has yet to be determined.


2. We should add a feature to the language to make it possible to catch 
multiple exception types with a single catch statement. There are two 
suggestions.

A. Do something similar to what Java 7 is doing and make it possible to simply 
list the exceptions that a particular catch statement accepts. That catch 
block will then catch any of the matching exceptions, and the type of the 
variable will be their most derived common type. One possible syntax would be

catch(e : Ex1, Ex2, Ex3) {}

B. Make it possible to add a condition which would be run when the catch 
statements are processed to allow you to catch based on other stuff in addition 
to the type. The condition would be a condition run at runtime rather than a 
compile time condition like template constraints. e.g.

catch(MyException e) if(e.prop == value) {}

It is unclear which we want to do or whether we want to do both. So, that 
still needs to be discussed further. But it's fairly clear that the majority 
want a feature along the lines of one or both of those two suggestions. 
However, regardless of which we choose, someone is going to have to take the 
time to implement it, since odds are that Walter isn't going to do it. So, 
whether we end up with a feature along these lines is highly dependent on 
whether anyone is willing to take the time to implement it and get it accepted 
by Walter.


3. We should add a Variant[string] property to Exception. Every derived class 
will then populate it with the values of its member variables in their 
constructors, and code outside of the exception classes can choose to insert 
other values into the table which are useful for the particular programs that 
they're in but which don't make sense to put in the exception types 
themselves.

A free function will then be designed and implemented which will take some 
kind of format string along with an exception and will then allow you to 
generate a message from that exception using whatever formatting you want 
using the hash table to generically obtain the values from the exception 
without needing to catch or know the exact type of the exception. e.g.

catch(Exception e)
{
    writeln(magicFormattingFunction(formatStr, e));
}

This Variant[string] property would _not_ replace having direct member 
variables in derived exceptions. Rather, it would allow us to make sure that 
we only put member variables in derived exceptions when they belong there, and 
code which wants additional data but not a new exception type can use the hash 
table to hold it. And of course, it also gives us the foundation to build the 
fancier string formatting capabalities.


There were other ideas that were discussed in the thread, but I think that 
these are the ones that we have at least some consensus on. However, given the 
mess that thread is, we really should make it clear in a separate thread (this 
thread) that we have a consensus that these are indeed the things that we want 
to pursue to improve exceptions in D and Phobos. Thoughts? Opinions?

- Jonathan M Davis