Exception/Error division in D

[Denis Shelomovskij]

Let's talk about an abstract situation without caring about breaking 
existing code, current docs, implementation etc.

Definitions:
* an Exception is something that tigers scope guards and executes 
catch/finally blocks if thrown;
* an Error is something that doesn't do it.

As a result _we can't do any clean-up if an Error is thrown_ because 
scope guards and catch/finally blocks aren't executed and the program is 
in invalid state because of this. Of course it's theoretically possible 
to code without scope guards and catch/finally blocks but isn't 
applicably for a real project. E.g. in some editor if and Error is 
thrown there is no ability to save opened documents.


Main Question: What do you think can be an Error?

Can "Integer Divide by Zero" be an Error? Definitely, not.

Can "Access Violation" be an Error? No, because it's very common to 
access a field/virtual member function of a null object.

Can "Out of memory" be an Error? No, because e.g. if I read a user file 
that require me to create a large array (> 100 MiB, e.g.) I don't want 
to crash, but just tell, that "Dear user, the file can't be opened 
because it requires..."


So what am I think can be an Error? IMHO, nothing. Because throwing 
everything can indicate that program in very good/terribly bad state and 
compiler doesn't know anything about that. And because fatal error is 
fatal the program should just try to print error and close instead of 
throwing something.


Let's now return to the real D world. Current implementation treats 
Errors as Exceptions for now. Documentation keeps silence. All listed 
"can't be an error" cases are Errors (and it's terrible).

So why do we has Exception/Error division in D? Because of nothrow. 
Personally I don't need nothrow for that high cost of making D unusable 
for me. Lets realize and solve Exception/Error problem and solve nothrow 
in the second place.


Related links:
http://forum.dlang.org/thread/1566418.J7qGkEti3s@lyonel
http://d.puremagic.com/issues/show_bug.cgi?id=8135
http://d.puremagic.com/issues/show_bug.cgi?id=8136
http://d.puremagic.com/issues/show_bug.cgi?id=8137


P.S.
By the way, the only problem I see in current implementation is a luck 
of "Object finalized" assertion ("True disposable objects (add 
"Finalized!" assertion)" NG thread that didn't interest anybody).

-- 
Денис В. Шеломовский
Denis V. Shelomovskij