Le 24/05/2012 12:27, Denis Shelomovskij a écrit :
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).
The fact that error don't trigger scope and everything is nonsensial.
Today, we know how to implement exception with NO RUNTIME COST when
exception isn't thrown. No reason not to do it, except executable size.
As this is a specific constraint, we may want to enable it by a compiler
switch, but not by default.
I see Error as problem that can occur anywhere in any piece of code.
I think D have some flaw in Exception management. See Segfault vs
NullPointerException discussions in that very forum. Segfault may be OK
for some application, but not for a server app that need to be robust.
Error exists because of nothrow. As some exceptions can be thrown
ANYWHERE, we need a way to separate what is expected to fail, and that
have to be handled to be nothrow, and what can go wrong anywhere
(basically, when druntime cannot do its job for some reasons).
