Exception/Error division in D

Steven Schveighoffer schveiguy at yahoo.com
Wed May 30 08:05:39 PDT 2012


On Wed, 30 May 2012 05:32:00 -0400, Don Clugston <dac at nospam.com> wrote:

> On 30/05/12 10:40, Jonathan M Davis wrote:
>> On Wednesday, May 30, 2012 10:26:36 deadalnix wrote:
>>> The fact that error don't trigger scope and everything is nonsensial.
>>
>> If an Error is truly unrecoverable (as they're generally supposed to  
>> be), then
>> what does it matter? Something fatal occured in your program, so it
>> terminates. Because it's an Error, you can get a stack trace and report
>> something before the program actually terminates, but continuing  
>> execution
>> after an Error is considered to be truly _bad_ idea, so in general, why  
>> does
>> it matter whether scope statements, finally blocks, or destructors get
>> executed? It's only rarer cases where you're trying to do something like
>> create a unit test framework on top of assert that you would need to  
>> catch an
>> Error, and that's questionable enough as it is. In normal program  
>> execution,
>> an error is fatal, so cleanup is irrelevant and even potentially  
>> dangerous,
>> because your program is already in an invalid state.
>
> That's true for things like segfaults, but in the case of an  
> AssertError, there's no reason to believe that cleanup would cause any  
> damage.

There's also no reason to assume that orderly cleanup *doesn't* cause any  
damage.  In fact, it's not reasonable to assume *anything*.

Which is the point.  If you want to recover from an error, you have to do  
it manually.  It should be doable, but the default handling should not  
need to be defined (i.e. implementations should be free to do whatever  
they want).

But there is no reasonable *default* for handling an error that the  
runtime can assume.

I'd classify errors/exceptions into three categories:

1. corruption/segfault -- not recoverable under any reasonable  
circumstances.  Special cases exist (such as a custom paging mechanism).
2. program invariant errors (i.e. assert errors) --  Recovery is not  
defined by the runtime, so you must do it manually.  Any decision the  
runtime makes will be arbitrary, and could be wrong.
3. try/catch exceptions -- these are planned for and *expected* to occur  
because the program cannot control it's environment.  e.g. EOF when none  
was expected.

The largest problem with the difference between 2 and 3 is the actual  
decision of whether an exceptional case is categorized as 2 or 3 can be  
decoupled from the code that decides between them.

For example:

double invert(double x)
{
    assertOrEnfoce?(x != 0); // which should it be?
    return 1.0/x;
}

case 1:

void main()
{
     writeln(invert(0)); // clearly a program error
}

case 2:

int main(string[] args)
{
    writeln(invert(to!double(args[1])); // clearly a catchable error
}

I don't know of a good way to solve that...

-Steve


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