Let's improve D's exceptions

[Steven Schveighoffer via Digitalmars-d]

On 5/13/15 3:24 PM, Jacob Carlborg wrote:
> On 2015-05-13 17:08, Adam D. Ruppe wrote:
>> Have you ever done:
>>
>> if(something) {
>>     import std.conv;
>>     throw new Exception("some error " ~ to!string(some_value));
>> }
>>
>> Don't you hate it?
>>
>> * having to import std.conv to see data from your exception is a pain
>> * it allocates eagerly and thus isn't suitable for a lot of places
>> * inspecting the data can be a pain as the string is unstructured
>>
>> This assumes the data is even bothered to be added. Anyone who has
>> gotten a RangeError in D knows important information is often just
>> dropped!
>>
>> A good solution is to make a new exception subclass for each error type,
>> storing details as data members. However, that's a bit of a pain in D
>> because of all the work you have to do to make a complete subclass:
>
> Yeah, I really hate that people are using plain Exception instead of
> creating a subclass. I'm trying to point this out in pull requests and
> similar but it's hard to get people to listen.
>
> One thing that is _not_ making things better is "enforce" which, if I
> recall correctly, throws Exception by default.

enforce is one of the most needless pieces of phobos:

enforce(cond, message);
vs.
if(!cond) throw new Exception(message);

And the second doesn't mess up inlining.

I think enforce could be boiler-plated better. The only verbose part of 
the if version is the throwing and newing.

template throe(Etype = Exception)
{
    void throe(Args...)(Args args, string file = __FILE__, size_t line = 
__LINE__)
    {
    	throw new Etype(args, file, line);
    }
}

if(!cond) throe(message);

Wait, you're in an io package, and you want to always throw IO exceptions?

alias except = throe!IOException;

if(!cond) except(args, to, ioexception);

Sure, it doesn't return the thing that caused the exception if nothing 
happens. Grepping phobos, this feature is used with enforce about 1% of 
the time. In fact, I didn't even know it had that feature until looking 
it up in the docs just now.

-Steve