What is the point of nothrow?

[Bauss]

On Monday, 11 June 2018 at 00:47:27 UTC, Jonathan M Davis wrote:
> On Sunday, June 10, 2018 23:59:17 Bauss via Digitalmars-d-learn 
> wrote:
>> What is the point of nothrow if it can only detect when 
>> Exception is thrown and not when Error is thrown?
>>
>> It seems like the attribute is useless because you can't 
>> really use it as protection to write bugless, safe code since 
>> the nasty bugs will pass by just fine.
>>
>> I'm aware that it's a feature that nothrow can throw Error, 
>> but it makes the attribute completely useless because you 
>> basically have no safety to guard against writing code that 
>> throws Error.
>>
>> To an extend @safe works, but there are tons of stuff that 
>> throws Error which you can only detect and guard against 
>> manually.
>>
>> So what is the point of nothrow when it can only detect 
>> exceptions you'd catch anyway.
>>
>> To me it would be so much more useful if you could detect code 
>> that could possibly throw Error.
>
> Why do you care about detecting code that can throw an Error? 
> Errors are supposed to kill the program, not get caught. As 
> such, why does it matter if it can throw an Error?
>
> Now, personally, I'm increasingly of the opinion that the fact 
> that we have Errors is kind of dumb given that if it's going to 
> kill the program, and it's not safe to do clean-up at that 
> point, because the program is in an invalid state, then why not 
> just print the message and stack trace right there and then 
> kill the program instead of throwing anything? But 
> unforntunately, that's not what happens, which does put things 
> in the weird state where code can catch an Error even though it 
> shouldn't be doing that.
>
> As for the benefits of nothrow, as I understand it, they're 
> twofold:
>
> 1. You know that you don't have to worry about any Exceptions 
> being thrown from that code. You don't have to worry about 
> doing any exception handling or having to ensure that anything 
> gets cleaned up because of an Exception being thrown.
>
> 2. If the compiler knows that a function can't throw an 
> Exception, then it doesn't have to insert any of the Exception 
> handling mechanism stuff that it normally does when a function 
> is called. It can assume that nothing ever gets thrown. If an 
> Error does get thrown, then none of the proper clean-up will 
> get done (e.g. constructors or scope statements), but because 
> an Error being thrown means that the program is in an invalid 
> state, it's not actually safe to be doing clean-up anyway. So, 
> the fact that a function is nothrow gives you a performance 
> benefit, because none of that extra Exception handling stuff 
> gets inserted. How large a benefit that is in practice, I don't 
> know, but it is a gain that can't be had with a function that 
> isn't nothrow.
>
> - Jonathan M Davis

Well at least from my point of view I would care about code that 
can throw Error, because if say nothrow could detect that then 
you could prevent writing that code that throws jt at all and 
thus you'd be writing less error prone code.

Maybe not necessarily nothrow, but something else that could 
ensure that your code is "100% safe" to run without any errors 
happening from ex. Accessing out of bounds, accessing invalid 
memory, attempting to access the member of an uninitialized class 
etc. Like you'd have to handle each such cases. Writing code in D 
today you have to think about each statement you write and 
whether it could possibly throw Error because you have little to 
no tools that helps you preventing writing such code.

I'm very well aware that Error is not supposed to be caught and 
that the program is in an invalid state, but ehat I'm trying to 
get at is that if nothrow or at least a feature similar existed 
that could detect code that may throw Error, then you could 
prevent writing code that throws it in the first place.

It would be a great tool to writing bugless code.