Multiple return values...

[Mantis]

09.03.2012 1:28, Manu пишет:
> [...]
> The problem is, that approach feels like a double negative to me. A 
> tuple is fundamentally a structure, returned by value. Implementing 
> hacks to subvert the standard behaviour of returning a structure by 
> value is unintuitive for a start, and you also lose the ability to 
> *actually* return a structure by value should that be what you intend.
> You're sacrificing a well defined, 'properly' implemented mechanic 
> to imitate something the language simply can't express. I just think 
> that's the wrong way to go about it.
>
> Let me try and make my case as I see it...
>
> These are 2 distinct concepts, returning multiple values, and 
> returning a struct by value.
>
> Returning a structure by value is currently well defined, and behaves 
> as any same programmer would expect; it is written to the stack with 
> memory layout according to the STRUCTURE. This is true for a tuple, 
> and it works as one expects. I have no issue here. If you return a 
> tuple, you SHOULD be able to take the pointer of the first item, 
> perform some arithmetic, and address some other struct member. It is a 
> struct, it ought to behave like one, precisely as any programmer will 
> expect.
>
> By contrast, multiple return values are quite the opposite. They are 
> explicitly NON-STRUCTURED. These serve a totally different purpose; to 
> return multiple unstructured things from a function.
> Imagine an inline function which returns 2 results, only one of which 
> is captured. It is easy and intuitive to eliminate the code path 
> leading to the ignored result. Not so simple if you're returning 
> structured data, because it could be indirectly addressed.
> In this case, using an explicit syntax to perform this specific task 
> doesn't suffer from the implicit problems associated with subverting 
> the structure syntax (what to do about memory layout/pointer 
> arithmetic? reserve stack space and generate code to store implicitly? 
> ick!), but it also clearly states the programmers intent, and also 
> clearly communicates a presumed behaviour. The presumption in this 
> case is that multiple return values would follow the exact same set of 
> rules as passing multiple args TO a function, but in reverse.
>
> Both operations seem useful and important in their own ways, they are 
> also both distinct operations, and they both warrant an expression in 
> the language. Returning a tuple if perfect how it is, it is just not 
> what I want to do in cases like those I list in my OP.
>
> How is any programmer supposed to intuitively assume that returning a 
> tuple by value would behave in that way? And how are you supposed to 
> trust it? It's an abuse of concept and syntax. It seems like a 
> convolution that could only possibly confuse people, they are 
> conceptually quite different things, and shouldn't be lumped into the 
> same syntax for my money.

Is tuple required to be anonymous struct? I thought it's implementation 
details that may be done the other way if tuples implemented in language 
rather then library. There's another problem with non-named return 
values, as this:
auto (sin_a, cos_a) = sincos( a );
is not equivalent to this:
auto (cos_a, sin_a) = sincos( a );
Here it doesn't matter much, but if you are returning more variables, 
that may become confusing and bloated, and will provoke juniors to make 
errors, which is obviously not good for your money =). Note also, since 
variables should be defined /before/ function call, your IDE will not 
help you to avoid such mistakes.

Another thing is that if you may 'save' packed variables for later use, 
such code can be made possible:
auto t = getSomeTuple(...);
someVar = t.var;
foreach( v; t[1..$] ) { // static foreach, gets unrolled for every value 
in t except for the first
     sum += v;
}
Not for everyday use, but sometimes may be useful.

Anyway, what's your suggestion for the syntax?