Pretty please: Named arguments

[Steven Schveighoffer]

On Mon, 28 Feb 2011 07:41:06 -0500, Michel Fortin  
<michel.fortin at michelf.com> wrote:

> On 2011-02-28 02:03:46 -0500, Bekenn <leaveme at alone.com> said:
>
>> Potential problems:  The only problems I can foresee here are  
>> variations on the situation when there are two (or more) versions of a  
>> function with the same number, type, and names of parameters, but in  
>> non-matching order, like this:
>>  void func(int a, char b);
>> void func(char b, int a);
>>  In such a case, the compiler should diagnose an error if named  
>> arguments are employed.
>>  Thoughts?
>
> Another problem is what happens if you override a function by using  
> different parameter names. For instance:
>
> 	class A {
> 		void func(int foo);
> 	}
> 	class B : A {
> 		void func(int bar);
> 	}
>
> Currently, the above is allowed. Would it now become an error?
>
> I think it'd be much easier to support named arguments if we didn't  
> allow reordering. I know I'm stripping the proposal from one of its main  
> benefits, but I think the essence remains. This would make the rather  
> common pattern of adding a comment for each argument compiler-verifiable  
> without disrupting things too much:
>
> 	draw(null, // text
> 	     null, // font
> 	     12, // size
> 	     0, // flags
> 	     );
>
> 	draw(text: null,
> 	     font: null,
> 	     size: 12,
> 	     flags: 0);
>
> It would be implemented as a check for equality of the argument names  
> after overload resolution. No ABI change, no interaction with  
> overloading, just an extra check that if an argument name is specified  
> it must match the one in the declaration.
>
> There'd still be a funny interaction with overriding (see my first  
> example), but you can always leave your argument unnamed if the  
> definition keep changing the name in unpredictable ways.
>

All these 'what if' ambiguities are simple to resolve -- if you use  
positional arguments, it works.  If you use named arguments, and it  
matches more than one overload, it fails.

In fact, the case you give is unambiguous -- if you have an A, you can use  
foo, if you have a B, you can use bar.  Note that the confusion is all to  
the person, and is completely avoidable.  It's like naming your arguments  
xy234m5  -- nobody knows what that means, so just choose a better name.   
But it's not illegal.

I would say this would be a huge improvement to D.  But I would also say  
 from my recollection, Walter is very against this.  I think it would take  
a motivated developer creating a pull request to get it done...

-Steve