Yet another MRV proposal!

[Simen Kjaeraas]

On Mon, 14 Apr 2008 11:23:50 +0200, downs <default_357-line at yahoo.de>  
wrote:

> Let's give this another try.
> The following proposal has the advantage that it's funded mostly on  
> existing syntax.
>
> An anonymous struct, in the position where you'd normally expect a  
> function/method return type, is usable as the return type instead.
>
> Example:
>
> struct { int a; float b; } test() { return(1, 2f); }
>
> writefln(test().a, test().b);
>
> The compiler would translate this into "current D" as follows:
>
> struct _D_anonymous_struct_1 { int a; float b; }
> _D_anonymous_struct_1 test() { return _D_anonymous_struct_1(1, 2f); }
>
> Because of the not-exactly-clear type name, it is necessary to store the  
> returned value in an auto/const/static variable.
>
> This looks like it could be ambiguous, but it really isn't - the two  
> conditions required here - an unnamed struct in the position where a  
> return type would be expected - are quite unambiguous :)
>
> Whaddya think?
>
>  --downs


Looks pretty good - there are reasons to allow returning anonymous
structs, and I'm sure they could find uses in other places as well.

One problem I see is that, compared to 'true' MRV, it requires extra
assignments if you want the values outside the struct.

   int a;
   float b;

   struct { int a; float b; } test() { return(1, 2f); }

   auto result = test();

   a = result.a; // These should
   b = result.b; // be redundant

compared to:

   int a;
   float b;

   // some weird syntax for MRVs
   [int, float] test() { return (1, 2f); }

   a, b = test(); // More prettiful

Now, if struct could have reference members...

   int a;
   float b;

   struct { int a, float b} test() { return(1, 2f); }

   struct { ref a; ref b; } = test();

or:

   int a;
   float b;

   [int, float] test() { return(1, 2f); }

   [ref a, ref b] = test();


With all that said, I really like your proposal, and I hope it gets  
implemented.

-- Simen