Multiple return values...

[Timon Gehr]

On 03/14/2012 11:06 PM, Derek Parnell wrote:
> On Thu, 15 Mar 2012 08:52:26 +1100, Andrei Alexandrescu
> <SeeWebsiteForEmail at erdani.org> wrote:
>
>> On 3/14/12 3:00 PM, Simen Kjærås wrote:
>>> template to(T...) {
>>> alias T to;
>>> }
>>>
>>> auto from(T...)(T t) {
>>> struct Result { T t; alias t this; }
>>> return Result( t );
>>> }
>>>
>>> void main( ) {
>>> int a = 3;
>>> int b = 4;
>>>
>>> to!(a, b) = from(b, a);
>>>
>>> assert( a == 4 );
>>> assert( b == 3 );
>>> }
>>
>> I got reborn inside a little when seeing this code. Congratulations!
>
> And I died a little ... I have no idea what this code is doing. What is
> the generated code produced by this and why?
>
> I'd like to break the nexus between science and magic here.
>

// this is used to access language built-in tuples
template to(T...) {
     alias T to;
}

// this builds a struct akin to std.typecons.tuple
// it contains the function parameters as fields t[0], t[1],...
auto from(T...)(T t) {
     struct Result { T t; alias t this; }
     return Result( t );
}

// to!(a,b) creates a tuple containing aliases to a and b

to!(a,b) = from(b,a);   // cannot assign Result to two fields
to!(a,b) = from(b,a).t; // try alias this
auto __tmp = from(b,a).t; // expand tuple assign to multiple assignment
a[0] = __tmp[0], a[1] = __tmp[1];