alias tuples?

[Reiner Pope]

Janice Caron wrote:
> We've got value tuples, and we've got type tuples. Could we also have
> alias tuples?
> 
> Here's what I mean. Right now I can do:
> 
> struct A
> {
>     X x;
>     Y y;
>     Z z;
> 
>     mixin Serialize!(x,y,z);
> }
> A a;
> 
> template Serialize(alias x,alias y,alias z)
> {
>     void serialize()
>     {
>         x.serialize();
>         y.serialize();
>         z.serialize();
>     }
> }
> 
> where the types X, Y and Z all have a serialize() function (and so on
> recursively for every object in need of serialization). Now it seems
> to me, if I wanted to make Serialize!() variadic, I'd need not a value
> tuple, nor a type tuple, but an "alias tuple". I'd want to be able to
> write something like:
> 
> struct A
> {
>     X x;
>     Y y;
>     Z z;
> 
>     mixin Serialize!(this.aliastupleof); /* expands to (x,y,z) */
> }
> A a;
> 
> template Serialize(alias T...)
> {
>     void serialize()
>     {
>         foreach(a;T)
>             a.serialize();
>     }
> }
> 
> Of course this won't work right now. The template won't compile
> because (alias T...) has no meaning, and even if it did, structs don't
> have an aliastupleof property.
> 
> Thoughts?

You can sort of fake it. You could write your first example as

struct A
{
     int x = 1;
     int y = 2;
     int z = 3;

     void serialize()
     {
         .serialize(this.tupleof); // or .serialize(x, z); if you want
     }
}

void serialize(T...)(T t)
{
     foreach (t2; t)
         typeSpecificSerialize(t2); // or t2.serialize(), but that won't 
work with int
}

void typeSpecificSerialize(T)(T t)
{
     writefln("%s: %s", T.stringof, t); // or whatever you want
}

---

But you don't get the variable names this way. If you want that, you can 
do the following, although you lose the variadic bit:

void serialize(T)(T t)
{
     foreach (i, t2; t.tupleof)
     {
         writefln("%s: [%s]", T.tupleof[i].stringof, t2);
     }
}

struct foo
{
     int a = 1;
     int b = 2;
}

void main()
{
     foo f;
     serialize(f);
}

---

I hope that helps, although it's not what you asked for. ;)

    -- Reiner