Hi all,
  I am wondering if there is any Phobos functionality for indexing into a list using a type. What I mean is something like:

assert(  somethingie!(float,  float, double, real)(1, 22, 333) == 1  );
assert(  somethingie!(double, float, double, real)(1, 22, 333) == 22  );
assert(  somethingie!(real,   float, double, real)(1, 22, 333) == 333  );

It would help with unittest definitions, such as:

foreach (F; TypeTuple!(float, double, real))
{
  Tuple!(F, int)[] vals =     // x, ilogb(x)
  [
    tuple(  2.0001    , 1           ),
    tuple(  1.9999    , 0           ),
    // ...
    tuple(-F.min_normal, somethingie!(F, float, double, real)(-126, -1022, -16382),
  ];

  foreach(elem; vals)
  {
    assert(ilogb(elem[0]) == elem[1]);
  }
}

Thanks!

(on IRC, I was advised to move the special case of min_normal outside of the loop. Perhaps more people like that better, but still I am interested in knowing if my "somethingie" exists, or whether you think it is useful too. (what if there are 10 such special cases?))

On Tuesday, 28 July 2015 at 21:12:13 UTC, Johan Engelen wrote:
> Hi all,
>   I am wondering if there is any Phobos functionality for indexing into a list using a type. What I mean is something like:
>
> assert(  somethingie!(float,  float, double, real)(1, 22, 333) == 1  );
> assert(  somethingie!(double, float, double, real)(1, 22, 333) == 22  );
> assert(  somethingie!(real,   float, double, real)(1, 22, 333) == 333  );

std.typetuple.staticIndexOf (going to be moved to std.meta in 2.068) can do this together with std.typecons.tuple:
----
void main()
{
    import std.typetuple: staticIndexOf;
    import std.typecons: tuple;
	
    assert(tuple(1, 22, 333)[staticIndexOf!(float,  float, double, real)] == 1);
    assert(tuple(1, 22, 333)[staticIndexOf!(double, float, double, real)] == 22);
    assert(tuple(1, 22, 333)[staticIndexOf!(real,   float, double, real)] == 333);
}
----

Or if you think that's worth encapsulating in somethingie:
----
template somethingie(T, TList...)
{
    auto somethingie(A...)(A args)
    {
        import std.typetuple: staticIndexOf;
        import std.typecons: tuple;

        return tuple(args)[staticIndexOf!(T, TList)];
    }
}

void main()
{
    assert(  somethingie!(float,  float, double, real)(1, 22, 333) == 1  );
    assert(  somethingie!(double, float, double, real)(1, 22, 333) == 22  );
    assert(  somethingie!(real,   float, double, real)(1, 22, 333) == 333  );
}
----