Passing Arguments on in Variadic Functions

[jmh530 via Digitalmars-d-learn]

In R, it is easy to have some optional inputs labeled as ... and 
then pass all those optional inputs in to another function. I was 
trying to get something similar to work in a templated D 
function, but I couldn't quite get the same behavior. What I have 
below is what I was able to get working.

This approach gives the correct result, but dmd won't deduce the 
type of the template. So for instance, the second to the last 
line of the unit test requires explicitly stating the types. I 
may as well use the alternate version that doesn't use the 
variadic function (which is simple for this trivial example, but 
maybe not more generally).

Do I have any options to get it more similar to the way R does 
things?

import std.algorithm : sum;
import core.vararg;

auto test(R)(R r)
{
        return sum(r);
}

auto test(R, E)(R r, ...)
{
        if (_arguments.length == 0)
                return test(r);
        else
        {
                auto seed = va_arg!(E)(_argptr);
                return sum(r, seed);
        }
}

auto test_alt(R, E)(R r, E seed)
{
        return sum(r, seed);
}

unittest
{
        int[] x = [10, 5, 15, 20, 30];
        assert(test(x) == 80);
        assert(test!(int[], float)(x, 0f) == 80f);
        assert(test_alt(x, 0f) == 80f);
}