Issue Turning Template into Variadic Template

[H. S. Teoh via Digitalmars-d-learn]

On Wed, Mar 30, 2016 at 06:12:40PM +0000, jmh530 via Digitalmars-d-learn wrote:
> I wrote a version of cartesianProduct that will return the cartesian
> product when the some of the types are not ranges. The original code
> is below.
> 
> My issue is that I can't figure out how to turn it into a variadic
> template.  The latest thing I tried is:
[...]

Does this do what you want?

	import std.algorithm.setops : cartesianProduct;
	import std.range : only;
	import std.range.primitives;
	import std.meta : AliasSeq;
	
	template ImplType(T...)
	{
		static if (T.length == 0)
			alias ImplType = AliasSeq!();
		else
		{
			static if (isInputRange!(T[0]))
				alias FirstType = T[0];
			else
				alias FirstType = typeof(only(T[0].init));
			alias ImplType = AliasSeq!(FirstType, ImplType!(T[1 .. $]));
		}
	}
	
	auto conditionalOnly(alias fun, T...)(T x) {
		ImplType!T y;
		foreach (i, e; x) {
			static if (isInputRange!(typeof(e)))
				y[i] = e;
			else
				y[i] = only(e);
		}
		return fun(y);
	}
	
	void main() {
		import std.stdio;
		writeln(conditionalOnly!cartesianProduct(1, [2, 3], 4, [5, 6]));
	}

Note that conditionalOnly isn't specifically tied to cartesianProduct;
you can use it on anything that receives a variadic number of range
arguments.


T

-- 
"I'm not childish; I'm just in touch with the child within!" - RL