Distinguishing between const and non-const variable in a template?

[renoX]

Hello,

I'm trying to improve format string by allowing the format
" ... %{x} ...", my problem is that when I give a non-const char[] 
parameter in
mixin(Putf!(foo));

then the template fail..

How can I reliably detect in a template if the parameter is a constant 
or not?

My goal is:
if the parameter isn't a const char[]: leave its name unchanged so that 
it's parsed by the writef at runtime, if it is a const char[]: parse it 
myself with the new syntax.

Regards,
renoX

template FindChar(char[] A, char B) {
     static if (A.length == 0) {
         const int FindChar = -1;
     } else static if (A[0] == B) {
         const int FindChar = 0;
     } else static if (-1 == FindChar!(A[1..$], B)) {
		const int FindChar = -1;
	} else {
		const int FindChar = 1 + FindChar!(A[1..$], B);
	}
}

template FmtString(char[] F, A...)
{
	static if (F.length == 0)
		static if (A.length)
			const char[] FmtString = "\"," ~ Fmt!(A);
		else
			const char[] FmtString = "\"";
	else static if (F.length == 1)
		static if (A.length)
			const char[] FmtString = F[0] ~ "\"," ~ Fmt!(A);
		else
			const char[] FmtString = F[0] ~ "\"";
	else static if (F[0..2] == "%%")
		const char[] FmtString = "%%" ~ FmtString!(F[2..$], A);
	else static if (F[0..2] == "%{")
	{
		// get the variable name between %{ and }
		static if (FindChar!(F, '}') <= 2)
			static assert(0, "format %{} incorrect in '" ~ F ~ "'");
		const char[] FmtString = "%s\"," ~ F[2..FindChar!(F,'}')] ~ ",\"" ~
			FmtString!(F[1+FindChar!(F,'}')..$], A);
	}
	else
		const char[] FmtString = F[0] ~ FmtString!(F[1..$], A);
}

template Fmt(A...)
{
	//static assert(0, cast(char*)A[0]);
	static if (A.length == 0)
		const char[] Fmt = "";
	else static if (is(typeof(A[0]) : char[]))
		const char[] Fmt = "\"" ~ FmtString!(A[0], A[1..$]);
	else static if (A.length == 1)
		const char[] Fmt = A[0].stringof;
	else
		const char[] Fmt = A[0].stringof ~ "," ~ Fmt!(A[1..$]);	
}

template Putf(A...)
{
	const char[] Putf = "writef(" ~ Fmt!(A) ~ ");";
}