• JS (52/52) Jul 20 2013 I am trying to write a simple template expansion of boolean

"JS" <js.mdnq gmail.com> writes:

I am trying to write a simple template expansion of boolean 
relations between values.

I've tried various ways but this is the only way I can get to 
work without a compile time error:

template A(string, T, R...)
{
	string A(string op, string s, T t, R r)
	{
		string n = "("~s~" == "~t~")";
		static if (r.length) return n~" "~op~" "~A(op, s, r);
		return n;
	}
}

But I do not think this method is ctfe as I can't use it as a 
mixin or else the arguments can't be read at compile time.


the idea is simple

A!("||", "a", d)

should expand out to the code(as a string at compile time)

(a == d[0]) || (a == d[1]) || ...

so I can do stuff like

void foo(T)(string a, T d...) {
if (a == "a" && !(mixin(A!("||", "a", d)))) ...


When I try to use pure ctfe I can't pass the variadic without 
template matching issues. I initially tried something like:

template A(string op, string s, string c, R...)
{
	string eval(string c) { return "AAA"; }
	string eval(string t, R r)
	{
		string n = "("~s~" == "~t~")";
		static if (r.length) return n~" "~op~" "~eval(op, s, r);
		return n;
	}
	enum A = eval(c);
}

I've tried

template A(alias S)
{
     string eval(T t...) { }
     enum A = eval(S);
}

but it can't be used with multiple parameters(we sort of need a 
variadic alias to make it work).


In any case, hopefully someone can come up with an efficient 
template that expands the (varadic) arguments at compile time(no 
overhead, same as if I typed the code in by hand).


I'm trying to use such a function in split(string s, T delims) 
where T can be a string or char).

so I can do split(s, ",", '.', " ") or split(s, " ") (accepts 
variable number of arguments) efficiently as possible. (no wasted 
overhead for multiple arguments more than necessary(such as 
calling a recursive function, etc...).