Custom Type Unions

[Dhruv Singal]

I'm trying to implement an interpreter in D as a class project. 
Having already written the the parser and interpreter in Racket, 
I'm having trouble figuring out if dlang has a way for me to 
create a union object that can be one of many object and have it 
be a first-class object, ie it can be the return type of 
functions. Is this something dlang supports? If not, is there a 
better way of implementing this?
Below is some rudimentary code I wrote to test this:
import std.stdio;
import std.string;
alias string = immutable(char)[];
union Value {real realV; bool booV; string stringV; primV prim;};

struct primV{
     real function(int, int) doer;
}
real add (int a, int b){
     real sum = a+b;
     return cast(real)sum;
}
void main(){
         writeln("Beginning Interp");
         primV adder = primV(&add);
         real x = adder.doer(5,6);
         writeln(x);
         }