Need Advice: Union or Variant?

[jwatson-CO-edu]

I have an implementation of the "[Little 
Scheme](https://mitpress.mit.edu/9780262560993/the-little-schemer/)" educational programming language written in D, [here](https://github.com/jwatson-CO-edu/SPARROW)".

It has many problems, but the one I want to solve first is the 
size of the "atoms" (units of data).

`Atom` is a struct that has fields for every possible type of 
data that the language supports. This means that a bool `Atom` 
unnecessarily takes up space in memory with fields for number, 
string, structure, etc.

Here is the 
[definition](https://github.com/jwatson-CO-edu/SPARROW/blob/main/lil_schemer.d#L55):

```d
enum F_Type{
     CONS, // Cons pair
     STRN, // String/Symbol
     NMBR, // Number
     EROR, // Error object
     BOOL, // Boolean value
     FUNC, // Function
}

struct Atom{
     F_Type  kind; // ---------------- What kind of atom this is
     Atom*   car; // ----------------- Left  `Atom` Pointer
     Atom*   cdr; // ----------------- Right `Atom` Pointer
     double  num; // ----------------- Number value
     string  str; // ----------------- String value, D-string 
underlies
     bool    bul; // ----------------- Boolean value
     F_Error err = F_Error.NOVALUE; // Error code
}

```
Question:
**Where do I begin my consolidation of space within `Atom`?  Do I 
use unions or variants?**