Need Advice: Union or Variant?

[jwatson-CO-edu]

On Thursday, 17 November 2022 at 21:19:56 UTC, Petar Kirov 
[ZombineDev] wrote:
> On Thursday, 17 November 2022 at 20:54:46 UTC, jwatson-CO-edu 
> wrote:
>> 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.
>>
>>[...]
>> Do I use unions or variants?**
>
> In general, I recommend 
> [`std.sumtype`](https://dlang.org/phobos/std_sumtype), as it is 
> one of the best D libraries for this purpose. It is implemented 
> as a struct containing two fields: the `kind` and a `union` of 
> all the possible types.
> That said, one difficulty you are likely to face is with 
> refactoring your code to use the 
> [`match`](https://dlang.org/phobos/std_sumtype#.match) and 
> [`tryMatch`](https://dlang.org/phobos/std_sumtype#.tryMatch) 
> functions, as `std.sumtype.SumType` does not expose the 
> underlying kind field.
>
> Other notable alternatives are:
> * [`mir-core`](https://code.dlang.org/packages/mir-core)'s 
> `mir.algebraic`: http://mir-core.libmir.org/mir_algebraic.html
> * 
> [`taggedalgebraic`](https://code.dlang.org/packages/taggedalgebraic): https://vibed.org/api/taggedalgebraic.taggedalgebraic/

Thank you!  This is intriguing.
The different flavors of `Atom` I need will have either {`car`, 
`cdr`} -or- {`num`} -or- {`str`} -or- {`bul`}.  Does SumType 
allow me to store the multiple fields {`car`, `cdr`} in one of 
the types, while the other types have only one field?

Since this is a dynamically-typed language, I need the atoms to 
both be interchangeable and to serve different purposes at the 
same time.