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.
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?**
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/
