Enum

Savvy supports fieldless enum to express the possible options for a parameter. For example, if you define such an enum with #[savvy],

/// @export
#[savvy]
enum LineType {
    Solid,
    Dashed,
    Dotted,
}

it will be available on R's side as this.

LineType$Solid
LineType$Dashed
LineType$Dotted

You can use the enum type as the argument of such a function like this

/// @export
#[savvy]
fn plot_line(x: IntegerSexp, y: IntegerSexp, line_type: &LineType) -> savvy::Result<()> {
    match line_type {
        LineType::Solid => {
            ...
        },
        LineType::Dashed => {
            ...
        },
        LineType::Dotted => {
            ...
        },
    }
}

so that the users can use it instead of specifying it by an integer or a character, which might be mistyped.

plot_line(x, y, LineType$Solid)

Of course, you can archive the same thing with i32 or &str as the input and match the value. The difference is that enum is typo-proof. But, you might feel it more handy to use a plain integer or character.

/// @export
#[savvy]
fn plot_line(x: IntegerSexp, y: IntegerSexp, line_type: &str) -> savvy::Result<()> {
    match line_type {
        "solid" => {
            ...
        },
        "dashed" => {
            ...
        },
        "dotted" => {
            ...
        },
        _ => {
            return Err("Unsupported line type!".into());
        }
    }
}

Limitation

As noted above, savvy supports only fieldless enum for simplicity. If you want to use an enum that contains some value, please wrap it with struct.

// You don't need to mark this with #[savvy]
enum AnimalEnum {
    Dog(String, f64),
    Cat { name: String, weight: f64 },
}

/// @export
#[savvy]
struct Animal(AnimalEnum);

Also, savvy currently doesn't support discriminants. For example, this one won't compile.

/// @export
#[savvy]
enum HttpStatus {
    Ok = 200,
    NotFound = 404,
}