Handling Scalar

Input

Scalar inputs are handled transparently. The corresponding types are shown in the table below.

/// @export
#[savvy]
fn scalar_input_int(x: i32) -> savvy::Result<()> {
    savvy::r_println!("{x}");
    Ok(())
}

NumericScalar

NumericScalar is a special type that can handle both integeer and double. You can get the value from it by as_i32() for i32, or as_f64() for f64. These method converts the value if the input type is different from the target type.

#[savvy]
fn times_two_numeric_i32_scalar(x: NumericScalar) -> savvy::Result<Sexp> {
    let v = x.as_i32()?;
    if v.is_na() {
        (i32::na()).try_into()
    } else {
        (v * 2).try_into()
    }
}

Note that, while as_f64() is infallible, as_i32() can fail when the conversion is from f64 to i32 and

• the value is Inf or -Inf
• the value is out of range for i32
• the value is not integer-ish (e.g. 1.1)

For convenience, NumericScalar also provides a conversion to usize by as_usize(). What's good is that this can handle integer-ish numeric, which means you can allow users to input a larger number than the integer max (2147483647)!

fn usize_to_string_scalar(x: NumericScalar) -> savvy::Result<Sexp> {
    let x_usize = x.as_usize()?;
    x_usize.to_string().try_into()
}

usize_to_string_scalar(2147483648)
#> [1] "2147483648"

Output

Just like a Rust vector, a Rust scalar value can be converted into Sexp by try_from(). It's as simple as.

/// @export
#[savvy]
fn scalar_output_int() -> savvy::Result<savvy::Sexp> {
    1.try_into()
}

Alternatively, the same conversion is available in the form of Owned{type}Sexp::try_from_scalar().

/// @export
#[savvy]
fn scalar_output_int() -> savvy::Result<savvy::Sexp> {
    let out = OwnedIntegerSexp::try_from_scalar(1)?;
    out.into()
}

Missing values

If the type of the input is scalar, NA is always rejected. This is inconsistent with the rule for vector input, but, this is my design decision in the assumption that a scalar missing value is rarely found useful on Rust's side.

/// @export
#[savvy]
fn identity_logical_single(x: bool) -> savvy::Result<savvy::Sexp> {
    let mut out = OwnedLogicalSexp::new(1)?;
    out.set_elt(0, x)?;
    out.into()
}

identity_logical_single(NA)
#> Error in identity_logical_single(NA) : 
#>   Must be length 1 of non-missing value