Key Ideas

Treating external SEXP and owned SEXP differently

Savvy is opinionated in many points. Among these, one thing I think should be explained first is that savvy uses separate types for SEXP passed from outside and that created within Rust function. The former, external SEXP, is read-only, and the latter, owned SEXP, is writable. Here's the list:

You might wonder why this is needed when we can just use mut to distinguish the difference of mutability. I mainly had two motivations for this:

1. avoid unnecessary protection: an external SEXP are already protected by the caller, while an owned SEXP needs to be protected by ourselves.
2. avoid unnecessary ALTREP checks: an external SEXP can be ALTREP, so it's better to handle them in ALTREP-aware way, while an owned SEXP is not.

This would be a bit lengthy, so let's skip here. You can read the details on my blog post. But, one correction is that I found the second reason might not be very important because a benchmark showed it's more efficient to be non-ALTREP-aware in most of the cases. Actually, the current implementation of savvy is non-ALTREP-aware for int, real, and logical (See #18).

No implicit conversions

Savvy doesn't provide conversion between types unless you do explicitly. For example, you cannot supply a double vector to a function with a IntegerSexp argument.

#[savvy]
fn identity_int(x: IntegerSexp) -> savvy::Result<savvy::Sexp> {
    let mut out = OwnedIntegerSexp::new(x.len())?;

    for (i, &v) in x.iter().enumerate() {
        out[i] = v;
    }

    out.into()
}

identity_int(c(1, 2))
#> Error in identity_int(c(1, 2)) : 
#>   Unexpected type: Cannot convert double to integer

While you probably feel this is inconvenient, this is also a design decision. My concerns on supporting these conversion are

• Complexity. It would make savvy's spec and implemenatation complicated.
• Hidden allocation. Conversion requires a new allocation for storing the converted values, which might be unhappy in some cases.

So, you have to write some wrapper R function like below. This might feel a bit tiring, but, in general, please do not avoid writing R code. Since you are creating an R package, there's a lot you can do in R code instead of making things complicated in Rust code. Especially, it's easier on R's side to show user-friendly error messages.

identity_int_wrapper <- function(x) {
  x <- vctrs::vec_cast(x, integer())
  identity_int(x)
}

Alternatively, you can use NumericSexp as input. This provides a method to convert the input either to i32 or to f64 on the fly. For more details, please read the section about NumericSexp

#[savvy]
fn identity_num(x: NumericSexp) -> savvy::Result<savvy::Sexp> {
    let mut out = OwnedIntegerSexp::new(x.len())?;

    for (i, &v) in x.iter_i32().enumerate() {
        out[i] = v;
    }

    out.into()
}