32  Integrating with

TipObjective

Create a vctrs compatible vector for our urls.

The format function is helpful for us to actually print it ourselves, but ideally, whenever we print our list of external pointers we see the url and not the external pointer.

Since we’re creating List objects from Rust, we can actually harness the S3 object system in R. See my video if you’re not familiar.

S3 works by looking at the classes of an object.

We can define our own format method for our list. To do this we need to do two things:

  1. Assign a class to the list
  2. Create an R function generic

The vctrs_vctr class

The vctrs package defines a generic vctrs_vctr class that gives sensible default behavior for vectors.

We can assign this class to our list so that we automatically get vctrs behaviors for free!

In R terms we’re going to add:

c("rurls", "vctrs_vctr", "list")

to the list.

Setting the class from Rust

We can assign a class to a mutable Robj with .set_class(). Because set_class() is fallible, the function returns a Result.

It needs to be mutable because we’re modifying the underlying R object.

Example

#[extendr]
fn parse_shapes(shapes: Strings) -> extendr_api::error::Result<List> {
    let mut res = shapes
        .into_iter()
        .map(|vi| RShape::new(vi).unwrap())
        .collect::<List>();

    res.set_class(&["shapes", "vctrs_vctr", "list"])?;
    Ok(res)
}

The R method

If you devtools::document() and devtools::load_all() from here you will see some basic print, but the format_rurls() won’t be used yet.

To do that we need to create a real R function!

Create a new R script (e.g. R/rurls.R), define format.rurls, and have it call your Rust helper:

#' @export
format.rurls <- function(x, ...) {
    format_rurls(x)
}

Exercise

  • Update url_parse() to return extendr_api::error::Result<List>
  • Set the class to c("rurls", "vctrs_vctr", "list") on the result
  • Add the format.rurls method above in an R script
  • devtools::document() and devtools::load_all(), then print a parsed vector
View solution 
#[extendr]
fn url_parse(x: Strings) -> extendr_api::Result<List> {
    let mut res = x
        .into_iter()
        .map(|xi| {
            if xi.is_na() {
                return ().into_robj();
            }

            match RUrl::new(xi) {
                Ok(v) => v.into_robj(),
                Err(_) => ().into_robj(),
            }
        })
        .collect::<List>();

    res.set_class(&["rurls", "vctrs_vctr", "list"])?;
    Ok(res)
}
#' @export
format.rurls <- function(x, ...) {
    format_rurls(x)
}