Project Structure

A extendr-powered R package has a fairly unique structure. This section briefly outlines the structure of an extendr package and the important files.

extendr works by creating a Rust library crate in src/rust that is defined by src/rust/Cargo.toml.

Cargo.toml

[package]
name = 'hellorust'
publish = false
version = '0.1.0'
edition = '2021'

[lib]
crate-type = [ 'staticlib' ]
name = 'hellorust'

[dependencies]
extendr-api = '*'

Note the crate-type = [ 'staticlib' ]. When this library is compiled, it creates a static library which can then be called from R.

Controlling exports to R: lib.rs

The lib.rs file determines what will be exposed to your R package. The extendr_module! macro in lib.rs controls what will have wrappers provided to your R package.

extendr_module! {
    mod hellorust;
    fn hello_world;
}

The mod hellorust is the name of the R package. Additional functions, impls, and modules can also be added to this macro.

Building the package: Makevars

When creating an R package that uses compiled code, a file called Makevars is used.

Note

See Using Makevars in Writing R Extensions for a thorough discussion.

Makevars is used as a preprocessing step for compiling an R package. The files Makevars and Makevars.win compile the Rust library in src/rust, and link to the library.

Tip

Makevars is used for *nix operating systems and Makevars.win is used for Windows.