23 Adding dependencies

Objective

Add a dependency to our package and import items from it.

Dependencies in Rust are managed by the Cargo.toml file. R will let you use packages that are on your machine even if you do not explicitly declare them as dependencies. Rust will not. You must declare dependencies.

`Cargo.toml`

The Cargo.toml is akin to the DESCRIPTION file. Dependencies are set via the [dependencies] table.

We can add dependencies manually, using cargo add crate-name, or via rextendr::use_crate("cratename").

[package]
name = "geohash"
publish = false
version = "0.1.0"
edition = "2021"
rust-version = "1.65"

[lib]
crate-type = [ "staticlib" ]
name = "geohashrs"

[dependencies]
extendr-api = "*"

Rust dependencies use semantic versionining (semver). The "*" is a shorthand for specifying the latest stable release.

Importing items from dependencies

When dependencies are added to a crate, the items from the dependency are accessed via namespace similar to R. If we look at lib.rs we see at the top

use extendr_api::prelude::*;

This has the format of crate_name::module::item. The * is a wildcard for importing everything from the module.

The prelude is a common module name. It re-exports the commonly used items from a crate all together so you do not have to manually list them.

Multiple items can be imported using {} for example:

use crate_name::{StructName, function_name};

Exercise

Add the geohash crate as a dependency
Import the Coord struct and the encode function from geohash in lib.rs

View solution

From the R terminal run:

rextendr::use_crate("geohash")

This will add the geohash dependency.

Open src/rust/src/lib.rs and add the following below use extendr_api::prelude::*;

use geohash::{Coord, encode};