Extension traits

In Grug, we make use of the extension trait pattern, which is well explained by this video.

To put it simply, a Rust library has two options on how to ship a functionality: to ship a function, or to ship a trait.

For instance, suppose our library needs to ship the functionality of converting Rust values to strings.

Shipping a function

The library exports a function:

#![allow(unused)]
fn main() {
pub fn to_json_string<T>(data: &T) -> String
where
    T: serde::Serialize,
{
    serde_json::to_string(data).unwrap_or_else(|err| {
        panic!("failed to serialize to JSON string: {err}");
    })
}
}

The consumer imports the function:

#![allow(unused)]
fn main() {
use grug::to_json_string;

let my_string = to_json_string(&my_data)?;
}

Shipping a trait

The library exports a trait, and implements the trait for all eligible types.

The trait is typically named {...}Ext where “Ext” stands for extension, because the effectively extends the functionality of types that implement it.

#![allow(unused)]
fn main() {
pub trait JsonSerExt {
    fn to_json_string(&self) -> String;
}

impl<T> JsonSerExt for T
where
    T: serde::Serialize,
{
    fn to_json_string(&self) -> String {
        serde_json::to_string(data).unwrap_or_else(|err| {
            panic!("failed to serialize to JSON string: {err}");
        })
    }
}
}

The consumer imports the trait:

#![allow(unused)]
fn main() {
use grug::JsonSerExt;

let my_string = my_data.to_json_string()?;
}