Backend

Rust Enums as Constructors

August 30, 20191 minute

In my previous post, Pattern Matching Multiple Variants in Rust, I talked about matching against multiple enums variants for a single branch.

Another handy enum feature in Rust is using the variants as a constructor.

For example,

#[derive(Debug)]
enum Avatar {
  Uploaded(String),
  NotUploaded,
  Hidden
}

let my_avatar = Avatar::Uploaded("https://avatar.com/123.jpg".to_owned());
let hidden_avatar = Avatar::Hidden;

dbg!(my_avatar);
dbg!(hidden_avatar);

Say you have a bunch of uploaded Avatars to process, but first you want them to be wrapped in the Uploaded variant. You can map them:

#[allow(dead_code)]

#[derive(Debug)]
enum Avatar {
    Uploaded(String),
    NotUploaded,
    Hidden,
}

fn main() {
    let avatars_to_process = vec![
        "https://avatar.com/some_avatar1.jpg",
        "https://avatar.com/some_avatar2.jpg",
        "https://avatar.com/some_avatar3.jpg",
    ];

    let uploaded_avatars_to_process = avatars_to_process
        .into_iter()
        .map(Into::into) // &str to String
        .map(Avatar::Uploaded)
        .collect::<Vec<Avatar>>();

    println!("Ready to process:");
    uploaded_avatars_to_process.into_iter().for_each(|a| println!("{:?}", a));
}
$ cargo run
Ready to process:
Uploaded("https://avatar.com/some_avatar1.jpg")
Uploaded("https://avatar.com/some_avatar2.jpg")
Uploaded("https://avatar.com/some_avatar3.jpg")

Here's the full example.

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© 2019 Dan Bruder