join vector of non-strings into a string rust

Question

I have a type T that implements display (so it has a .to_string() method)

let my_vec = vec![T(), T(), T()];
println!("{}", my_vec.join(", "));

sadly errors with "trait bounds not satisfied" because the separator, ", ", is not of the same type as the vector's items (I'm pretty sure).

I guess my workaround is then

println!("{}", my_vec.iter().map(|x| x.to_string()).collect::<Vec<_>>().join(", "));

But isn't there anything shorter and clearer that I can write out instead of this?

I've just written this function to help me out:

fn join<T, I>(vec: &[T], sep: I) -> String
where
    T: std::fmt::Display,
    I: std::fmt::Display,
{
    vec.iter().map(|x| x.to_string()).collect::<Vec<_>>().join(&sep.to_string())
}

But I'd rather not have to. There must be alternative solutions that: are already built-in, don't require manual implementation, at least don't require the creation of a top-level function that's then only called twice.

halfelf · Accepted Answer · 2022-11-29 05:56:37Z

AFAIK, your work around is good and sound without non-std libs.

Several cargos provide some helper functions for this issue, for example:

Playground

itertools and joinery

use itertools::Itertools;
use joinery::Joinable;

struct Color {
    red: u8,
    green: u8,
    blue: u8,
}

impl std::fmt::Display for Color {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        write!(
            f,
            "RGB ({}, {}, {}) 0x{:02X}{:02X}{:02X}",
            self.red, self.green, self.blue, self.red, self.green, self.blue
        )
    }
}

fn main() {
    let colors = vec![
        Color {
            red: 128,
            green: 255,
            blue: 90,
        },
        Color {
            red: 0,
            green: 3,
            blue: 254,
        },
    ];

    println!(
        "{}",
        colors
            .iter()
            .map(|x| x.to_string())
            .collect::<Vec<_>>()
            .join(", ")
    );

    println!("{}", colors.iter().join(", "));
    println!("{}", colors.join_with(", "));
}

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