Is there a way to print enum values?

Question

Is there an easy way to format and print enum values? I expected that they'd have a default implementation of std::fmt::Display, but that doesn't appear to be the case.

enum Suit {
    Heart,
    Diamond,
    Spade,
    Club
}

fn main() {
    let s: Suit = Suit::Heart;
    println!("{}", s);
}

Desired output: Heart

Error:

error[E0277]: the trait bound `Suit: std::fmt::Display` is not satisfied
  --> src/main.rs:10:20
   |
10 |     println!("{}", s);
   |                    ^ the trait `std::fmt::Display` is not implemented for `Suit`
   |
   = note: `Suit` cannot be formatted with the default formatter; try using `:?` instead if you are using a format string
   = note: required by `std::fmt::Display::fmt`

I wrote a crate for this exact problem: crates.io/crates/enum-display — SeedyROM
– SeedyROM, Commented Apr 1, 2023 at 19:28

Matilda Smeds · Accepted Answer · 2023-03-02 16:10:30Z

100

The Debug trait prints out the name of the Enumvariant.

If you need to format the output, you can implement Display for your Enum like so:

use std::fmt;

enum Suit {
    Heart,
    Diamond,
    Spade,
    Club
}

impl fmt::Display for Suit {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
       match self {
           Suit::Heart => write!(f, "♥"),
           Suit::Diamond => write!(f, "♦"),
           Suit::Spade => write!(f, "♠"),
           Suit::Club => write!(f, "♣"),
       }
    }
}

fn main() {
    let heart = Suit::Heart;
    println!("{}", heart);
}

edited Mar 2, 2023 at 16:10

answered Aug 26, 2017 at 8:32

Matilda Smeds

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4 Comments

wonderflow Over a year ago

Why when I try to write like this, it always print the first one ♥. Heart => write!(f, "♥") Diamond => write!(f, "♦")

Matilda Smeds Over a year ago

To print ♦ you need to create a reference to Suit::Diamond and then print it. For instance let diamond = Suit::Diamond; println!("{}", diamond);

Trí Phan Over a year ago

What is the need of dereferencing self in your answer?

Matilda Smeds Over a year ago

Nice catch, it is not needed!

Shepmaster · Accepted Answer · 2017-01-22 21:40:54Z

95

You can derive an implementation of std::format::Debug:

#[derive(Debug)]
enum Suit {
    Heart,
    Diamond,
    Spade,
    Club
}

fn main() {
    let s = Suit::Heart;
    println!("{:?}", s);
}

It is not possible to derive Display because Display is aimed at displaying to humans and the compiler cannot automatically decide what is an appropriate style for that case. Debug is intended for programmers, so an internals-exposing view can be automatically generated.

edited Jan 22, 2017 at 21:40

Shepmaster

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answered Jan 19, 2015 at 12:17

DK.

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3 Comments

Clemens Tolboom Over a year ago

This is not the answer

Someone Over a year ago

For enum's with values appended, e.g. Heart(u8), where u8 could be the card number from ace = 1, 2 to 13 (10, J, Q, K); is it possible to output only the enum name, not the value; Heart instead of Heart(7)?

Bryan Larsen Over a year ago

The OP asked for "an easy way to format and print enum values". This is the easiest of all the answers, and it format's and print's.. Answer is correct for OP.

njam · Accepted Answer · 2021-09-22 11:48:09Z

32

If you want to auto-generate Display implementations for enum variants you might want to use the strum crate:

#[derive(strum_macros::Display)]
enum Suit {
    Heart,
    Diamond,
    Spade,
    Club,
}

fn main() {
    let s: Suit = Suit::Heart;
    println!("{}", s); // Prints "Heart"
}

answered Sep 22, 2021 at 11:48

njam

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1 Comment

optevo Over a year ago

While it doesn't have many votes (currently), this is probably what you want as it's the least work to provide simple Display support to any enum.

raugfer · Accepted Answer · 2021-09-18 22:37:02Z

8

Combining both DK. and Matilda Smeds answers for a slightly cleaner version:

use std::fmt;

#[derive(Debug)]
enum Suit {
    Heart,
    Diamond,
    Spade,
    Club
}

impl fmt::Display for Suit {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
       write!(f, "{:?}", self)
    }
}

fn main() {
    let heart = Suit::Heart;
    println!("{}", heart);
}

answered Sep 18, 2021 at 22:37

raugfer

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2 Comments

fsan Over a year ago

I do not understand... if using derive(Debug) why use fmt::Display than ? It would be the same as: #[derive(Debug)] enum Suit { Heart } fn main() { let heart = Suit::Heart; println!("{:?}", heart); } which defeats the whole purpose of trying to make a Display.

raugfer Over a year ago

I am suggesting an implementation of std::fmt::Display, exactly because that is what is usually expected in most contexts.

Resfe · Accepted Answer · 2023-04-20 12:17:19Z

3

you need to use "{:#?}" for format. This will work now.

#[derive(Debug)]
enum Suit {
    Heart,
    Diamond,
    Spade,
    Club
}

fn main() {
    let s: Suit = Suit::Heart;
    println!("{:#?}", s); 
}

answered Apr 20, 2023 at 12:17

Resfe

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Comments

Bhavya Patel · Accepted Answer · 2024-09-14 06:26:07Z

0

I have tried to print values of enum using pattern matching, where based on the enum values I assing them to certain string which I can print using call_print_fun, where input args is variable assign using enum

fn main(){
   let direction=Dirs::North;
   let _direction=Dirs::South;
   call_print_fun(direction);
}
enum Dirs{
    North,
    South
}

fn call_print_fun(_dir: Dirs){
    println!("The Direction Class:");
    let girl: &str = match _dir {
        Dirs::North => "North",
        Dirs::South => "South",
    };
    print!("{}",girl)
}

answered Sep 14, 2024 at 6:26

Bhavya Patel

316 bronze badges

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