Is there a way to print enum values?
EnumsRustEnums Problem Overview
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`
Enums Solutions
Solution 1 - Enums
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.
Solution 2 - Enums
The Debug
trait prints out the name of the Enum
variant.
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);
}
Solution 3 - Enums
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"
}
Solution 4 - Enums
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);
}