A custom error enum in Rust

One enum per failure mode, with a Display impl that reads like a sentence.

use std::fmt;

#[derive(Debug)]
enum ConfigError {
    Missing(String),
    OutOfRange { field: String, value: i64 },
}

impl fmt::Display for ConfigError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            ConfigError::Missing(key) => write!(f, "missing key {key}"),
            ConfigError::OutOfRange { field, value } => {
                write!(f, "{field} out of range: {value}")
            }
        }
    }
}

fn main() {
    let err = ConfigError::OutOfRange {
        field: "port".to_string(),
        value: 99999,
    };
    println!("{err}");
    println!("{:?}", ConfigError::Missing("host".to_string()));
}

How it works

1. Each variant carries the detail that failure needs.
2. impl Display is what makes it printable.
3. Matching on the enum lets callers react per variant.

Keywords and builtins used here

• ConfigError
• Result
• String
• enum
• fmt
• fn
• for
• i64
• impl
• let
• main
• match
• mut
• self
• use

The run, in numbers

Lines

27

Characters to type

548

Tokens

153

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 87 seconds.

Type this snippet

Step 1 of 5 in Your own error types, step 8 of 15 in Error handling.

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