Error Trait

std::error::Error trait เป็น foundation สำหรับสร้าง custom error types ใน Rust

1. std::error::Error

การ implement Error trait ขั้นพื้นฐานต้อง implement Display และ Debug ด้วย

use std::error::Error;
use std::fmt;

#[derive(Debug)]
struct ValidationError {
    field: String,
    message: String,
}

// ============================================
// Display - user-friendly message
// ============================================

impl fmt::Display for ValidationError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "Validation error on '{}': {}", self.field, self.message)
    }
}

// ============================================
// Error trait - standard error behavior
// ============================================

impl Error for ValidationError {}

fn validate_name(name: &str) -> Result<(), ValidationError> {
    if name.is_empty() {
        return Err(ValidationError {
            field: "name".to_string(),
            message: "cannot be empty".to_string(),
        });
    }
    if name.len() < 2 {
        return Err(ValidationError {
            field: "name".to_string(),
            message: "must be at least 2 characters".to_string(),
        });
    }
    Ok(())
}

fn main() {
    for name in &["", "A", "Alice"] {
        match validate_name(name) {
            Ok(()) => println!("'{}' is valid!", name),
            Err(e) => println!("Error: {}", e),
        }
    }
}