use std::fmt;
use std::num::ParseIntError;

#[derive(Debug)]
enum AppError {
    Parse(ParseIntError),
    Validation(String),
}

impl fmt::Display for AppError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            AppError::Parse(e) => write!(f, "Parse error: {}", e),
            AppError::Validation(msg) => write!(f, "Validation: {}", msg),
        }
    }
}

impl From<ParseIntError> for AppError {
    fn from(e: ParseIntError) -> Self {
        AppError::Parse(e)
    }
}

fn parse_positive(input: &str) -> Result<u32, AppError> {
    let n: u32 = input.parse()?;
    if n == 0 {
        return Err(AppError::Validation("must be positive".into()));
    }
    Ok(n)
}

fn main() {
    println!("{:?}", parse_positive("42"));
    println!("{:?}", parse_positive("0"));
    println!("{:?}", parse_positive("abc"));
}