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HTML / CSS / JS

use std::num::ParseIntError;

#[derive(Debug)]
enum AppError {
    ParseError(ParseIntError),
    NegativeNumber(i64),
    TooBig(i64),
}

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

fn parse_and_validate(s: &str) -> Result<i64, AppError> {
    let n: i64 = s.trim().parse()?;   // ? converts ParseIntError via From
    if n < 0 {
        return Err(AppError::NegativeNumber(n));
    }
    if n > 1_000_000 {
        return Err(AppError::TooBig(n));
    }
    Ok(n)
}

fn double(s: &str) -> Result<i64, AppError> {
    let n = parse_and_validate(s)?;
    Ok(n * 2)
}

fn main() {
    let inputs = vec!["42", "  100  ", "-5", "999999999", "abc"];

for input in inputs {
        match double(input) {
            Ok(v) => println!("double({input:?}) = {v}"),
            Err(AppError::ParseError(e)) => println!("{input:?} parse error: {e}"),
            Err(AppError::NegativeNumber(n)) => println!("{input:?} is negative: {n}"),
            Err(AppError::TooBig(n)) => println!("{input:?} is too big: {n}"),
        }
    }

// Combinators
    let opt: Option<i64> = double("7").ok();
    println!("as option: {:?}", opt);

let mapped = double("3").map(|v| v + 1);
    println!("mapped: {:?}", mapped);
}