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use std::fs;
use std::io;
use std::path::Path;

// Chain multiple fallible operations with ?
fn read_number_from_file(path: &Path) -> Result<i64, Box<dyn std::error::Error>> {
    let content = fs::read_to_string(path)?;    // io::Error -> Box<dyn Error>
    let n: i64 = content.trim().parse()?;        // ParseIntError -> Box<dyn Error>
    Ok(n)
}

fn compute(path: &Path) -> Result<i64, Box<dyn std::error::Error>> {
    let n = read_number_from_file(path)?;
    if n < 0 {
        return Err("number must be non-negative".into());
    }
    Ok(n * n)
}

// main can return Result — ? works here too
fn main() -> Result<(), Box<dyn std::error::Error>> {
    // Write a temp file for demonstration
    let path = Path::new("number.txt");
    fs::write(path, "42
")?;

match compute(path) {
        Ok(v) => println!("42 squared = {v}"),
        Err(e) => println!("error: {e}"),
    }

// Try a bad file
    let bad = Path::new("missing.txt");
    match compute(bad) {
        Ok(v) => println!("got {v}"),
        Err(e) => println!("expected error: {e}"),
    }

// Cleanup
    fs::remove_file(path)?;
    Ok(())
}