#![allow(unused)]

pub mod alphabet_iter {
    use std::convert::TryFrom;
    pub fn alphabet(start: char, end: char) -> Option<impl Iterator<Item=char>> {
        // Check if there are invalid code points in the range.
        if start <= '\u{D7FF}' && end >= '\u{E000}' {
            None
        } else {
            Some(_impl_alphabet_iter(start as u32, end as u32))
        }
    }
    fn _impl_alphabet_iter(start: u32, end: u32) -> impl Iterator<Item=char> {
        let mut cursor = start;
        std::iter::from_fn(move || {
            if cursor > end {
                None
            } else {
                cursor += 1;
                // Unwrap is ok here as we've already checked it's a char
                Some( char::try_from(cursor - 1).unwrap() )
            }
        })
    }
}

use alphabet_iter::alphabet;
fn main() {
    if let Some(mut iter) = alphabet('a', 'z') {
        for letter in iter {
            println!("{}", letter);
        }
    } else {
        println!("Not a valid range!");
    }
}