pub type Spanned<Tok, Loc, Error> = Result<(Loc, Tok, Loc), Error>;

#[derive(Debug)]

pub enum Tok {

    Linefeed,

}

#[derive(Debug)]

pub enum LexicalError {

    // Not possible

}

use std::str::CharIndices;

pub struct Lexer<'input> {

    chars: CharIndices<'input>,

}

impl<'input> Lexer<'input> {

    pub fn new(input: &'input str) -> Self {

        Lexer { chars: input.char_indices() }

    }

}

impl<'input> Iterator for Lexer<'input> {

    type Item = Spanned<Tok, usize, LexicalError>;

    fn next(&mut self) -> Option<Self::Item> {

        loop {

            match self.chars.next() {

                Some((i, '\n')) => return Some(Ok((i, Tok::Linefeed, i+1))),

                None => return None, // End of file

                _ => continue, // Comment; skip this character

            }

        }

    }

}