rust-nom-beginning-of-line/src/main.rs

use nom::{
    bytes::complete::{tag, take_while},
    combinator::recognize,
    sequence::{pair, preceded},
    IResult,
};

/**
 * Using Rust Nom, show how to detect "content that begins at the start of a line." This particular
 * variant rolls forward until it finds any content *other than* the start of a line, which is
 * defined as "the input token after any \n".
 *
 */
fn is_beginning_of_line(input: &str) -> IResult<&str, &str> {
    if input.is_empty() {
        // It took me an absolutely ridiculous amount of time to find a simple "how do you construct
        // a standard error in Rust Nom" example. Shout-out to Daniel Imfeld (@dimfeld)
        // (https://imfeld.dev/writing/parsing_with_nom) for being the *23rd* entry Google offered
        // to answer that question, and being the *first* one with an example that wasn't "How to
        // write a custom Nom error" or "How to handle errors in Nom."
        return Err(nom::Err::Error(nom::error::Error::new(
            input,
            nom::error::ErrorKind::Eof,
        )));
    }
    
    let (remaining, recognized) = recognize(preceded(
        take_while(|c| c == '\n'),
        // The empty string tag always succeeds without consuming any of the input. So we're
        // skipping the start-of-line markers before "trivially" succeeding, always successfully
        // checking the empty token, so we don't actually consume the next token.
        tag(""),
    ))(input)?;

    // Remaining is now pointing to the first token *after* the line feed, which could be literally
    // where the input started, or remaining is where the input needs to be next *after* the input
    // started with "\n". (I.e, we rolled forward one or more "\n"'s, but didn't meet anything else
    // so we can't be anywhere but at column 0).
    if std::ptr::eq(input, remaining) || input.starts_with('\n') {
        Ok((remaining, recognized))
    } else {
        // I wasn't really sure what error to return here. This is one of those parsers you'll
        // probably use in an alt or something and we're consuming a hazy concept of end-of-line
        // until we're sure we're at the beginning of a line. "Tag" was the closest thing that
        // seemed right.
        Err(nom::Err::Error(nom::error::Error::new(
            input,
            nom::error::ErrorKind::Tag,
        )))
    }
}

// And this is how you use it; you're looking for *content* at the *start* of the line, not the
// start itself.
fn pattern_at_beginning_of_line(input: &str) -> IResult<&str, &str> {
    preceded(
        is_beginning_of_line,
        tag("BEGIN"),
    )(input)
}

fn pattern_at_beginning_after_blank_line(input: &str) -> IResult<&str, &str> {
    let (remaining, (recognized, rest)) = pair(
        is_beginning_of_line,
        tag("BEGIN"))(input)?;
    if recognized.chars().filter(|c| *c == '\n').count() < 2 {
        Err(nom::Err::Error(nom::error::Error::new(
            input,
            nom::error::ErrorKind::Tag,
        )))
    } else {
        Ok((remaining, rest))
    }
}


fn main() {
    let input = "\nBEGINThis is where your input will be next.";
    // Note that `pattern_at_beginning_of_line` matches the parser *after* `is_beginning_of_line`.
    // Using `preceded` (above) throws out all the line feeds.
    match pattern_at_beginning_of_line(input) {
        Ok((remaining, matched)) => println!("Matched: '{}', Remaining: '{}'", matched, remaining),
        Err(err) => println!("Error: {:?}", err),
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn predicate_empty_in_not_sol() {
        let result = is_beginning_of_line("");
        assert!(result.is_err());
    }

    #[test]
    fn predicate_sol() {
        let result = is_beginning_of_line("\n");
        assert!(result.is_ok());
    }

    #[test]
    fn predicate_not_sol() {
        let result = is_beginning_of_line("TEST");
        assert!(result.is_ok());
        let (remaining, _) = result.unwrap();
        assert_eq!(remaining, "TEST");
    }

    #[test]
    fn predicate_more_than_sol() {
        let result = is_beginning_of_line("\nTEST");
        assert!(result.is_ok());
        let (remaining, _) = result.unwrap();
        assert_eq!(remaining, "TEST");
    }
    
    #[test]
    fn sample_test() {
        let result = pattern_at_beginning_of_line("BEGIN: the rest");
        assert!(result.is_ok());
        let (remaining, input) = result.unwrap();
        println!("{:?}", input);
        assert_eq!(remaining, ": the rest");
    }

    #[test]
    fn with_leading_return() {
        let result = pattern_at_beginning_of_line("\nBEGIN: the rest");
        assert!(result.is_ok());
        let (remaining, _) = result.unwrap();
        assert_eq!(remaining, ": the rest");
    }

    #[test]
    fn with_multiple_leading_return() {
        let result = pattern_at_beginning_of_line("\n\n\nBEGIN: the rest");
        assert!(result.is_ok());
        let (remaining, _) = result.unwrap();
        assert_eq!(remaining, ": the rest");
    }

    #[test]
    fn with_space_leading_return() {
        let result = pattern_at_beginning_of_line(" \nBEGIN: the rest");
        assert!(result.is_err());
    }
}