Nom example

main.rs

use nom::branch::alt;
use nom::bytes::complete::tag;
use nom::character::complete;
use nom::sequence::preceded;
use nom::{error::ParseError, AsChar, IResult, InputIter, Parser, Slice};

use std::str::FromStr;

#[derive(Clone, Copy, PartialEq, Debug)]
enum Flip {
    Heads,
    Tails,
}

impl FromStr for Flip {
    type Err = &'static str;

    fn from_str(text: &str) -> Result<Self, Self::Err> {
        match text {
            "HEADS" => Ok(Flip::Heads),
            "TAILS" => Ok(Flip::Tails),
            _ => Err("Invalid result"),
        }
    }
}

type ParseResult<'a, T> = IResult<&'a str, T>;

fn after_slash<I, O, F, E: ParseError<I>>(parser: F) -> impl FnMut(I) -> IResult<I, O, E>
where
    F: Parser<I, O, E>,
    I: Slice<std::ops::RangeFrom<usize>> + InputIter,
    <I as InputIter>::Item: AsChar,
{
    preceded(complete::char('/'), parser)
}

fn main() {
    const INPUT: &str = "/HEADS/TAILS";
    
    let result: ParseResult<Flip> =
        tuple((after_slash(<&str>::parse_to), after_slash(<&str>::parse_to)))(INPUT);
    
    // Current best working alternative:
    // tuple(( after_slash( alt( tag("HEADS"), tag("TAILS") ) repeat a million times
    
    println!("Got: {:?}", result);
}