Function gchemol_parser::parsers::peek

pub fn peek<I, O, E, F>(f: F) -> impl Fn(I) where
    E: ParseError<I>,
    F: Fn(I) -> Result<(I, O), Err<E>>,
    I: Clone, 

tries to apply its parser without consuming the input

use nom::combinator::peek;
use nom::character::complete::alpha1;

let parser = peek(alpha1);

assert_eq!(parser("abcd;"), Ok(("abcd;", "abcd")));
assert_eq!(parser("123;"), Err(Err::Error(("123;", ErrorKind::Alpha))));