Function peek

pub fn peek<Input, Output, Error, ParseNext>(
    parser: ParseNext,
) -> impl Parser<Input, Output, Error>
where
    Input: Stream,
    Error: ParserError<Input>,
    ParseNext: Parser<Input, Output, Error>,

Apply the parser without advancing the input.

To lookahead and only advance on success, see opt.

Example

use winnow::combinator::peek;
use winnow::ascii::alpha1;

fn parser<'i>(input: &mut &'i str) -> ModalResult<&'i str> {
    peek(alpha1).parse_next(input)
}

assert_eq!(parser.parse_peek("abcd;"), Ok(("abcd;", "abcd")));
assert!(parser.parse_peek("123;").is_err());