Function signed 

pub fn signed<'a, N, F, P, I, C>(
    parser: F,
    plus_sign: bool,
) -> impl Parser<I, Output = N> + 'a
where
    F: FnMut(bool) -> P + 'a,
    P: Parser<I, Output = N> + 'a,
    I: Input<Ok = C> + ?Sized + 'a,
    C: Character + 'a,

Takes a function returns a integer parser, returns a parser of signed integer.

The taken function must return negative result if the argument is true, and vice versa. If plus_sign is true, it allows plus signs besides minus signs, has no effects.