Function combine::combinator::satisfy

pub fn satisfy<I, P>(predicate: P) -> Satisfy<I, P> where
    I: Stream,
    P: FnMut(I::Item) -> bool, 

Parses a token and succeeds depending on the result of predicate.

let mut parser = satisfy(|c| c == '!' || c == '?');
assert_eq!(parser.parse("!").map(|x| x.0), Ok('!'));
assert_eq!(parser.parse("?").map(|x| x.0), Ok('?'));