Function combine::char::letter

pub fn letter<I>() -> Letter<I> where
    I: Stream<Item = char>,
    I::Error: ParseError<I::Item, I::Range, I::Position>, 

Parses an alphabet letter according to std::char::is_alphabetic.

use combine::Parser;
use combine::char::letter;
assert_eq!(letter().parse("a"), Ok(('a', "")));
assert_eq!(letter().parse("A"), Ok(('A', "")));
assert!(letter().parse("9").is_err());