Parsing combinators

Traditional, without documentation

• type P a
  
  nil   :: a -> P a
  token :: (String->Maybe a) -> P a
  (!)   :: P a -> P a -> P a
  (<@)  :: P (a->b) -> P a -> P b
  
  parse :: P a -> [String] -> Either ErrorMessage a
  

• Note: the command line is of type [String], so the token type is String