Implementation of our command line parsing combinators

• To be able to extract the grammar, implement all combinators as constructors in a data type!

• data P a where
      Nil   :: a -> P a
      Ap    :: P (b->a) -> P b -> P a
      Token :: (String->Maybe a) -> String -> P a
      (:--) :: P a -> String -> P a
      (:!)  :: P a -> P a -> P a
      Many  :: P a -> P [a]
      ...
  

• But...