applicative parser

parser.hs

import Control.Applicative
import Data.Char

newtype Parser a = P (String -> [(a, String)])

instance Functor Parser where
  fmap g p = P (\inp -> case parse p inp of
                     []         -> []
                     [(v, out)] -> [(g v, out)])

instance Applicative Parser where
  pure v = P (\inp -> [(v, inp)])
  pg <*> px = P (\inp -> case parse pg inp of
                      [] -> []
                      [(g, out)] -> parse (fmap g px) out)

parse (P p) inp = p inp

item = P (\inp -> case inp of
               [] -> []
               (x:xs) -> [(x, xs)])

three = pure g <*> item <*> item <*> item
        where g x y z = (x, z)