wtf.haskell

wtf.hs

-- Ex. 2 - implement an Applicative instance for Parser
--
-- A. pure a represents the parser which consumes no input
--    and successfully returns a result of a.
--
-- B. p1 <*> p2 represents the parser which first runs p1
--    (which will consume some input and produce a
--    function), <----- doesn't this produce a Maybe (a, String) ????

--    ...then passes the remaining input to p2
--    (which consumes more input and produces some value),
--    then returns the result of applying the function to
--    the value. However, if either p1 or p2 fails then the
--    whole thing should also fail (put another way, p1 <*>
--    p2 only succeeds if both p1 and p2 succeed).

instance Applicative Parser where
  pure x = Parser $ \s -> Just (x, s)
  (Parser p1) <*> (Parser p2) = Parser f
    where
      f s = case p1 s of
        (Just (x, xs)) -> case p2 xs of
          (Just (y, ys)) -> Nothing
          Nothing -> Nothing
        Nothing -> Nothing