Stupid Reader Monad Instance

StupidReader.hs

{-# LANGUAGE ApplicativeDo #-}

newtype Reader t a = Reader { runReader :: t -> a }

instance Functor (Reader t) where
  fmap f (Reader g) = Reader (\x -> f (g x))

instance Applicative (Reader t) where
  pure = return
  Reader f <*> Reader g = Reader $ \x -> (f x) (g x)

instance Monad (Reader t) where
  return x = Reader (\_ -> x)
  Reader f >>= g = Reader (\x -> runReader (roll x) x)
    where roll = g . f

ask :: (t -> a) -> Reader t a
ask f = Reader (\x -> f x)

data Enviroment = Env { name :: String, price :: String, desc :: String }
  deriving (Eq, Show)

dogeR :: Reader Enviroment String
dogeR = do
  n <- ask name
  p <- ask price
  d <- ask desc
  return (n ++ ": $" ++ p ++ "\n" ++ d)

doge :: String
doge = runReader dogeR (Env "World" "50000" "Beautiful world!")