Haskell-book/26/MaybeT/src/Reader.hs


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module Reader where

import MonadIO
import MonadTrans

newtype ReaderT r m a =
    ReaderT { runReaderT :: r -> m a }

instance (Functor m)
      => Functor (ReaderT r m) where
    fmap f (ReaderT rma) =
        ReaderT $ (fmap . fmap) f rma

instance (Applicative m)
      => Applicative (ReaderT r m) where
    pure a = ReaderT (pure (pure a))

    (ReaderT fmab) <*> (ReaderT rma) =
        ReaderT $ (<*>) <$> fmab <*> rma

instance (Monad m)
      => Monad (ReaderT r m) where
    return = pure

    (ReaderT rma) >>= f =
        ReaderT $ \r -> do
            a <- rma r
            runReaderT (f a) r

instance MonadTrans (ReaderT r) where
    lift = ReaderT . const

instance (MonadIO m)
  => MonadIO (ReaderT r m) where
      liftIO = lift . liftIO