Add remaining haskell book exercises

1 files changed, 35 insertions, 0 deletions

diff --git a/Haskell-book/26/MaybeT/src/Reader.hs b/Haskell-book/26/MaybeT/src/Reader.hs
new file mode 100644
index 0000000..38fded9
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+++ b/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