Add remaining haskell book exercises

1 files changed, 43 insertions, 0 deletions

diff --git a/Haskell-book/26/MaybeT/src/Identity.hs b/Haskell-book/26/MaybeT/src/Identity.hs
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index 0000000..8189c18
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+++ b/Haskell-book/26/MaybeT/src/Identity.hs
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+module Identity where
+
+import MonadIO
+import MonadTrans
+
+newtype Identity a =
+    Identity { runIdentity :: a }
+    deriving (Eq, Show)
+
+instance Functor Identity where
+    fmap f (Identity a) = Identity (f a)
+
+instance Applicative Identity where
+    pure = Identity
+    (Identity f) <*> (Identity a) = Identity (f a)
+
+newtype IdentityT f a =
+    IdentityT { runIdentityT :: f a }
+    deriving (Eq, Show)
+
+instance (Functor m)
+  => Functor (IdentityT m) where
+      fmap f (IdentityT fa) = IdentityT (fmap f fa)
+
+instance (Applicative m)
+  => Applicative (IdentityT m) where
+      pure x = IdentityT (pure x)
+
+      (IdentityT fab) <*> (IdentityT fa) =
+          IdentityT (fab <*> fa)
+
+instance (Monad m)
+  => Monad (IdentityT m) where
+      return = pure
+
+      (IdentityT ma) >>= f = IdentityT $ ma >>= runIdentityT . f
+
+instance (MonadIO m)
+  => MonadIO (IdentityT m) where
+      liftIO = IdentityT . liftIO
+
+instance MonadTrans IdentityT where
+    lift = IdentityT