Add remaining haskell book exercises

Haskell-book/18/Functions.hs

@@ -0,0 +1,21 @@
+module Functions where
+
+j :: Monad m => m (m a) -> m a
+j = flip (>>=) id
+
+l1 :: Monad m => (a -> b) -> m a -> m b
+l1 = fmap
+
+l2 :: Monad m => (a -> b -> c) -> m a -> m b -> m c
+l2 f xs ys = f <$> xs <*> ys
+
+a :: Monad m => m a -> m (a -> b) -> m b
+a xs f = f <*> xs
+
+meh :: Monad m => [a] -> (a -> m b) -> m [b]
+meh xs f = rec $ fmap f xs
+    where rec [] = return []
+          rec (x:xs) = (:) <$> x <*> (rec xs)
+
+flipType :: Monad m => [m a] -> m [a]
+flipType = flip meh id