Add first two solutions to chapter 12 exercises

Author: evturn

12-monads-and-more/12.05-exercises.hs

@@ -0,0 +1,25 @@
+-- 1.
+data Tree a = Leaf
+            | Node (Tree a) a (Tree a)
+            deriving Show
+
+instance Functor Tree where
+  fmap f (Node l x r) = Node (fmap f l) (f x) (fmap f r)
+  fmap _ Leaf         = Leaf
+
+-----------------------------------------------------------------------------
+-- 2.
+--
+-- |
+-- Avoid conflict with library definition for `((->) r)`
+newtype R r a = R
+              { run :: r -> a }
+
+instance Functor (R r) where
+  fmap f  (R r) = R $ f . r
+
+applyR :: (Int -> Int) -> R Int Int -> Int -> Int
+applyR f r x = run (fmap f r) x
+
+runR :: Int
+runR = applyR (+100) (R $ (+2)) 3