Add solutions for chapter exercise seven

Author: evturn

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

@@ -29,3 +29,43 @@ instance Functor ZipList where
 instance Applicative ZipList where
   pure x = Z $ repeat x
   Z fs <*> Z xs = Z [f x | (f, x) <- zip fs xs]
+
+-----------------------------------------------------------------------------
+-- 5.
+-- What?
+
+-----------------------------------------------------------------------------
+-- 6.
+instance Monad ((->) a) where
+  return = pure
+  mx >>= f = \x -> f (mx x) x
+
+-----------------------------------------------------------------------------
+-- 7.
+data Expr a = Var a
+            | Val Int
+            | Add (Expr a) (Expr a)
+            deriving Show
+
+instance Functor Expr where
+  fmap f (Var x)   = Var (f x)
+  fmap _ (Val x)   = Val x
+  fmap f (Add x y) = Add (fmap f x) (fmap f y)
+
+instance Applicative Expr where
+  pure = Var
+  _       <*> Val x   = Val x
+  Val x   <*> _       = Val x
+  Var f   <*> Var x   = Var (f x)
+  Var f   <*> Add x y = Add (fmap f x) (fmap f y)
+  Add f g <*> x       = Add (f <*> x) (g <*> x)
+
+instance Monad Expr where
+  return = pure
+  Val x   >>= _ = Val x
+  Var x   >>= f = f x
+  Add x y >>= f = Add (x >>= f) (y >>= f)
+
+-----------------------------------------------------------------------------
+-- 8.
+