diff --git a/ch07/ex10.hs b/ch07/ex10.hs
index dcd533d..8d3a6c4 100644
--- a/ch07/ex10.hs
+++ b/ch07/ex10.hs
@@ -9,4 +9,4 @@ luhnDouble x =
     else x * 2
 
 luhn :: [Int] -> Bool
-luhn xs = sum (altMap luhnDouble id xs) `mod` 10 == 0
+luhn xs = sum (altMap id luhnDouble $ reverse xs) `mod` 10 == 0
diff --git a/ch12/ex7.hs b/ch12/ex7.hs
index f3baa15..0b4c848 100644
--- a/ch12/ex7.hs
+++ b/ch12/ex7.hs
@@ -4,22 +4,20 @@ data Expr a
   | Add (Expr a) (Expr a)
   deriving (Show)
 
-instance Functor Expr
+instance Functor Expr where
   -- fmap :: (a -> b) -> Expr a -> Expr b
-                                          where
   fmap _ (Val x) = Val x
   fmap g (Var x) = Var (g x)
   fmap g (Add a b) = Add (fmap g a) (fmap g b)
 
-instance Applicative Expr
+instance Applicative Expr where
   -- pure :: a -> Expr a
-                         where
   pure = Var
+  
   -- (<*>) :: Expr (a -> b) -> Expr a -> Expr b
-  _ <*> 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)
+  Val x <*> _ = error "Cannot apply a non-function value"
+  _ <*> Val x = error "Cannot apply a non-function value"
+  Var f <*> x = fmap f x
   Add f g <*> x = Add (f <*> x) (g <*> x)
 
 instance Monad Expr where
@@ -28,6 +26,7 @@ instance Monad Expr where
   Val x >>= _ = Val x
   Var x >>= f = f x
   Add a b >>= f = Add (a >>= f) (b >>= f)
+
 --examples
 -- Var 5 >>= \x -> Var (x*3)
 -- result: Var 15