Page440 solution to exercise in HPFFP

Page440.hs

module Page440 where

data BinaryTree a =
    Leaf
  | Node (BinaryTree a) a (BinaryTree a)
  deriving (Eq, Ord, Show)

mapTree :: (a -> b)
        -> BinaryTree a
        -> BinaryTree b
mapTree _ Leaf = Leaf
mapTree f (Node left a right) =
    Node (mapTree f left) (f a) (mapTree f right)

testTree' :: BinaryTree Integer
testTree' =
  Node (Node Leaf 3 Leaf)
       1
       (Node Leaf 4 Leaf)

mapExpected =
  Node (Node Leaf 4 Leaf)
       2
       (Node Leaf 5 Leaf)

-- acceptance test for mapTree
mapOkay =
  if mapTree (+1) testTree' == mapExpected
  then print "yup okay!"
  else error "test failed!"