Heap.hs

data Tree = Leaf Int | Node Int Tree Tree deriving Show

getValue tree =
 case tree of
  Leaf value -> value
  Node value tree1 tree2 -> value

isHeap tree =
 case tree of
  Leaf value -> True
  Node value tree1 tree2 ->
   isHeap tree1 && isHeap tree2 &&
    value <= getValue tree1 && value <= getValue tree2

tree1 = Node 2 (Node 3 (Leaf 4) (Leaf 7)) (Leaf 6)
tree2 = Node 20 (Node 3 (Leaf 4) (Leaf 7)) (Leaf 6)
tree3 = Node 2 (Node 30 (Leaf 4) (Leaf 7)) (Leaf 6)

results = map isHeap [tree1,tree2,tree3]