Haskell Cookbook

How it works...

1. The binary tree can either be empty (no value, no children) or can have exactly two child trees and a value. 
2. The binary tree is parameterized by a type argument. The type argument denotes the type of the value that each node stores.

3. The binary tree is defined as a sum type. Following are the alternatives for the sum type:
   1. The empty tree is denoted by data constructor Leaf.
   2. The binary tree node is a product type (BinaryTree) implemented using record syntax, with the following fields:
      • left: This denotes the left binary tree
      • val: This indicates the value of the node
      • right: This denotes the right binary tree
   3. Since left and right  are of type BinaryTree, this is a recursively defined data type.
4. The helper function empty ...