Haskell Cookbook

How to do it...

1. Create a new Haskell project binary-tree-functor using the simple Stack template:

        stack new binary-tree-functor simple

2. Open src/Main.hs. This is the file that will be used for our purposes.
3. After adding module definition for Main, add the following import:

        module Main where        import Data.Functor

4. Define the binary tree and utility functions to create the tree:

        -- The tree can be empty (Leaf) or a node with a         -- value, left and right trees.        data Tree a = Leaf            | Node (Tree a) a (Tree a)             deriving (Show, Eq)

        -- Create a tree given a value, left tree and a right tree        node :: Tree a -> a -> Tree a -> Tree a        node l x r = Node l x r

       -- Induct a value into a new tree (node with empty left and         right trees) singleton :: a ...