To write a program one follows a recipe of steps. Here are the steps we will use for all our programs.

• Each step is a concrete activity
• Each step has a product that can be inspected

Here are the steps we will follow:

• Understand the problem. Often the analysis is inserted in the program as a comment.
• Write a contract for each function about how its inputs and outputs behave. In Haskell, a contract is written using a typing prototype.
• Create a set of examples. We will use HUnit assertions as the product that encodes examples.
• Write the body of each function. Use the guideline that the structure of the body follows the structure of the input data.
• Testing. Test your program. We will use HUnit tests as the testing product. Later we will learn how to use QiuckCheck random testing as additonal testing products.

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