Becoming Functional

Chapter 8. Pattern Matching

Mostly when we, as programmers, think of pattern matching we think of regular expressions. But in the context of functional programming, this terminology takes on a new meaning. Instead of regular expression matching, we’re going to be looking at matching objects against other objects.

Using pattern matching, you can extract from objects, match on members of objects, and verify that objects are of specific types—all within a statement. Pattern matching allows for fewer lines of variable assignment and more lines of understandable code. With pattern matching, you can match on members of an object, which allows you to write more concise logic for when a specific segment of code should be executed.

Simple Matches

Now that the code has started shaping up, our boss has asked us to create a new function that will create a new Customer. The requirements are as follows:

• name cannot be blank.
• state cannot be blank.
• domain cannot be blank.
• enabled must be true to start with.
• contract will be created based on today’s date.
• contacts should be created as a blank list for now.

Our basic method, as shown in Example 8-1, uses a large if structure to return null in the event that an invalid value is passed in. We’re currently printing to the console, but we should also log the message being sent.

Example 8-1. Imperative createCustomer method using an if structure

if(name.isEmpty) {
  println("Name cannot be blank")
  null
} else if(state.isEmpty) {
  println("State cannot be ...