Clean Functions and Methods in Scala

Welcome to your next step in mastering Clean Code! 🚀 Previously, we emphasized the significance of naming conventions in clean coding. Now, we delve into the realm of functions and methods, which serve as the backbone of application logic and are crucial for code organization and execution. Structuring these functions effectively is vital for enhancing the clarity and maintainability of a codebase. In this lesson, we'll explore best practices and techniques to ensure our code remains clean, efficient, and readable.

Let's outline the key principles for writing clean functions:

• Keep functions small. Small functions are easier to read, comprehend, and maintain.
• Focus on a single task. A function dedicated to one task is more reliable and simpler to debug.
• Limit arguments to three or fewer. Excessive arguments complicate the function signature and make it difficult to understand and use.
• Avoid boolean flags. Boolean flags can obscure the code's purpose; consider separate methods for different behaviors.
• Eliminate side effects. Functions should avoid altering external state or depending on external changes to ensure predictability.
• Implement the DRY principle. Employ helper functions to reuse code, minimizing redundancy and enhancing maintainability.

Now, let's take a closer look at each of these rules.

Functions should remain small, and if they become too large, consider splitting them into multiple, focused functions. While there's no fixed rule on what counts as large, a common guideline is around 15 to 25 lines of code, often defined by team conventions.

Below, you can see the processOrder function, which is manageable but has the potential to become unwieldy over time:

def processOrder(order: Order, inventory: Inventory, logger: Logger): Unit =
  // Step 1: Validate the order
  if !order.isValid() then
    logger.log("Invalid Order")
    return

  // Step 2: Process payment
  if !order.processPayment() then
    logger.log("Payment failed")
    return

  // Step 3: Update inventory
  inventory.update(order.getItems())

  // Step 4: Notify customer
  order.notifyCustomer()

  // Step 5: Log order processing
  logger.log("Order processed successfully")

Given that this process involves multiple steps, it can be improved by extracting each step into a dedicated private function, as shown below:

def processOrder(order: Order, inventory: Inventory, logger: Logger): Unit =
  if !validateOrder(order, logger) then return
  if !processPayment(order, logger) then return
  updateInventory(order, inventory)
  notifyCustomer(order)
  logOrderProcessing(logger)

private def validateOrder(order: Order, logger: Logger): Boolean =
  if !order.isValid() then
    logger.log("Invalid Order")
    false
  else
    true

private def processPayment(order: Order, logger: Logger): Boolean =
  if !order.processPayment() then
    logger.log("Payment failed")
    false
  else
    true

private def updateInventory(order: Order, inventory: Inventory): Unit =
  inventory.update(order.items)

private def notifyCustomer(order: Order): Unit =
  order.notifyCustomer()

private def logOrderProcessing(logger: Logger): Unit =
  logger.log("Order processed successfully")

A function should embody the principle of doing one thing only. If a function handles multiple responsibilities, it may include several logical sections. Below, you can see the saveAndNotifyUser function which is both too lengthy and does multiple different things at once:

def saveAndNotifyUser(user: User, dataSource: DataSource, webClient: WebClient): Unit =
  val sql = "INSERT INTO users (name, email) VALUES (?, ?)"
  val connection = dataSource.getConnection()
  val statement = connection.prepareStatement(sql)

  try
    statement.setString(1, user.name)
    statement.setString(2, user.email)
    statement.executeUpdate()
  catch
    case e: SQLException => e.printStackTrace() // Handle SQL exception
  finally
    connection.close()

  webClient.post()
           .uri("/sendWelcomeEmail")
           .bodyValue(user)
           .retrieve()
           .onStatus(_.isError, _ => throw new RuntimeException("Failed to send email"))
           .block()

To enhance this code, you can create two dedicated functions for saving the user and sending the welcome email. This results in dedicated responsibilities for each function and clearer code coordination:

def saveAndNotifyUser(user: User, dataSource: DataSource, webClient: WebClient): Unit =
  saveUser(user, dataSource)
  notifyUser(user, webClient)

private def saveUser(user: User, dataSource: DataSource): Unit =
  val sql = "INSERT INTO users (name, email) VALUES (?, ?)"
  val connection = dataSource.getConnection()
  val statement = connection.prepareStatement(sql)

  try
    statement.setString(1, user.name)
    statement.setString(2, user.email)
    statement.executeUpdate()
  catch
    case e: SQLException => e.printStackTrace() // Handle SQL exception
  finally
    connection.close()

private def notifyUser(user: User, webClient: WebClient): Unit =
  webClient.post()
           .uri("/sendWelcomeEmail")
           .bodyValue(user)
           .retrieve()
           .onStatus(_.isError, _ => throw new RuntimeException("Failed to send email"))
           .block()

Try to keep the number of function arguments to a maximum of three, as having too many can make functions less understandable and harder to use effectively. 🤔

Consider the saveAddress function below with five arguments, which makes the function less clean:

def saveAddress(street: String, city: String, state: String, zipCode: String, country: String): Unit =
  // Logic to save address

A cleaner version encapsulates the details into an Address case class, reducing the number of arguments and making the function signature clearer:

case class Address(street: String, city: String, state: String, zipCode: String, country: String)

def saveAddress(address: Address): Unit =
  // Logic to save address

Boolean flags in functions can create confusion, as they often suggest multiple pathways or behaviors within a single function; instead, use separate methods for distinct behaviors.

The setFlag function below uses a boolean flag to indicate user status, leading to potential complexity:

def setFlag(user: User, isAdmin: Boolean): Unit =
  // Logic based on flag

A cleaner approach is to have distinct methods representing the different behaviors:

def grantAdminPrivileges(user: User): Unit =
  // Logic for admin rights

def revokeAdminPrivileges(user: User): Unit =
  // Logic to remove admin rights

A side effect occurs when a function modifies some state outside its scope or relies on something external. This can lead to unpredictable behavior and reduce code reliability.

Below, the addToTotal function demonstrates a side effect by modifying an external state:

// Not Clean - Side Effect
var total: Int = 0 // external state

def addToTotal(value: Int): Int =
  total += value // modifies external state
  total

A cleaner version, calculateTotal, performs the operation without altering any external state:

// Clean - No Side Effect 
def calculateTotal(initial: Int, value: Int): Int =
  initial + value

Avoid code repetition by introducing helper functions to reduce redundancy and improve maintainability.

The printUserInfo and printManagerInfo functions below repeat similar logic, violating the DRY principle:

def printUserInfo(user: User): Unit =
  println(s"Name: ${user.name}")
  println(s"Email: ${user.email}")

def printManagerInfo(manager: Manager): Unit =
  println(s"Name: ${manager.name}")
  println(s"Email: ${manager.email}")

To adhere to DRY principles, use a generalized printInfo function that operates on a parent Person type:

def printInfo(person: Person): Unit =
  println(s"Name: ${person.name}")
  println(s"Email: ${person.email}")

In this lesson, we learned that clean functions are key to maintaining readable and maintainable code. By keeping functions small, adhering to the Single Responsibility Principle, limiting arguments, avoiding side effects, and embracing the DRY principle, you set a strong foundation for clean coding. Next, we'll practice these principles to further sharpen your coding skills! 🎓