Welcome to the third lesson of our course, where we explore the concept of using structs as Parameter Objects to address a common code smell: complex function signatures in Go. Throughout this course, we strive to eliminate code smells to enhance the readability, maintainability, and scalability of your codebase, with a strong emphasis on Test Driven Development (TDD) using Go and Testify.

In our previous lessons, we addressed code smells such as duplicated code using the Extract Function technique to manage long functions. Today, we are focusing on the code smell associated with long parameter lists and how to effectively address it using Go's structural capabilities. You will learn to leverage Go structs to encapsulate related parameters and improve the clarity and usability of your functions. Mastering the TDD cycle — Red, Green, Refactor — will be crucial as we continue to refactor and improve your codebase in this lesson.

Long parameter lists are considered a code smell because they complicate function signatures, making the code harder to read, maintain, and test. This complexity arises from several issues:

1. Readability: When a function has too many parameters, it becomes challenging to understand each parameter's purpose without referring to documentation or the function's implementation. This clutter makes the code less intuitive.

2. Maintainability: Modifying a function with a long parameter list becomes cumbersome. Adding or removing parameters can introduce errors in existing function calls throughout the codebase, increasing the risk of bugs.

3. Error-Prone: Parameters can easily be mixed up or misordered, especially when they have similar types. Even with type-checking, logical errors can occur if parameters are passed incorrectly.

4. Testing Challenges: Long parameter lists complicate writing and maintaining tests, as test cases need to supply many arguments. This complexity can lead to brittle tests that are more challenging to maintain.

5. Lack of Cohesion: Long parameter lists might indicate unrelated parameters or a function that is doing too much, violating the single responsibility principle.

By recognizing long parameter lists as a code smell and refactoring them into structs as Parameter Objects, we can improve code maintainability and readability, making it more robust and easier to understand.

Let's start by identifying a problematic long parameter list in an existing Go code example. Consider the ProcessExamScore function. The numerous parameters make it difficult to read and navigate effectively.

This complexity can lead to issues such as incorrectly ordered parameters during function calls and increased difficulty in refactoring or adding new features.

Furthermore, these long parameter lists complicate the testing process, as seen below:

Notice how difficult it is to understand this test! The meaning of values like 0.7, 0.1, 2, and 75 are not immediately clear.

Notice the following:

• Re-ordering parameters: Related values are grouped in structs, enhancing readability and logical flow.
• Parameter Objects Using Structs: Function signature has been simplified by encapsulating related parameters into cohesive structs such as ScoreWeights and CoursePolicy.

• DefaultScoreWeights() Function: Provides default settings for exam, homework, and attendance weightings.
• DefaultCoursePolicy() Function: Does the same, except it does it for the Course Policy
• DefaultIndividualPerformance(examScore float64) Function: The most likely to change is the ExamScore so we can pass that in here

The refactored test below highlights improved readability:

This concise form minimizes cognitive load, reduces potential errors related to parameter misordering, and increases maintainability, making the test more intuitive and focused on its intent.

In real-world scenarios, structs as Parameter Objects are exceptionally useful in Go when methods require multiple related parameters. For example, consider handling user settings or configuration parameters where grouping related values enhances the readability of function calls and improves test manageability.

Alternative approaches like maps or builder patterns might be more suitable when parameters don't naturally compose a cohesive group, demonstrating Go's flexibility.

In this lesson, we learned to refactor long parameter lists using structs as Parameter Objects in Go:

• Recognized problems caused by long parameter lists.
• Created Go structs to encapsulate and bundle related parameters, simplifying function signatures.
• Re-organized parameters for improved clarity.
• Used Go idioms to provide sensible defaults.
• Updated tests to better accommodate the refactored design using Testify.

Prepare to apply these techniques in upcoming exercises where you'll enhance your skills in refactoring and testing using Go structs. These exercises will reinforce the TDD workflow — Red, Green, Refactor — and help maintain a robust, scalable codebase.