When do you declare variables in anonymous functions in Golang?

Question

I use some anonymous functions in my code and I’m trying to understand the difference (if there is one) between these two code snipets being called in a function:

defer func(s *Service, ID string) {
    err := s.Deprovision(ID)
    if err != nil {
        s.logger.Error(err)
    }
}(service, identifier)


defer func() {
    err := service.Deprovision(identifier)
    if err != nil {
        service.logger.Error(err)
    }
}()

One directly uses variables available in the parent func and the other essentially declares these variables in the anonymous function and then specifies the parent variables to pass in.

Does this make a difference? If so, when do I need to be mindful of this?

Answer 1

The two code snippets you provided are both using anonymous functions with defer statements. However, they differ in how they capture and use variables from the enclosing function. Let's break down the differences and discuss when you need to be mindful of them:

1. Direct Use of Variables (First Snippet): In the first code snippet, the anonymous function directly uses the parameters s and ID from the enclosing function's scope. This means that these variables are captured by the closure and can be accessed directly within the anonymous function without being passed as arguments.

2. Passing Variables as Arguments (Second Snippet): In the second code snippet, the anonymous function does not use the parameters from the enclosing function's scope. Instead, it captures the service and identifier variables by value from the enclosing function's scope and then uses them within the function body. This is essentially equivalent to declaring and initializing new local variables within the anonymous function.

Does this make a difference?

Yes, there is a difference between the two approaches, and the choice between them depends on your specific use case.

• Scoping and Clarity: The first approach is more explicit and may be clearer to someone reading the code, as it shows exactly which variables are being used from the enclosing scope. This can help prevent potential mistakes and improve code readability.

• Performance and Memory: The second approach, where variables are captured by value, can have a small impact on performance and memory usage, especially if the enclosing function's scope contains large objects. This is because each closure will have its own copy of the captured variables, which could increase memory consumption.

When to Be Mindful:

1. Variable Changes: If the values of the variables (service and identifier in your case) change after the anonymous function is defined but before it is executed (due to loops or other control structures), the behavior of the second approach might be unexpected.

2. Closures and Goroutines: When using goroutines in conjunction with closures, the captured variables can have a subtle impact on concurrency behavior. Make sure you understand how closures capture variables and how they can affect concurrent execution.

3. Performance Considerations: If memory usage is a concern or if capturing by value causes performance issues, you might want to consider the first approach, which directly uses the existing variables.

In most cases, the differences between the two approaches might not be significant, but it's good to be aware of these considerations when making your choice. Choose the approach that best fits your specific use case and promotes code clarity and maintainability.

Answer 2

A demo on the differences between the two methods. Both functions modify the value, and both print the address of result.

func something(){
    result := 1
    foo := func() {
        result = 2
        fmt.Println("No parameters: ", &result)
    }

    bar := func(res int) {
        res = 3
        fmt.Println("Passed parameters: ", &res)
    }

    fmt.Println("Original: ", &result)
    foo()
    bar(result)
    fmt.Println("Final Value: ", result)
}

results in the following output

Original:  0xc00000a0b8
No parameters:  0xc00000a0b8
Passed parameters:  0xc00000a0f0
Final Value:  2

notice that the first two memory addresses are the same, but the third is different. This means it is creating a copy of the variable, and operations on the new copy won't affect the value of the original. This is why the value at the end is still 2. Your needs will determine which approach is best.