When Are Empty Parentheses Required for Function Invocation in Go Closures?

Function Invocation Parentheses in Go Closures

In Go, closures are anonymous functions that can reference variables from their enclosing scope. However, after the closure body, one may encounter the use of empty parentheses "()". This usage, often seen in function literals and defer statements, raises questions about its purpose.

Function Literals

Function literals are anonymous functions assigned to variables or passed as arguments. Consider the example:

func(ch chan int) { ch <- ACK }

Here, the empty parentheses are used to invoke the function immediately. By adding the parentheses, we effectively call the anonymous function, passing in the replyChan channel.

Defer Statements

The defer statement executes a function at the end of the surrounding function, after the return statement. In the following example:

func f() (result int) {
    defer func() {
        result++
    }()

The empty parentheses are required because the defer statement requires a function call as its argument. By adding the parentheses, we invoke the anonymous function and immediately increment the result variable.

Why is Function Invocation Necessary?

The reason for requiring function invocation in defer statements is to ensure that the function is executed at the end of the surrounding function, regardless of how execution exits the function (e.g., return, panic). By calling the function immediately, the closure captures the current state of the variables it references, ensuring the intended behavior when the function executes later.

Note:

While the use of empty parentheses is common after closures in defer statements, it is not limited to closures. Any function call must be enclosed in parentheses to invoke the function.

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