When Are Parentheses Required After a Go Closure Body?

Closure Body Parentheses in Go

In Go, understanding the usage of parentheses after the body of a closure can be perplexing. Let's delve deeper into this concept by examining two examples.

Function Literals:

In the function literal example:

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

The parentheses immediately after the closure body, (replyChan), invoke the function literal and pass the argument replyChan to it.

Defer Statements:

In the defer statement example:

func f() (result int) {
    defer func() {
        result++
    }() // why and how?
    return 0
}

The parentheses after the closure body, (), are not exclusively related to closures but are a part of the defer statement syntax. According to the Go language specifications, the expression in a defer statement must always be a function call. Therefore, it necessitates the parentheses to indicate that the closure is invoked immediately.

Importance of Function Call Syntax:

In Go, there is a distinct difference between a function value and a function call. A function value refers to the actual function definition, while a function call represents the execution of the function.

Consider the following code:

func f() int { return 42 }

a := f // function value
b := f() // function call

In the first line, a is assigned the function value of f, while in the second line, b is assigned the result of invoking f.

Similarly, in a defer statement, the expression must be a function call, not just a function value. Hence, the parentheses are required to indicate the function invocation.

Additional Note:

The parentheses can also encompass an expression list, including an empty list. This distinction is significant because it determines when the closure executes and with which arguments.

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