Can Go directly represent function calls as a type for use as parameters?

Go Type for Function Call

When utilizing keywords like go and defer, function calls are required as parameters. However, is there a specific type that can be used in a similar manner, particularly for creating a function that expects a function call (not simply a function itself) as an argument?

Limitations and Workarounds

Go does not provide a type that directly allows for this functionality. Instead, one can employ function-typed variables or values and call them later as if they were functions. For instance:

<code class="go">func myFunc() {
    fmt.Println("hi")
}

func main() {
    var f func()
    f = myFunc
    f() // This calls the function value stored in f: myFunc in this example
}</code>

To achieve the desired functionality mentioned in the comment, wrap the function call and arguments within a func() and utilize that. For example:

<code class="go">func launch(f func()) {
    fmt.Println("Before launch")
    go func() {
        defer fmt.Println("After completion")
        f()
    }()
}</code>

Usage:

<code class="go">func main() {
    launch(func() {
        fmt.Println("Hello, playground")
    })

    time.Sleep(time.Second)
}</code>

Output:

Before launch
Hello, playground
After completion

This method does not involve a direct workaround. If parameters change, make a copy before calling launch() and employ the copy within the function literal (closure).

Mimicking Automatic Parameter Saving

For a specific function type, create a helper function with identical signature, returning a function without parameters. The returned function acts as a closure, calling the original function with the parameters. Invoking the helper function effectively saves the parameters, mimicking the behavior of defer:

<code class="go">func wrapPrintln(s string) func() {
    return func() {
        fmt.Println(s)
    }
}</code>

Usage:

<code class="go">launch(wrapPrintln(s))</code>

Using Reflection

Reflection can eliminate the need for manual copies, but this approach involves passing functions as parameters instead of calling them. It is also slower due to reflection overhead.

<code class="go">func launch(f interface{}, params ...interface{}) {
    fmt.Println("Before launch")
    go func() {
        defer fmt.Println("After completion")
        pv := make([]reflect.Value, len(params))
        for i, v := range params {
            pv[i] = reflect.ValueOf(v)
        }
        reflect.ValueOf(f).Call(pv)
    }()
}</code>

Example usage:

<code class="go">func main() {
    i, s := 1, "Hello, playground"

    launch(fmt.Printf, "%d %q\n", i, s)
    i, s = 2, "changed"

    time.Sleep(time.Second)
}</code>

Output:

Before launch
1 "Hello, playground"
After completion

Exception: Method Value

One exception where automatic parameter saving can be utilized is with method values. When x has static type T and T's method set contains method M, x.M (without calling it) represents a method value that captures a copy of x as the receiver when the expression's result (a function value) is called.

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