The following is the fully excutable go program of "Hello World!"

package main

import "fmt"

func main() {
    fmt.Println("Hello, World!")
}

First, note that at very beginning, we have the line

package main

Every go script begins with the

package ${filename}

where ${filename} is the name of the go script. In this way, a go project is structured as a collection of packages。

this is $\mathscr{A}$