Why use package variables for "flag" values?

feralmosquito · January 20, 2017, 12:26am

I’m reading The Go Programming Language, and in all the examples I’ve seen using the flag package, package-level variables are used to hold the flags. Examples:

github.com

adonovan/gopl.io/blob/master/ch2/echo4/main.go#L16




// Echo4 prints its command-line arguments.
package main


import (
	"flag"
	"fmt"
	"strings"
)


var n = flag.Bool("n", false, "omit trailing newline")
var sep = flag.String("s", " ", "separator")


func main() {
	flag.Parse()
	fmt.Print(strings.Join(flag.Args(), *sep))
	if !*n {
		fmt.Println()
	}
}

The forum won’t let me make this a link, but here’s another: https://github.com/adonovan/gopl.io/blob/master/ch7/sleep/sleep.go#L16

My question is, why? Making it a package variable is non-obvious to me. The examples seem to work just as well if I move the declarations into main().

Thanks in advance for any insight!

dfc · January 20, 2017, 4:26am

It makes it easier to do something like

var db string

func main() {
      flag.StringVar(*db, "db", "localhost:3389", "database host")
      flag.Parse()
      processData()
}

func processData() {
      conn, err := openDatabaseConnection(db, ...)
      ...
}

IMO, it’s clearer to explicitly pass db into processData and scope the declaration of db to the same function that calls flag.StringVar

system · April 20, 2017, 4:26am

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