package main

import (
	"fmt"
	"os"
	"io"
)

func main() {
	osArgs := os.Args[1:]

	if err := greet(os.Stdout, osArgs); err != nil {
		fmt.Printf("ERROR: %s.\n", err)
	}
}

func greet(w io.Writer, n []string) error {
	var name string

	if len(n) > 1 {
		return fmt.Errorf("unexpected number of arguments passed to the greeter: want 0 or 1 argument, got %d", len(n))
	} else if len(n) == 0 {
		name = "World"
	} else {
		name = n[0]
	}

	fmt.Fprintf(w, "Hello, %s!\n", name)

	return nil
}