greet/greet.go

package main

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

type numberOfArgumentError struct {
	length int
}

func (e numberOfArgumentError) Error() string {
	return fmt.Sprintf("unexpected number of arguments passed to the greeter: want 0 or 1 argument, got %d", e.length)
}

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

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

func greet(writer io.Writer, args []string) error {
	var name string

	if len(args) > 1 {
		return numberOfArgumentError{length: len(args)}
	}

	if len(args) == 0 {
		name = "World"
	} else {
		name = args[0]
	}

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

	return nil
}