parse.go
func NewParser(config Config, dests ...interface{}) (*Parser, error) {
	p := Parser{
		config: config,
	}
	for _, dest := range dests {
		if dest, ok := dest.(Versioned); ok {
			p.version = dest.Version()
		}
		if dest, ok := dest.(Described); ok {
			p.description = dest.Description()
		}
		v := reflect.ValueOf(dest)
		if v.Kind() != reflect.Ptr {
			panic(fmt.Sprintf("%s is not a pointer (did you forget an ampersand?)", v.Type()))
		}
		v = v.Elem()
		if v.Kind() != reflect.Struct {
			panic(fmt.Sprintf("%T is not a struct pointer", dest))
		}

		var errs []string
		walkFields(v, func(field reflect.StructField, val reflect.Value, t reflect.Type) bool {
			// Check for the ignore switch in the tag
			tag := field.Tag.Get("arg")
			if tag == "-" {
				return false
			}

			// If this is an embedded struct then recurse into its fields
			if field.Anonymous && field.Type.Kind() == reflect.Struct {
				return true
			}

			spec := spec{
				long: strings.ToLower(field.Name),
				dest: val,
			}

			help, exists := field.Tag.Lookup("help")
			if exists {
				spec.help = help
			}

			// Check whether this field is supported. It's good to do this here rather than
			// wait until ParseValue because it means that a program with invalid argument
			// fields will always fail regardless of whether the arguments it received
			// exercised those fields.
			var parseable bool
			parseable, spec.boolean, spec.multiple = canParse(field.Type)
			if !parseable {
				errs = append(errs, fmt.Sprintf("%s.%s: %s fields are not supported",
					t.Name(), field.Name, field.Type.String()))
				return false
			}

			// Look at the tag
			if tag != "" {
				for _, key := range strings.Split(tag, ",") {
					key = strings.TrimLeft(key, " ")
					var value string
					if pos := strings.Index(key, ":"); pos != -1 {
						value = key[pos+1:]
						key = key[:pos]
					}

					switch {
					case strings.HasPrefix(key, "---"):
						errs = append(errs, fmt.Sprintf("%s.%s: too many hyphens", t.Name(), field.Name))
					case strings.HasPrefix(key, "--"):
						spec.long = key[2:]
					case strings.HasPrefix(key, "-"):
						if len(key) != 2 {
							errs = append(errs, fmt.Sprintf("%s.%s: short arguments must be one character only",
								t.Name(), field.Name))
							return false
						}
						spec.short = key[1:]
					case key == "required":
						spec.required = true
					case key == "positional":
						spec.positional = true
					case key == "separate":
						spec.separate = true
					case key == "help": // deprecated
						spec.help = value
					case key == "env":
						// Use override name if provided
						if value != "" {
							spec.env = value
						} else {
							spec.env = strings.ToUpper(field.Name)
						}
					default:
						errs = append(errs, fmt.Sprintf("unrecognized tag '%s' on field %s", key, tag))
						return false
					}
				}
			}
			p.specs = append(p.specs, &spec)

			// if this was an embedded field then we already returned true up above
			return false
		})

		if len(errs) > 0 {
			return nil, errors.New(strings.Join(errs, "\n"))
		}
	}
	if p.config.Program == "" {
		p.config.Program = "program"
		if len(os.Args) > 0 {
			p.config.Program = filepath.Base(os.Args[0])
		}
	}
	return &p, nil
}
func NewParser(config Config, dests ...interface{}) (*Parser, error) {
	p := Parser{
		config: config,
	}
	for _, dest := range dests {
		if dest, ok := dest.(Versioned); ok {
			p.version = dest.Version()
		}
		if dest, ok := dest.(Described); ok {
			p.description = dest.Description()
		}
		v := reflect.ValueOf(dest)
		if v.Kind() != reflect.Ptr {
			panic(fmt.Sprintf("%s is not a pointer (did you forget an ampersand?)", v.Type()))
		}
		v = v.Elem()
		if v.Kind() != reflect.Struct {
			panic(fmt.Sprintf("%T is not a struct pointer", dest))
		}

		var errs []string
		walkFields(v, func(field reflect.StructField, val reflect.Value, t reflect.Type) bool {
			// Check for the ignore switch in the tag
			tag := field.Tag.Get("arg")
			if tag == "-" {
				return false
			}

			// If this is an embedded struct then recurse into its fields
			if field.Anonymous && field.Type.Kind() == reflect.Struct {
				return true
			}

			spec := spec{
				long: strings.ToLower(field.Name),
				dest: val,
			}

			help, exists := field.Tag.Lookup("help")
			if exists {
				spec.help = help
			}

			// Check whether this field is supported. It's good to do this here rather than
			// wait until ParseValue because it means that a program with invalid argument
			// fields will always fail regardless of whether the arguments it received
			// exercised those fields.
			var parseable bool
			parseable, spec.boolean, spec.multiple = canParse(field.Type)
			if !parseable {
				errs = append(errs, fmt.Sprintf("%s.%s: %s fields are not supported",
					t.Name(), field.Name, field.Type.String()))
				return false
			}

			// Look at the tag
			if tag != "" {
				for _, key := range strings.Split(tag, ",") {
					key = strings.TrimLeft(key, " ")
					var value string
					if pos := strings.Index(key, ":"); pos != -1 {
						value = key[pos+1:]
						key = key[:pos]
					}

					switch {
					case strings.HasPrefix(key, "---"):
						errs = append(errs, fmt.Sprintf("%s.%s: too many hyphens", t.Name(), field.Name))
					case strings.HasPrefix(key, "--"):
						spec.long = key[2:]
					case strings.HasPrefix(key, "-"):
						if len(key) != 2 {
							errs = append(errs, fmt.Sprintf("%s.%s: short arguments must be one character only",
								t.Name(), field.Name))
							return false
						}
						spec.short = key[1:]
					case key == "required":
						spec.required = true
					case key == "positional":
						spec.positional = true
					case key == "separate":
						spec.separate = true
					case key == "help": // deprecated
						spec.help = value
					case key == "env":
						// Use override name if provided
						if value != "" {
							spec.env = value
						} else {
							spec.env = strings.ToUpper(field.Name)
						}
					default:
						errs = append(errs, fmt.Sprintf("unrecognized tag '%s' on field %s", key, tag))
						return false
					}
				}
			}
			p.specs = append(p.specs, &spec)

			// if this was an embedded field then we already returned true up above
			return false
		})

		if len(errs) > 0 {
			return nil, errors.New(strings.Join(errs, "\n"))
		}
	}
	if p.config.Program == "" {
		p.config.Program = "program"
		if len(os.Args) > 0 {
			p.config.Program = filepath.Base(os.Args[0])
		}
	}
	return &p, nil
}
// NewParser constructs a parser from a list of destination structs