parse.go
func process(specs []*spec, args []string) error {
	// track the options we have seen
	wasPresent := make(map[*spec]bool)

	// construct a map from --option to spec
	optionMap := make(map[string]*spec)
	for _, spec := range specs {
		if spec.positional {
			continue
		}
		if spec.long != "" {
			optionMap[spec.long] = spec
		}
		if spec.short != "" {
			optionMap[spec.short] = spec
		}
	}

	// deal with environment vars
	for _, spec := range specs {
		if spec.env == "" {
			continue
		}

		value, found := os.LookupEnv(spec.env)
		if !found {
			continue
		}

		if spec.multiple {
			// expect a CSV string in an environment
			// variable in the case of multiple values
			values, err := csv.NewReader(strings.NewReader(value)).Read()
			if err != nil {
				return fmt.Errorf(
					"error reading a CSV string from environment variable %s with multiple values: %v",
					spec.env,
					err,
				)
			}
			if err = setSlice(spec.dest, values, !spec.separate); err != nil {
				return fmt.Errorf(
					"error processing environment variable %s with multiple values: %v",
					spec.env,
					err,
				)
			}
		} else {
			if err := scalar.ParseValue(spec.dest, value); err != nil {
				return fmt.Errorf("error processing environment variable %s: %v", spec.env, err)
			}
		}
		wasPresent[spec] = true
	}

	// process each string from the command line
	var allpositional bool
	var positionals []string

	// must use explicit for loop, not range, because we manipulate i inside the loop
	for i := 0; i < len(args); i++ {
		arg := args[i]
		if arg == "--" {
			allpositional = true
			continue
		}

		if !isFlag(arg) || allpositional {
			positionals = append(positionals, arg)
			continue
		}

		// check for an equals sign, as in "--foo=bar"
		var value string
		opt := strings.TrimLeft(arg, "-")
		if pos := strings.Index(opt, "="); pos != -1 {
			value = opt[pos+1:]
			opt = opt[:pos]
		}

		// lookup the spec for this option
		spec, ok := optionMap[opt]
		if !ok {
			return fmt.Errorf("unknown argument %s", arg)
		}
		wasPresent[spec] = true

		// deal with the case of multiple values
		if spec.multiple {
			var values []string
			if value == "" {
				for i+1 < len(args) && !isFlag(args[i+1]) {
					values = append(values, args[i+1])
					i++
					if spec.separate {
						break
					}
				}
			} else {
				values = append(values, value)
			}
			err := setSlice(spec.dest, values, !spec.separate)
			if err != nil {
				return fmt.Errorf("error processing %s: %v", arg, err)
			}
			continue
		}

		// if it's a flag and it has no value then set the value to true
		// use boolean because this takes account of TextUnmarshaler
		if spec.boolean && value == "" {
			value = "true"
		}

		// if we have something like "--foo" then the value is the next argument
		if value == "" {
			if i+1 == len(args) {
				return fmt.Errorf("missing value for %s", arg)
			}
			if !nextIsNumeric(spec.dest.Type(), args[i+1]) && isFlag(args[i+1]) {
				return fmt.Errorf("missing value for %s", arg)
			}
			value = args[i+1]
			i++
		}

		err := scalar.ParseValue(spec.dest, value)
		if err != nil {
			return fmt.Errorf("error processing %s: %v", arg, err)
		}
	}

	// process positionals
	for _, spec := range specs {
		if !spec.positional {
			continue
		}
		if len(positionals) == 0 {
			break
		}
		wasPresent[spec] = true
		if spec.multiple {
			err := setSlice(spec.dest, positionals, true)
			if err != nil {
				return fmt.Errorf("error processing %s: %v", spec.long, err)
			}
			positionals = nil
		} else {
			err := scalar.ParseValue(spec.dest, positionals[0])
			if err != nil {
				return fmt.Errorf("error processing %s: %v", spec.long, err)
			}
			positionals = positionals[1:]
		}
	}
	if len(positionals) > 0 {
		return fmt.Errorf("too many positional arguments at '%s'", positionals[0])
	}

	// finally check that all the required args were provided
	for _, spec := range specs {
		if spec.required && !wasPresent[spec] {
			name := spec.long
			if !spec.positional {
				name = "--" + spec.long
			}
			return fmt.Errorf("%s is required", name)
		}
	}

	return nil
}
func process(specs []*spec, args []string) error {
	// track the options we have seen
	wasPresent := make(map[*spec]bool)

	// construct a map from --option to spec
	optionMap := make(map[string]*spec)
	for _, spec := range specs {
		if spec.positional {
			continue
		}
		if spec.long != "" {
			optionMap[spec.long] = spec
		}
		if spec.short != "" {
			optionMap[spec.short] = spec
		}
	}

	// deal with environment vars
	for _, spec := range specs {
		if spec.env == "" {
			continue
		}

		value, found := os.LookupEnv(spec.env)
		if !found {
			continue
		}

		if spec.multiple {
			// expect a CSV string in an environment
			// variable in the case of multiple values
			values, err := csv.NewReader(strings.NewReader(value)).Read()
			if err != nil {
				return fmt.Errorf(
					"error reading a CSV string from environment variable %s with multiple values: %v",
					spec.env,
					err,
				)
			}
			if err = setSlice(spec.dest, values, !spec.separate); err != nil {
				return fmt.Errorf(
					"error processing environment variable %s with multiple values: %v",
					spec.env,
					err,
				)
			}
		} else {
			if err := scalar.ParseValue(spec.dest, value); err != nil {
				return fmt.Errorf("error processing environment variable %s: %v", spec.env, err)
			}
		}
		wasPresent[spec] = true
	}

	// process each string from the command line
	var allpositional bool
	var positionals []string

	// must use explicit for loop, not range, because we manipulate i inside the loop
	for i := 0; i < len(args); i++ {
		arg := args[i]
		if arg == "--" {
			allpositional = true
			continue
		}

		if !isFlag(arg) || allpositional {
			positionals = append(positionals, arg)
			continue
		}

		// check for an equals sign, as in "--foo=bar"
		var value string
		opt := strings.TrimLeft(arg, "-")
		if pos := strings.Index(opt, "="); pos != -1 {
			value = opt[pos+1:]
			opt = opt[:pos]
		}

		// lookup the spec for this option
		spec, ok := optionMap[opt]
		if !ok {
			return fmt.Errorf("unknown argument %s", arg)
		}
		wasPresent[spec] = true

		// deal with the case of multiple values
		if spec.multiple {
			var values []string
			if value == "" {
				for i+1 < len(args) && !isFlag(args[i+1]) {
					values = append(values, args[i+1])
					i++
					if spec.separate {
						break
					}
				}
			} else {
				values = append(values, value)
			}
			err := setSlice(spec.dest, values, !spec.separate)
			if err != nil {
				return fmt.Errorf("error processing %s: %v", arg, err)
			}
			continue
		}

		// if it's a flag and it has no value then set the value to true
		// use boolean because this takes account of TextUnmarshaler
		if spec.boolean && value == "" {
			value = "true"
		}

		// if we have something like "--foo" then the value is the next argument
		if value == "" {
			if i+1 == len(args) {
				return fmt.Errorf("missing value for %s", arg)
			}
			if !nextIsNumeric(spec.dest.Type(), args[i+1]) && isFlag(args[i+1]) {
				return fmt.Errorf("missing value for %s", arg)
			}
			value = args[i+1]
			i++
		}

		err := scalar.ParseValue(spec.dest, value)
		if err != nil {
			return fmt.Errorf("error processing %s: %v", arg, err)
		}
	}

	// process positionals
	for _, spec := range specs {
		if !spec.positional {
			continue
		}
		if len(positionals) == 0 {
			break
		}
		wasPresent[spec] = true
		if spec.multiple {
			err := setSlice(spec.dest, positionals, true)
			if err != nil {
				return fmt.Errorf("error processing %s: %v", spec.long, err)
			}
			positionals = nil
		} else {
			err := scalar.ParseValue(spec.dest, positionals[0])
			if err != nil {
				return fmt.Errorf("error processing %s: %v", spec.long, err)
			}
			positionals = positionals[1:]
		}
	}
	if len(positionals) > 0 {
		return fmt.Errorf("too many positional arguments at '%s'", positionals[0])
	}

	// finally check that all the required args were provided
	for _, spec := range specs {
		if spec.required && !wasPresent[spec] {
			name := spec.long
			if !spec.positional {
				name = "--" + spec.long
			}
			return fmt.Errorf("%s is required", name)
		}
	}

	return nil
}
// process goes through arguments one-by-one, parses them, and assigns the result to
// the underlying struct field