func (f *Int64SliceFlag) Apply(set *flag.FlagSet) error {
	if val, ok := flagFromEnvOrFile(f.EnvVars, f.FilePath); ok {
		f.Value = &Int64Slice{}
		for _, s := range strings.Split(val, ",") {
			if err := f.Value.Set(strings.TrimSpace(s)); err != nil {
				return fmt.Errorf("could not parse %q as int64 slice value for flag %s: %s", val, f.Name, err)
			}
		}
		f.HasBeenSet = true
	}
	for _, name := range f.Names() {
		if f.Value == nil {
			f.Value = &Int64Slice{}
		}
		set.Var(f.Value, name, f.Usage)
	}
	return nil
}
Apply populates the flag given the flag set and environment
package cli
import (
	"encoding/json"
	"flag"
	"fmt"
	"strconv"
	"strings"
)
type Int64Slice struct {
	slice      []int64
	hasBeenSet bool
}
func NewInt64Slice(defaults ...int64) *Int64Slice {
	return &Int64Slice{slice: append([]int64{}, defaults...)}
}
func (i *Int64Slice) Set(value string) error {
	if !i.hasBeenSet {
		i.slice = []int64{}
		i.hasBeenSet = true
	}
	if strings.HasPrefix(value, slPfx) {
		_ = json.Unmarshal([]byte(strings.Replace(value, slPfx, "", 1)), &i.slice)
		i.hasBeenSet = true
		return nil
	}
	tmp, err := strconv.ParseInt(value, 0, 64)
	if err != nil {
		return err
	}
	i.slice = append(i.slice, tmp)
	return nil
}
func (i *Int64Slice) String() string {
	return fmt.Sprintf("%#v", i.slice)
}
func (i *Int64Slice) Serialize() string {
	jsonBytes, _ := json.Marshal(i.slice)
	return fmt.Sprintf("%s%s", slPfx, string(jsonBytes))
}
func (i *Int64Slice) Value() []int64 {
	return i.slice
}
func (i *Int64Slice) Get() interface{} {
	return *i
}
type Int64SliceFlag struct {
	Name        string
	Aliases     []string
	Usage       string
	EnvVars     []string
	FilePath    string
	Required    bool
	Hidden      bool
	Value       *Int64Slice
	DefaultText string
	HasBeenSet  bool
}
func (f *Int64SliceFlag) IsSet() bool {
	return f.HasBeenSet
}
func (f *Int64SliceFlag) String() string {
	return FlagStringer(f)
}
func (f *Int64SliceFlag) Names() []string {
	return flagNames(f.Name, f.Aliases)
}
func (f *Int64SliceFlag) IsRequired() bool {
	return f.Required
}
func (f *Int64SliceFlag) TakesValue() bool {
	return true
}
func (f Int64SliceFlag) GetUsage() string {
	return f.Usage
}
func (f *Int64SliceFlag) GetValue() string {
	if f.Value != nil {
		return f.Value.String()
	}
	return ""
}