package main

import (
    "fmt"
    "os"
    "reflect"
)

type AppConfig struct {
    Pg    string `cli:"pg"    env:"PG"    default:"host=host.local dbname=db user=user password=password" description:"Connection to PostgreSQL"`
    Redis string `cli:"redis" env:"REDIS" default:"host.local"                                            description:"Redis server"`
}

func main() {
    config := GetConfig(&AppConfig{})
    fmt.Println(config)
}

func GetConfig(config *AppConfig) *AppConfig {
    ref := reflect.TypeOf(*config)
    value := reflect.ValueOf(*config)
    for i := 0; i < value.NumField(); i++ {
        field := ref.Field(i)

        name := field.Tag.Get("env")
        if name != "" {
            env := os.Getenv(name)
            if env != "" {
                value.Field(i).SetString(env)
            } else {
                def := field.Tag.Get("default")
                if def != "" {
                    value.Field(i).SetString(def)
                }
            }
        }
        /*
            cli := field.Tag.Get("cli")
            if cli != "" {
                flag.Var(value.Field(i).Pointer(), cli, field.Tag.Get("description"))
            }
        */
    }
    // flag.Parse()
    return config
}

panic: reflect: reflect.Value.SetString using unaddressable value