How to do it...

1. Open the console and create the folder chapter01/recipe12.
2. Navigate to the directory.
3. Create the main.go file with the following content:

        package main

        import (
          "encoding/json"
          "fmt"
          "os"
        )

        type Client struct {
          consulIP string
          connString string
        }

        func (c *Client) String() string {
          return fmt.Sprintf("ConsulIP: %s , Connection String: %s",
                             c.consulIP, c.connString)
        }

        var defaultClient = Client{
          consulIP: "localhost:9000",
          connString: "postgres://localhost:5432",
        }

        // ConfigFunc works as a type to be used
        // in functional options
        type ConfigFunc func(opt *Client)

        // FromFile func returns the ConfigFunc
        // type. So this way it could read the configuration
        // from the json.
        func FromFile(path string) ConfigFunc {
          return func(opt *Client) {
            f, err := os.Open(path)
            if err != nil {
              panic(err)
            }
            defer f.Close()
            decoder := json.NewDecoder(f)

            fop := struct {
              ConsulIP string `json:"consul_ip"`
            }{}
            err = decoder.Decode(&fop)
            if err != nil {
              panic(err)
            }
            opt.consulIP = fop.ConsulIP
          }
        }

        // FromEnv reads the configuration
        // from the environmental variables
        // and combines them with existing ones.
        func FromEnv() ConfigFunc {
          return func(opt *Client) {
            connStr, exist := os.LookupEnv("CONN_DB")
            if exist {
              opt.connString = connStr
            }
          }
        }
 
        func NewClient(opts ...ConfigFunc) *Client {
          client := defaultClient
          for _, val := range opts {
            val(&client)
          }
          return &client
        }

        func main() {
          client := NewClient(FromFile("config.json"), FromEnv())
          fmt.Println(client.String())
        }

1. In the same folder, create the file config.json with content:

        {
          "consul_ip":"127.0.0.1"
        }

1. Execute the code by the command CONN_DB=oracle://local:5921 go run main.go.
2. See the output: