Build your first configuration

What you will learn

You will load a YAML file into a Go struct. The same program will use a default port and let APP_PORT override it.

Before you start

Finish the installation step. Create an empty Go package with a main.go file and a config.yaml file.

Step 1: Define the configuration struct

Put this in main.go:

package main

import (
    "fmt"
    "log"

    "github.com/arloliu/fuda"
)

type Config struct {
    Host string `yaml:"host" default:"localhost" env:"APP_HOST"`
    Port int    `yaml:"port" default:"8080" env:"APP_PORT"`
}

func main() {
    loader, err := fuda.New().FromFile("config.yaml").Build()
    if err != nil {
        log.Fatal(err)
    }

    var cfg Config
    if err := loader.Load(&cfg); err != nil {
        log.Fatal(err)
    }

    fmt.Printf("Server: %s:%d\n", cfg.Host, cfg.Port)
}

Step 2: Add a YAML file

Put this in config.yaml:

host: 0.0.0.0

The file sets Host. It leaves Port out, so Fuda uses the default:"8080" tag.

Step 3: Run the program

Run:

go run .

You should see:

Server: 0.0.0.0:8080

Step 4: Override the port with an environment variable

Run the same program with APP_PORT:

APP_PORT=9090 go run .

You should see:

Server: 0.0.0.0:9090

What happened

Fuda reads Host from YAML. No YAML value exists for Port, so it starts with 8080 from the default tag. When APP_PORT exists, the env tag replaces the file or default value.

Common mistakes

Use the field names in the yaml tags as keys in config.yaml. Set APP_PORT in the same command that runs the program. On Windows PowerShell, use $env:APP_PORT = "9090" before go run ..

Next step

Choose a value to see the precedence rule in one place.