Very simple example of reading a config file into an object which provides an interface for fetching config values

config.go

package main

import (
	"bufio"
	"fmt"
	"os"
	"strings"
)

type Config interface {
	Get(string) (string, error)
}

type AppConfig struct {
	data map[string]string
}

func (ac AppConfig) Get(key string) (string, error) {
	value, ok := ac.data[key]
	if !ok {
		return "", fmt.Errorf("attempt to read nonexistent config value %s", key)
	}
	return value, nil
}

func NewAppConfig(filename string) AppConfig {
	fin, err := os.Open(filename)
	if err != nil {
		panic("error opening config file: " + err.Error())
	}
	defer func() {
		cerr := fin.Close()
		if cerr != nil {
			panic("error closing config file: " + cerr.Error())
		}
	}()
	scanner := bufio.NewScanner(fin)
	scanner.Split(bufio.ScanLines)
	config := AppConfig{}
	config.data = make(map[string]string, 0)
	for scanner.Scan() {
		line := scanner.Text()
		chunks := strings.Split(line, "=")
		if len(chunks) == 2 {
			config.data[chunks[0]] = chunks[1]
		}
	}
	return config
}

func dumpConfig(conf Config) {
	x, err := conf.Get("keyname")
	if err != nil {
		panic(err)
	}
	fmt.Println("keyname = " + x)
	x, err = conf.Get("invalidkey")
	if err != nil {
		panic(err)
	}
	fmt.Println("invalidkey = " + x)
}

func main() {
	config := NewAppConfig("configfile.txt")
	dumpConfig(config)
}

configfile.txt

keyname=My value
otherkey=Something else