golang 'future' example (https://en.wikipedia.org/wiki/Futures_and_promises)

golang-future.go

package main

import (
	"encoding/json"
	"fmt"
	"log"
	"net/http"
	"sync"
)

type CurrencyRateFuture func() (float64, error)

func NewCurrencyRateFuture() CurrencyRateFuture {
	var (
		val float64
		err error
		mtx sync.Mutex
	)

	mtx.Lock()
	go func() {
		defer mtx.Unlock()
		v, e := ReadCurrencyRateFromRBC()
		val = v
		if e != nil {
			err = fmt.Errorf("reading currency rate: %s", e)
		}
	}()

	return func() (float64, error) {
		mtx.Lock()
		defer mtx.Unlock()
		return val, err
	}
}

func main() {
	currRateFuture := NewCurrencyRateFuture()

	rate, err := currRateFuture()
	if err != nil {
		panic(err)
	}
	fmt.Println(rate)

	rate, err = currRateFuture()
	if err != nil {
		panic(err)
	}
	fmt.Println(rate)
}

func ReadCurrencyRateFromRBC() (float64, error) {
	log.Print("currency rate reading begin")
	defer log.Print("currency rate reading end")

	const convAPI = "https://cash.rbc.ru/cash/json/converter_currency_rate/?currency_from=USD&currency_to=RUR&source=cbrf&sum=2&date="
	resp, err := http.Get(convAPI)
	if err != nil {
		return 0, err
	}
	defer resp.Body.Close()

	if resp.StatusCode != 200 {
		return 0, fmt.Errorf("bad HTTP status code: %d", resp.StatusCode)
	}

	reply := struct {
		Data   struct{ Rate1 float64 }
		Status int
	}{}

	err = json.NewDecoder(resp.Body).Decode(&reply)
	if err != nil {
		return 0, fmt.Errorf("bad API response: %s", err)
	}

	if reply.Status != 200 {
		return 0, fmt.Errorf("bad API reply status code: %d", reply.Status)
	}

	return reply.Data.Rate1, nil
}