multiWeatherProvider.go

func (w multiWeatherProvider) temperature(city string) (float64, error) {
	// Make a channel for temperatures, and a channel for errors.
	// Each provider will push a value into only one.
	temps := make(chan float64, len(w))
	errs := make(chan error, len(w))

	// For each provider, spawn a goroutine with an anonymous function.
	// That function will invoke the temperature method, and forward the response.
	for _, provider := range w {
		go func(p weatherProvider) {
			k, err := p.temperature(city)
			if err != nil {
				errs <- err
				return
			}
			temps <- k
		}(provider)
	}

	sum := 0.0

	// Collect a temperature or an error from each provider.
	for i := 0; i < len(w); i++ {
		select {
		case temp := <-temps:
			sum += temp
		case err := <-errs:
			return 0, err
		}
	}

	// Return the average, same as before.
	return sum / float64(len(w)), nil
}