public Task <IActionResult> Convert(decimal amount, string from, string to)
=> RatesApi.GetRateAsync(from + to)
.OrElse(() => CurrencyLayer.GetRateAsync(from + to))
.Map(rate => amount * rate)
.Map(
Faulted: ex => StatusCode(500, Errors.UnexpectedError),
Completed: result => Ok(result) as IActionResult);