Scala：如何组合返回Future的方法

Question

Given a Map that stores exchange rates from EUR/USD to BTC...

val btcRates = Map("EUR" -> 0.0036, "USD" -> 0.0045)

... and the following two methods...

// returns a Future containing a Map of value to convert to BTC
def getAmounts = Future(Map("EUR" -> 500.0, "USD" -> 550.0, "CHF" -> 400))

// returns a Future containing the exchange rate for the specified currency
def getBtcRate(refCurrency: String) = Future(btcRates(refCurrency))

How do I invoke getAmounts and then for each element of the Map it returns invoke getBtcRate to convert the amount to BTC? And how do I sum all the converted amounts?

def getTotal: Future[Double] = {
  getAmounts.flatMap { _.map { case (currency, amount) =>
    getBtcRate(currency).flatMap { rate =>
      amount * rate // how do I sum this and how do I return the result?
    }
  }}
}

Answer 1

I've used for-comprehension for getTotal since it's a little easier on the eyes than a chain of flatmap s:

  def getTotal: Future[Double] = for {
    amounts <- getAmounts
    totals <- Future.sequence(
      amounts.map { case (currency, amount) => 
        getBtcRate(currency).map(rate => amount * rate)
      }
    )
  } yield totals.sum

What this does is that amounts is the result of getAmounts (really this is the body of your flatmap ). You then map over that tuple to create a sequence of futures by calling getBtcRate for each currency . Each of those futures is then mapped to get the total.

Now you have a Iterable[Future[Double]] and using Future.sequence you transform it into a Future[Iterable[Double]]

Now you can just yield that Iterable and sum it, yielding the desired Future[Double] .