Question

I’m trying to define a clean interface for clients to use my library. Some sample client code is below.

for (security <- allSecurities) {
  val askLast = ask
}

The problem is I would like “ask” to automatically be passed “security”. My attempt at doing this in the parent class is as follows

 var lastSecurity = ""  
  private val lastAsk = new HashMap[...]
  def allSecurities = for {
    security <- lastTrade.keySet.toList
  } yield {
    lastSecurity = security
    security
  }

  def ask = lastAsk(lastSecurity).price

Unfortunately it doesn’t quite work as I envisaged since in the client lastSecurity has the same value throughout the entire loop instead of being dynamically updated.. So basically I’m trying to allow clients to do

val askLast = ask

instead of

val askLast = ask(security)

Can I do this in Scala?

Answer 1

You can get what you want with a lazy “view” of the sequence,

  var lastSecurity: Int = _
  val securities = Seq(1, 2).view.map { a => 
    lastSecurity = a
    a
  }

  for (s <- securities) {
    println("%d %d".format(s, lastSecurity))
  }

prints

1 1
2 2

The key idea here is that with a lazy view, the map is not evaluated immediately, but only as elements are needed by the for loop.

---

Here’s another approach. I’m not recommending this as good code, but it does demonstrate the flexibility of Scala,

  var lastSecurity: Int = _

  class SecurityWrapper(s: Seq[Int]) {
    def foreach(f: Int => Unit) {
      s.foreach { a =>
        lastSecurity = a
        f(a)
      }
    }
  }

  for (security <- new SecurityWrapper(Seq(1, 2))) {
    println("%d %d".format(security, lastSecurity))
  }

The class SecurityWrapper contains a custom foreach method. In each iteration through the loop, it writes to the lastSecurity variable. Scala’s for comprehension sugar will automatically use SecurityWrapper.foreach without any extra work on our part.