Question

I need to build a sequence of objects that are loaded from an external resource. This loading being an expensive operation needs to be delayed until the time the objects are needed. After the collection is built, I need an indexed access to the contained objects. Does Scala standard library provide a collection suited to this use case? If not, what will be the best way to implement it?

Edit:

The indexed lookup should preferably be an O(1) operation.

Answer 1

Strange, Miles recently tweeted about this. A reply then points out to Need at the end of Name.scala in scalaz and another one points to specs’ LazyParameter.

Little testing with Need:

import scalaz._
import Scalaz._
val longOp = { var x = 0; () => {println("longOp"); x += 1; x }}
val seq = Seq(Need(longOp()), Need(longOp()))
val sum = seq.map(_.value).sum
val sum = seq.map(_.value).sum

val v = Need(longOp())
val s = v.map("got " + _)
println(s.value)
println(s.value)

prints:

longOp: () => Int = <function0>
seq: Seq[scalaz.Name[Int]] = List(
  scalaz.Name$$anon$2@1066d88, scalaz.Name$$anon$2@1011f1f)
longOp
longOp
sum: Int = 3
sum: Int = 3
v: scalaz.Name[Int] = scalaz.Name$$anon$2@133ef6a
s: scalaz.Name[java.lang.String] = scalaz.Name$$anon$2@11696ec
longOp
got 3
got 3

So longOp is only called once on first access of value.