immutable

Type Members

1. abstract class ArraySeq[+T] extends AbstractSeq[T] with immutable.IndexedSeq[T]

   

   An immutable array.

   An immutable array.

   Supports efficient indexed access and has a small memory footprint.

2. final class LazyList[+A] extends AbstractSeq[A] with immutable.LinearSeq[A] with GenericTraversableTemplate[A, LazyList] with LinearSeqOptimized[A, LazyList[A]] with Serializable

   

   This class implements an immutable linked list that evaluates elements in order and only when needed.

   This class implements an immutable linked list that evaluates elements in order and only when needed. Here is an example:

   import scala.math.BigInt
   object Main extends App {
   
     val fibs: LazyList[BigInt] = BigInt(0) #:: BigInt(1) #:: fibs.zip(fibs.tail).map { n => n._1 + n._2 }
   
     fibs take 5 foreach println
   }
   
   // prints
   //
   // 0
   // 1
   // 1
   // 2
   // 3

   A LazyList, like the one in the example above, may be infinite in length. Aggregate methods, such as count, sum, max or min on such infinite length sequences will not terminate. Filtered infinite lazy lists are also effectively infinite in length.

   Elements of a LazyList are memoized; that is, the value of each element is computed only once. To illustrate, we will alter body of the fibs value above and take some more values:

   import scala.math.BigInt
   object Main extends App {
   
     val fibs: LazyList[BigInt] = BigInt(0) #:: BigInt(1) #:: fibs.zip(
       fibs.tail).map(n => {
         println("Adding %d and %d".format(n._1, n._2))
         n._1 + n._2
       })
   
     fibs take 5 foreach println
     fibs take 6 foreach println
   }
   
   // prints
   //
   // 0
   // 1
   // Adding 0 and 1
   // 1
   // Adding 1 and 1
   // 2
   // Adding 1 and 2
   // 3
   
   // And then prints
   //
   // 0
   // 1
   // 1
   // 2
   // 3
   // Adding 2 and 3
   // 5

   There are a number of subtle points to the above example.

   • The definition of fibs is a val not a method. The memoization of the LazyList requires us to have somewhere to store the information and a val allows us to do that.
   • While the LazyList is actually being modified during access, this does not change the notion of its immutability. Once the values are memoized they do not change and values that have yet to be memoized still "exist", they simply haven't been realized yet.
   • One must be cautious of memoization; you can very quickly eat up large amounts of memory if you're not careful. The reason for this is that the memoization of the LazyList creates a structure much like scala.collection.immutable.List. So long as something is holding on to the head, the head holds on to the tail, and so it continues recursively. If, on the other hand, there is nothing holding on to the head (e.g. we used def to define the LazyList) then once it is no longer being used directly, it disappears.
   • Note that some operations, including drop, dropWhile, flatMap or collect may process a large number of intermediate elements before returning. These necessarily hold onto the head, since they are methods on LazyList, and a lazy list holds its own head. For computations of this sort where memoization is not desired, use Iterator when possible.

   // For example, let's build the natural numbers and do some silly iteration
   // over them.
   
   // We'll start with a silly iteration
   def loop(s: String, i: Int, iter: Iterator[Int]): Unit = {
     // Stop after 200,000
     if (i < 200001) {
       if (i % 50000 == 0) println(s + i)
       loop(s, iter.next(), iter)
     }
   }
   
   // Our first LazyList definition will be a val definition
   val lazylist1: LazyList[Int] = {
     def loop(v: Int): LazyList[Int] = v #:: loop(v + 1)
     loop(0)
   }
   
   // Because lazylist1 is a val, everything that the iterator produces is held
   // by virtue of the fact that the head of the LazyList is held in lazylist1
   val it1 = lazylist1.toIterator
   loop("Iterator1: ", it1.next(), it1)
   
   // We can redefine this LazyList such that all we have is the Iterator left
   // and allow the LazyList to be garbage collected as required.  Using a def
   // to provide the LazyList ensures that no val is holding onto the head as
   // is the case with lazylist1
   def lazylist2: LazyList[Int] = {
     def loop(v: Int): LazyList[Int] = v #:: loop(v + 1)
     loop(0)
   }
   val it2 = lazylist2.toIterator
   loop("Iterator2: ", it2.next(), it2)
   
   // And, of course, we don't actually need a LazyList at all for such a simple
   // problem.  There's no reason to use a LazyList if you don't actually need
   // one.
   val it3 = new Iterator[Int] {
     var i = -1
     def hasNext = true
     def next(): Int = { i += 1; i }
   }
   loop("Iterator3: ", it3.next(), it3)

   • The fact that tail works at all is of interest. In the definition of fibs we have an initial (0, 1, LazyList(...)) so tail is deterministic. If we defined fibs such that only 0 were concretely known then the act of determining tail would require the evaluation of tail which would cause an infinite recursion and stack overflow. If we define a definition where the tail is not initially computable then we're going to have an infinite recursion:

   // The first time we try to access the tail we're going to need more
   // information which will require us to recurse, which will require us to
   // recurse, which...
   lazy val sov: LazyList[Vector[Int]] = Vector(0) #:: sov.zip(sov.tail).map { n => n._1 ++ n._2 }

   The definition of fibs above creates a larger number of objects than necessary depending on how you might want to implement it. The following implementation provides a more "cost effective" implementation due to the fact that it has a more direct route to the numbers themselves:

   lazy val fib: LazyList[Int] = {
     def loop(h: Int, n: Int): LazyList[Int] = h #:: loop(n, h + n)
     loop(1, 1)
   }

   A

   the type of the elements contained in this lazy list.

   Annotations

   @SerialVersionUID()

   See also

   "Scala's Collection Library overview" section on LazyLists for more information.