LinkedHashMap
is used to preserve insertion order in the map, but this only works for mutable maps. Which is the immutable Map
implementation that preserves insertion order?
ListMap implements an immutable map using a list-based data structure, and thus preserves insertion order.
scala> import collection.immutable.ListMap
import collection.immutable.ListMap
scala> ListMap(1 -> 2) + (3 -> 4)
res31: scala.collection.immutable.ListMap[Int,Int] = Map(1 -> 2, 3 -> 4)
scala> res31 + (6 -> 9)
res32: scala.collection.immutable.ListMap[Int,Int] = Map(1 -> 2, 3 -> 4, 6 -> 9)
The following extension method - Seq#toListMap
can be quite useful when working with ListMap
s.
scala> import scalaz._, Scalaz._, Liskov._
import scalaz._
import Scalaz._
import Liskov._
scala> :paste
// Entering paste mode (ctrl-D to finish)
implicit def seqW[A](xs: Seq[A]) = new SeqW(xs)
class SeqW[A](xs: Seq[A]) {
def toListMap[B, C](implicit ev: A <~< (B, C)): ListMap[B, C] = {
ListMap(co[Seq, A, (B, C)](ev)(xs) : _*)
}
}
// Exiting paste mode, now interpreting.
seqW: [A](xs: Seq[A])SeqW[A]
defined class SeqW
scala> Seq((2, 4), (11, 89)).toListMap
res33: scala.collection.immutable.ListMap[Int,Int] = Map(2 -> 4, 11 -> 89)