Scala Center?
next() Collections
Julien Richard-Foy , Scala Center
Scala.io – November 2, 2017
http://julienrf.github.io/2017/collections
EPronovost 
Ichoran 
LPTK 
NthPortal 
SethTisue 
allanrenucci 
biboudis 
esarbe 
julienrf 
marcelocenerine 
nicolasstucki 
odd 
odersky 
olafurpg 
pnf 
shawjef3 
sjrd 
smarter 
stephennancekivell 
szeiger 
tpolecat 
xavier-fernandez List Hierarchy
List (Actual) Hierarchy
List New Hierarchy
Ops classestrait IterableOps[A, CC[_], C] {
def take(n: Int): C = …
def map[B](f: A => B): CC[B] = …
}trait List[A] extends Iterable[A] with IterableOps[A, List, List[A]]Ops classestrait IterableOps[A, CC[_], C] {
def take(n: Int): C = …
def map[B](f: A => B): CC[B] = …
}trait StringOps extends IterableOps[Char, IndexedSeq, String]map Implementationtrait TraversableLike[+A, +Repr] {
def map[B, That](f: A => B)(implicit bf: CanBuildFrom[Repr, B, That]): That = {
val b = bf(this)
for (x <- this) b += f(x)
b.result
}
}this collection and of the resulting collectionStream or views?map Implementation on Viewstrait TraversableViewLike[+A, +Repr] extends TraversableLike[A, Repr] {
override def map[B, That](f: A => B)(implicit bf: CanBuildFrom[This, B, That]): That = {
newMapped(f).asInstanceOf[That]
}
}object TraversableView {
implicit def canBuildFrom[A] =
new CanBuildFrom[TraversableView[_], A TraversableView[A]] {
def apply() = new Builder[A, Nothing] {
def +=(elem: A): this.type = this
def result() = throw new UnsupportedOperationException("TraversableView.Builder.result")
}
}
}CanBuildFrom instancetrait IterableOps[A, CC[_], C] {
def map[B](f: A => B): CC[B] = fromIterable(View.Map(this, f))
}Viewsobject View {
case class Map[A, B](underlying: Iterable[A], f: A => B) extends View[B] {
def iterator() = underlying.iterator().map(f)
}
}IteratorIterators, Views can be aliased without dangertrait IterableOps[A, CC[_], C] {
def map[B](f: A => B): CC[B] = fromIterable(View.Map(this, f))
def fromIterable[E](it: Iterable[E]): CC[E]
}fromIterable implementation decides whether to evaluate the collection elements or notfromIterable trait List[+A] extends Iterable[A] with IterableOps[A, List, List[A]] {
def fromIterable[E](it: Iterable[E]): List[E] =
(List.newBuilder[E]() ++= it).result()
}trait View[+A] extends Iterable[A] with IterableOps[A, View, View[A]] {
def fromIterable[E](it: Iterable[E]): View[E] =
new View[E] {
def iterator() = it.iterator()
}
}CanBuildFromCanBuildFrommap Operation on List[A]
map Operation on TreeMap[A, B]
map Operation on TreeMap[A, B]
CanBuildFrom is powerful trait GenTraversableLike[+A, +Repr] {
def map[B, That](f: A => B)(implicit bf: CanBuildFrom[Repr, B, That]): That
}One definition.
Works with String, List[A], TreeSet[A], HashMap[K, V], SortedMap[K, V].
map Operation on List[A]
map Operation on TreeMap[K, V]
CanBuildFrom was more than that!CanBuildFrom was one of the basic building block for generic programming over collections:
BuildFromFactoryCanBuildFrom trait BuildFrom[From, A, C] {
def fromSpecificIterable(from: From)(it: Iterable[A]): C
def newBuilder(from: From): Builder[A, C]
}From is the type of a source collectionCanBuildFrom trait BuildFrom[From, A, C] {
def fromSpecificIterable(from: From)(it: Iterable[A]): C
def newBuilder(from: From): Builder[A, C]
}implicit class HasDistinctBy[A, CC[X] <: Iterable[X]](coll: CC[A]) {
def distinctBy[B, C](f: A => B)(implicit bf: BuildFrom[CC[A], A, C]): C =
bf.fromSpecificIterable(coll)(new View[A] {
def iterator() = …
})
}trait Factory[-A, +C] extends Any {
def fromSpecific(it: IterableOnce[A]): C
def newBuilder(): Builder[A, C]
}BuildFrom, but without Fromimport play.api.libs.json._
implicit def iterableReads[A, C <: Iterable[A]](implicit
factory: Factory[A, C],
ra: Reads[A]
): Reads[C] = Reads[C] {
case JsArray(elts) => JsResult.traverse(elts)(ra.reads)(factory)
case _ => JsError("error.expected.jsarray")
}
val readsList = Reads.of[List[Int]]
val readsSortedMap = Reads.of[SortedMap[Instant, BigDecimal]]Traversable
Stack, UnrolledBuffer, LinkedList nor “paged” collection…val xs = List(1 -> 'a', 2 -> 'b', 3 -> 'c')
val vector = xs.to(Vector) // Before: xs.to[Vector]
val map = xs.to(Map) // Before: xs.toMapto operation now takes a value as parametergroupMap case class User(name: String, age: Int)
// Before:
def namesByAge(users: Seq[User]): Map[Int, Seq[String]] =
users.groupBy(_.age).mapValues(_.map(_.name))
// With `groupMap`:
def namesByAge(users: Seq[User]): Map[Int, Seq[String]] =
users.groupMap(_.age)(_.name)LazyList Stream has been remamed to LazyListTuple2Zipped scala> val xs = Vector(1, 2, 3)
xs: Vector[Int] = Vector(1, 2, 3)
scala> (xs, xs).zipped.map(_ + _)
res0: Vector[Int] = Vector(2, 4, 6)(xs zip xs).map { case (x1, x2) => x1 + x2 }Tuple2Zipped
Tuple2ZippedLazyZip2 val xs = Vector(1, 2, 3)
(xs lazyZip xs).map(_ + _)
val buffer = ArrayBuffer(1, 2, 3)
buffer.mapInPlace(x => x + 1)InPlace do not create a copy of the collectionscala.Seq[A] = ? scala.Seq[A] = scala.collection.immutable.Seq[A]ImmutableArrayArraySeq?lazyFoldRight?