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Scala example source code file (TraversableOnce.scala)

This example Scala source code file (TraversableOnce.scala) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Scala source code examples by using tags.

All credit for the original source code belongs to scala-lang.org; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Scala tags/keywords

a, a1, annotation, b, boolean, cc, int, reflection, string, stringbuilder, traversableonce, unsupportedoperationexception

The TraversableOnce.scala Scala example source code

/*                     __                                               *\
**     ________ ___   / /  ___     Scala API                            **
**    / __/ __// _ | / /  / _ |    (c) 2003-2013, LAMP/EPFL             **
**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
** /____/\___/_/ |_/____/_/ | |                                         **
**                          |/                                          **
\*                                                                      */

package scala
package collection

import mutable.{ Buffer, Builder, ListBuffer, ArrayBuffer }
import generic.CanBuildFrom
import scala.annotation.unchecked.{ uncheckedVariance => uV }
import scala.language.{implicitConversions, higherKinds}
import scala.reflect.ClassTag

/** A template trait for collections which can be traversed either once only
 *  or one or more times.
 *  $traversableonceinfo
 *
 *  @author Martin Odersky
 *  @author Paul Phillips
 *  @version 2.8
 *  @since   2.8
 *
 *  @define coll traversable or iterator
 *
 *  @tparam A    the element type of the collection
 *
 *  @define traversableonceinfo
 *  This trait exists primarily to eliminate code duplication between
 *  `Iterator` and `Traversable`, and thus implements some of the common
 *  methods that can be implemented solely in terms of foreach without
 *  access to a `Builder`. It also includes a number of abstract methods
 *  whose implementations are provided by `Iterator`, `Traversable`, etc.
 *  It contains implementations common to `Iterators` and
 *  `Traversables`, such as folds, conversions, and other operations which
 *  traverse some or all of the elements and return a derived value.
 *  Directly subclassing `TraversableOnce` is not recommended - instead,
 *  consider declaring an `Iterator` with a `next` and `hasNext` method,
 *  creating an `Iterator` with one of the methods on the `Iterator` object,
 *  or declaring a subclass of `Traversable`.
 *
 *  @define coll traversable or iterator
 *  @define orderDependent
 *
 *    Note: might return different results for different runs, unless the underlying collection type is ordered.
 *  @define orderDependentFold
 *
 *    Note: might return different results for different runs, unless the
 *    underlying collection type is ordered or the operator is associative
 *    and commutative.
 *  @define mayNotTerminateInf
 *
 *    Note: may not terminate for infinite-sized collections.
 *  @define willNotTerminateInf
 *
 *    Note: will not terminate for infinite-sized collections.
 */
trait TraversableOnce[+A] extends Any with GenTraversableOnce[A] {
  self =>

  /** Self-documenting abstract methods. */
  def foreach[U](f: A => U): Unit
  def isEmpty: Boolean
  def hasDefiniteSize: Boolean

  // Note: We could redefine this in TraversableLike to always return `repr`
  // of type `Repr`, only if `Repr` had type bounds, which it doesn't, because
  // not all `Repr` are a subtype `TraversableOnce[A]`.
  // The alternative is redefining it for maps, sets and seqs. For concrete implementations
  // we don't have to do this anyway, since they are leaves in the inheritance hierarchy.
  // Note 2: This is implemented in all collections _not_ inheriting `Traversable[A]`
  //         at least indirectly. Currently, these are `ArrayOps` and `StringOps`.
  //         It is also implemented in `TraversableOnce[A]`.
  /** A version of this collection with all
   *  of the operations implemented sequentially (i.e. in a single-threaded manner).
   *
   *  This method returns a reference to this collection. In parallel collections,
   *  it is redefined to return a sequential implementation of this collection. In
   *  both cases, it has O(1) complexity.
   *
   *  @return a sequential view of the collection.
   */
  def seq: TraversableOnce[A]

  /** Presently these are abstract because the Traversable versions use
   *  breakable/break, and I wasn't sure enough of how that's supposed to
   *  function to consolidate them with the Iterator versions.
   */
  def forall(p: A => Boolean): Boolean
  def exists(p: A => Boolean): Boolean
  def find(p: A => Boolean): Option[A]
  def copyToArray[B >: A](xs: Array[B], start: Int, len: Int): Unit

  // for internal use
  protected[this] def reversed = {
    var elems: List[A] = Nil
    self foreach (elems ::= _)
    elems
  }

  def size: Int = {
    var result = 0
    for (x <- self) result += 1
    result
  }

  def nonEmpty: Boolean = !isEmpty

  def count(p: A => Boolean): Int = {
    var cnt = 0
    for (x <- this)
      if (p(x)) cnt += 1

    cnt
  }

  /** Finds the first element of the $coll for which the given partial
   *  function is defined, and applies the partial function to it.
   *
   *  $mayNotTerminateInf
   *  $orderDependent
   *
   *  @param pf   the partial function
   *  @return     an option value containing pf applied to the first
   *              value for which it is defined, or `None` if none exists.
   *  @example    `Seq("a", 1, 5L).collectFirst({ case x: Int => x*10 }) = Some(10)`
   */
  def collectFirst[B](pf: PartialFunction[A, B]): Option[B] = {
    // make sure to use an iterator or `seq`
    self.toIterator.foreach(pf.runWith(b => return Some(b)))
    None
  }

  def /:[B](z: B)(op: (B, A) => B): B = foldLeft(z)(op)

  def :\[B](z: B)(op: (A, B) => B): B = foldRight(z)(op)

  def foldLeft[B](z: B)(op: (B, A) => B): B = {
    var result = z
    this foreach (x => result = op(result, x))
    result
  }

  def foldRight[B](z: B)(op: (A, B) => B): B =
    reversed.foldLeft(z)((x, y) => op(y, x))

  /** Applies a binary operator to all elements of this $coll,
   *  going left to right.
   *  $willNotTerminateInf
   *  $orderDependentFold
   *
   *  @param  op    the binary operator.
   *  @tparam  B    the result type of the binary operator.
   *  @return  the result of inserting `op` between consecutive elements of this $coll,
   *           going left to right:
   *           {{{
   *             op( op( ... op(x_1, x_2) ..., x_{n-1}), x_n)
   *           }}}
   *           where `x,,1,,, ..., x,,n,,` are the elements of this $coll.
   *  @throws `UnsupportedOperationException` if this $coll is empty.   */
  def reduceLeft[B >: A](op: (B, A) => B): B = {
    if (isEmpty)
      throw new UnsupportedOperationException("empty.reduceLeft")

    var first = true
    var acc: B = 0.asInstanceOf[B]

    for (x <- self) {
      if (first) {
        acc = x
        first = false
      }
      else acc = op(acc, x)
    }
    acc
  }

  def reduceRight[B >: A](op: (A, B) => B): B = {
    if (isEmpty)
      throw new UnsupportedOperationException("empty.reduceRight")

    reversed.reduceLeft[B]((x, y) => op(y, x))
  }

  def reduceLeftOption[B >: A](op: (B, A) => B): Option[B] =
    if (isEmpty) None else Some(reduceLeft(op))

  def reduceRightOption[B >: A](op: (A, B) => B): Option[B] =
    if (isEmpty) None else Some(reduceRight(op))

  def reduce[A1 >: A](op: (A1, A1) => A1): A1 = reduceLeft(op)

  def reduceOption[A1 >: A](op: (A1, A1) => A1): Option[A1] = reduceLeftOption(op)

  def fold[A1 >: A](z: A1)(op: (A1, A1) => A1): A1 = foldLeft(z)(op)

  def aggregate[B](z: =>B)(seqop: (B, A) => B, combop: (B, B) => B): B = foldLeft(z)(seqop)

  def sum[B >: A](implicit num: Numeric[B]): B = foldLeft(num.zero)(num.plus)

  def product[B >: A](implicit num: Numeric[B]): B = foldLeft(num.one)(num.times)

  def min[B >: A](implicit cmp: Ordering[B]): A = {
    if (isEmpty)
      throw new UnsupportedOperationException("empty.min")

    reduceLeft((x, y) => if (cmp.lteq(x, y)) x else y)
  }

  def max[B >: A](implicit cmp: Ordering[B]): A = {
    if (isEmpty)
      throw new UnsupportedOperationException("empty.max")

    reduceLeft((x, y) => if (cmp.gteq(x, y)) x else y)
  }

  def maxBy[B](f: A => B)(implicit cmp: Ordering[B]): A = {
    if (isEmpty)
      throw new UnsupportedOperationException("empty.maxBy")

    var maxF: B = null.asInstanceOf[B]
    var maxElem: A = null.asInstanceOf[A]
    var first = true

    for (elem <- self) {
      val fx = f(elem)
      if (first || cmp.gt(fx, maxF)) {
        maxElem = elem
        maxF = fx
        first = false
      }
    }
    maxElem
  }
  def minBy[B](f: A => B)(implicit cmp: Ordering[B]): A = {
    if (isEmpty)
      throw new UnsupportedOperationException("empty.minBy")

    var minF: B = null.asInstanceOf[B]
    var minElem: A = null.asInstanceOf[A]
    var first = true

    for (elem <- self) {
      val fx = f(elem)
      if (first || cmp.lt(fx, minF)) {
        minElem = elem
        minF = fx
        first = false
      }
    }
    minElem
  }

  /** Copies all elements of this $coll to a buffer.
   *  $willNotTerminateInf
   *  @param  dest   The buffer to which elements are copied.
   */
  def copyToBuffer[B >: A](dest: Buffer[B]): Unit = dest ++= seq

  def copyToArray[B >: A](xs: Array[B], start: Int): Unit =
    copyToArray(xs, start, xs.length - start)

  def copyToArray[B >: A](xs: Array[B]): Unit =
    copyToArray(xs, 0, xs.length)

  def toArray[B >: A : ClassTag]: Array[B] = {
    if (isTraversableAgain) {
      val result = new Array[B](size)
      copyToArray(result, 0)
      result
    }
    else toBuffer.toArray
  }

  def toTraversable: Traversable[A]

  def toList: List[A] = to[List]

  def toIterable: Iterable[A] = toStream

  def toSeq: Seq[A] = toStream

  def toIndexedSeq: immutable.IndexedSeq[A] = to[immutable.IndexedSeq]

  def toBuffer[B >: A]: mutable.Buffer[B] = to[ArrayBuffer].asInstanceOf[mutable.Buffer[B]]

  def toSet[B >: A]: immutable.Set[B] = to[immutable.Set].asInstanceOf[immutable.Set[B]]

  def toVector: Vector[A] = to[Vector]

  def to[Col[_]](implicit cbf: CanBuildFrom[Nothing, A, Col[A @uV]]): Col[A @uV] = {
    val b = cbf()
    b ++= seq
    b.result()
  }

  def toMap[T, U](implicit ev: A <:< (T, U)): immutable.Map[T, U] = {
    val b = immutable.Map.newBuilder[T, U]
    for (x <- self)
      b += x

    b.result()
  }

  def mkString(start: String, sep: String, end: String): String =
    addString(new StringBuilder(), start, sep, end).toString

  def mkString(sep: String): String = mkString("", sep, "")

  def mkString: String = mkString("")

  /** Appends all elements of this $coll to a string builder using start, end, and separator strings.
   *  The written text begins with the string `start` and ends with the string `end`.
   *  Inside, the string representations (w.r.t. the method `toString`)
   *  of all elements of this $coll are separated by the string `sep`.
   *
   * Example:
   *
   * {{{
   *      scala> val a = List(1,2,3,4)
   *      a: List[Int] = List(1, 2, 3, 4)
   *      
   *      scala> val b = new StringBuilder()
   *      b: StringBuilder = 
   *      
   *      scala> a.addString(b , "List(" , ", " , ")")
   *      res5: StringBuilder = List(1, 2, 3, 4)
   * }}}
   *
   *  @param  b    the string builder to which elements are appended.
   *  @param start the starting string.
   *  @param sep   the separator string.
   *  @param end   the ending string.
   *  @return      the string builder `b` to which elements were appended.
   */
  def addString(b: StringBuilder, start: String, sep: String, end: String): StringBuilder = {
    var first = true

    b append start
    for (x <- self) {
      if (first) {
        b append x
        first = false
      }
      else {
        b append sep
        b append x
      }
    }
    b append end

    b
  }

  /** Appends all elements of this $coll to a string builder using a separator string.
   *  The written text consists of the string representations (w.r.t. the method `toString`)
   *  of all elements of this $coll, separated by the string `sep`.
   *
   * Example:
   *
   * {{{
   *      scala> val a = List(1,2,3,4)
   *      a: List[Int] = List(1, 2, 3, 4)
   *      
   *      scala> val b = new StringBuilder()
   *      b: StringBuilder =
   *
   *      scala> a.addString(b, ", ")
   *      res0: StringBuilder = 1, 2, 3, 4
   * }}}
   *
   *  @param  b    the string builder to which elements are appended.
   *  @param sep   the separator string.
   *  @return      the string builder `b` to which elements were appended.
   */
  def addString(b: StringBuilder, sep: String): StringBuilder = addString(b, "", sep, "")

  /** Appends all elements of this $coll to a string builder.
   *  The written text consists of the string representations (w.r.t. the method
   * `toString`) of all elements of this $coll without any separator string.
   *
   * Example:
   *
   * {{{
   *      scala> val a = List(1,2,3,4)
   *      a: List[Int] = List(1, 2, 3, 4)
   *      
   *      scala> val b = new StringBuilder()
   *      b: StringBuilder =
   *
   *      scala> val h = a.addString(b)
   *      h: StringBuilder = 1234
   * }}}

   *  @param  b    the string builder to which elements are appended.
   *  @return      the string builder `b` to which elements were appended.
   */
  def addString(b: StringBuilder): StringBuilder = addString(b, "")
}


object TraversableOnce {
  implicit def alternateImplicit[A](trav: TraversableOnce[A]) = new ForceImplicitAmbiguity
  implicit def flattenTraversableOnce[A, CC[_]](travs: TraversableOnce[CC[A]])(implicit ev: CC[A] => TraversableOnce[A]) =
    new FlattenOps[A](travs map ev)

  /* Functionality reused in Iterator.CanBuildFrom */
  private[collection] abstract class BufferedCanBuildFrom[A, CC[X] <: TraversableOnce[X]] extends generic.CanBuildFrom[CC[_], A, CC[A]] {
    def bufferToColl[B](buff: ArrayBuffer[B]): CC[B]
    def traversableToColl[B](t: GenTraversable[B]): CC[B]

    def newIterator: Builder[A, CC[A]] = new ArrayBuffer[A] mapResult bufferToColl

    /** Creates a new builder on request of a collection.
     *  @param from  the collection requesting the builder to be created.
     *  @return the result of invoking the `genericBuilder` method on `from`.
     */
    def apply(from: CC[_]): Builder[A, CC[A]] = from match {
      case xs: generic.GenericTraversableTemplate[_, _] => xs.genericBuilder.asInstanceOf[Builder[A, Traversable[A]]] mapResult {
        case res => traversableToColl(res.asInstanceOf[GenTraversable[A]])
      }
      case _ => newIterator
    }

    /** Creates a new builder from scratch
     *  @return the result of invoking the `newBuilder` method of this factory.
     */
    def apply() = newIterator
  }

  /** With the advent of `TraversableOnce`, it can be useful to have a builder which
   *  operates on `Iterator`s so they can be treated uniformly along with the collections.
   *  See `scala.util.Random.shuffle` or `scala.concurrent.Future.sequence` for an example.
   */
  class OnceCanBuildFrom[A] extends BufferedCanBuildFrom[A, TraversableOnce] {
    def bufferToColl[B](buff: ArrayBuffer[B]) = buff.iterator
    def traversableToColl[B](t: GenTraversable[B]) = t.seq
  }

  /** Evidence for building collections from `TraversableOnce` collections */
  implicit def OnceCanBuildFrom[A] = new OnceCanBuildFrom[A]

  class FlattenOps[A](travs: TraversableOnce[TraversableOnce[A]]) {
    def flatten: Iterator[A] = new AbstractIterator[A] {
      val its = travs.toIterator
      private var it: Iterator[A] = Iterator.empty
      def hasNext: Boolean = it.hasNext || its.hasNext && { it = its.next().toIterator; hasNext }
      def next(): A = if (hasNext) it.next() else Iterator.empty.next()
    }
  }

  class ForceImplicitAmbiguity

  implicit class MonadOps[+A](trav: TraversableOnce[A]) {
    def map[B](f: A => B): TraversableOnce[B] = trav.toIterator map f
    def flatMap[B](f: A => GenTraversableOnce[B]): TraversableOnce[B] = trav.toIterator flatMap f
    def withFilter(p: A => Boolean) = trav.toIterator filter p
    def filter(p: A => Boolean): TraversableOnce[A] = withFilter(p)
  }
}

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