scala.collection.TraversableOnce

trait TraversableOnce[+A] extends GenTraversableOnce[A]

A template trait for collections which can be traversed either once only or one or more times.

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 .

Abstract Value Members From scala.collection.TraversableOnce

abstract def exists(p: (A) ⇒ Boolean): Boolean

Tests whether a predicate holds for at least one element of this traversable or iterator.

Note: may not terminate for infinite-sized collections.

  • p
    • the predicate used to test elements.
  • returns
    • true if the given predicate p is satisfied by at least one element of this traversable or iterator, otherwise false
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

abstract def find(p: (A) ⇒ Boolean): Option[A]

Finds the first element of the traversable or iterator satisfying a predicate, if any.

Note: may not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered.

  • p
    • the predicate used to test elements.
  • returns
    • an option value containing the first element in the traversable or iterator that satisfies p , or None if none exists.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

abstract def forall(p: (A) ⇒ Boolean): Boolean

Tests whether a predicate holds for all elements of this traversable or iterator.

Note: may not terminate for infinite-sized collections.

  • p
    • the predicate used to test elements.
  • returns
    • true if this traversable or iterator is empty or the given predicate p holds for all elements of this traversable or iterator, otherwise false .
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

abstract def seq: 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.

  • returns
    • a sequential view of the collection.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

Concrete Value Members From scala.collection.TraversableOnce

def /:[B](z: B)(op: (B, A) ⇒ B): B

Applies a binary operator to a start value and all elements of this traversable or iterator, going left to right.

Note: /: is alternate syntax for foldLeft ; z /: xs is the same as xs foldLeft z .

Examples:

Note that the folding function used to compute b is equivalent to that used to compute c.

scala> val a = List(1,2,3,4)
a: List[Int] = List(1, 2, 3, 4)

scala> val b = (5 /: a)(_+_)
b: Int = 15

scala> val c = (5 /: a)((x,y) => x + y)
c: Int = 15

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

  • B
    • the result type of the binary operator.
  • z
    • the start value.
  • op
    • the binary operator.
  • returns
    • the result of inserting op between consecutive elements of this traversable or iterator, going left to right with the start value z on the left:
    op(...op(op(z, x_1), x_2), ..., x_n)
    
where `x1, ..., xn` are the elements of this traversable or iterator.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def :\[B](z: B)(op: (A, B) ⇒ B): B

Applies a binary operator to all elements of this traversable or iterator and a start value, going right to left.

Note: :\ is alternate syntax for foldRight ; xs :\ z is the same as xs foldRight z .

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

Examples:

Note that the folding function used to compute b is equivalent to that used to compute c.

scala> val a = List(1,2,3,4)
a: List[Int] = List(1, 2, 3, 4)

scala> val b = (a :\ 5)(_+_)
b: Int = 15

scala> val c = (a :\ 5)((x,y) => x + y)
c: Int = 15
  • B
    • the result type of the binary operator.
  • z
    • the start value
  • op
    • the binary operator
  • returns
    • the result of inserting op between consecutive elements of this traversable or iterator, going right to left with the start value z on the right:
    op(x_1, op(x_2, ... op(x_n, z)...))
    
where `x1, ..., xn` are the elements of this traversable or iterator.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def addString(b: StringBuilder): StringBuilder

Appends all elements of this traversable or iterator to a string builder. The written text consists of the string representations (w.r.t. the method toString ) of all elements of this traversable or iterator 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
  • b
    • the string builder to which elements are appended.
  • returns
    • the string builder b to which elements were appended.

(defined at scala.collection.TraversableOnce)

def addString(b: StringBuilder, sep: String): StringBuilder

Appends all elements of this traversable or iterator 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 traversable or iterator, 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
  • b
    • the string builder to which elements are appended.
  • sep
    • the separator string.
  • returns
    • the string builder b to which elements were appended.

(defined at scala.collection.TraversableOnce)

def addString(b: StringBuilder, start: String, sep: String, end: String): StringBuilder

Appends all elements of this traversable or iterator 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 traversable or iterator 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)
  • b
    • the string builder to which elements are appended.
  • start
    • the starting string.
  • sep
    • the separator string.
  • end
    • the ending string.
  • returns
    • the string builder b to which elements were appended.

(defined at scala.collection.TraversableOnce)

def aggregate[B](z: ⇒ B)(seqop: (B, A) ⇒ B, combop: (B, B) ⇒ B): B

Aggregates the results of applying an operator to subsequent elements.

This is a more general form of fold and reduce . It is similar to foldLeft in that it doesn’t require the result to be a supertype of the element type. In addition, it allows parallel collections to be processed in chunks, and then combines the intermediate results.

aggregate splits the traversable or iterator into partitions and processes each partition by sequentially applying seqop , starting with z (like foldLeft ). Those intermediate results are then combined by using combop (like fold ). The implementation of this operation may operate on an arbitrary number of collection partitions (even 1), so combop may be invoked an arbitrary number of times (even 0).

As an example, consider summing up the integer values of a list of chars. The initial value for the sum is 0. First, seqop transforms each input character to an Int and adds it to the sum (of the partition). Then, combop just needs to sum up the intermediate results of the partitions:

List('a', 'b', 'c').aggregate(0)({ (sum, ch) => sum + ch.toInt }, { (p1, p2) => p1 + p2 })
  • B
    • the type of accumulated results
  • z
    • the initial value for the accumulated result of the partition - this will typically be the neutral element for the seqop operator (e.g. Nil for list concatenation or 0 for summation) and may be evaluated more than once
  • seqop
    • an operator used to accumulate results within a partition
  • combop
    • an associative operator used to combine results from different partitions
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def collectFirst[B](pf: PartialFunction[A, B]): Option[B]

Finds the first element of the traversable or iterator for which the given partial function is defined, and applies the partial function to it.

Note: may not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered.

  • pf
    • the partial function
  • returns
    • 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)

(defined at scala.collection.TraversableOnce)

def copyToArray[B >: A](xs: Array[B]): Unit

[use case]

Copies the elements of this traversable or iterator to an array. Fills the given array xs with values of this traversable or iterator. Copying will stop once either the end of the current traversable or iterator is reached, or the end of the target array is reached.

Note: will not terminate for infinite-sized collections.

  • xs
    • the array to fill.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

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

[use case]

Copies the elements of this traversable or iterator to an array. Fills the given array xs with values of this traversable or iterator, beginning at index start . Copying will stop once either the end of the current traversable or iterator is reached, or the end of the target array is reached.

Note: will not terminate for infinite-sized collections.

  • xs
    • the array to fill.
  • start
    • the starting index.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

abstract def copyToArray[B >: A](xs: Array[B], start: Int, len: Int): Unit

[use case]

Copies the elements of this traversable or iterator to an array. Fills the given array xs with at most len elements of this traversable or iterator, starting at position start . Copying will stop once either the end of the current traversable or iterator is reached, or the end of the target array is reached, or len elements have been copied.

Note: will not terminate for infinite-sized collections.

  • xs
    • the array to fill.
  • start
    • the starting index.
  • len
    • the maximal number of elements to copy.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def copyToBuffer[B >: A](dest: Buffer[B]): Unit

Copies all elements of this traversable or iterator to a buffer.

Note: will not terminate for infinite-sized collections.

  • dest
    • The buffer to which elements are copied.

(defined at scala.collection.TraversableOnce)

def count(p: (A) ⇒ Boolean): Int

Counts the number of elements in the traversable or iterator which satisfy a predicate.

  • p
    • the predicate used to test elements.
  • returns
    • the number of elements satisfying the predicate p .
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def foldLeft[B](z: B)(op: (B, A) ⇒ B): B

Applies a binary operator to a start value and all elements of this traversable or iterator, going left to right.

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

  • B
    • the result type of the binary operator.
  • z
    • the start value.
  • op
    • the binary operator.
  • returns
    • the result of inserting op between consecutive elements of this traversable or iterator, going left to right with the start value z on the left:
    op(...op(z, x_1), x_2, ..., x_n)
    
where `x1, ..., xn` are the elements of this traversable or iterator.
Returns `z` if this traversable or iterator is empty.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def foldRight[B](z: B)(op: (A, B) ⇒ B): B

Applies a binary operator to all elements of this traversable or iterator and a start value, going right to left.

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

  • B
    • the result type of the binary operator.
  • z
    • the start value.
  • op
    • the binary operator.
  • returns
    • the result of inserting op between consecutive elements of this traversable or iterator, going right to left with the start value z on the right:
    op(x_1, op(x_2, ... op(x_n, z)...))
    
where `x1, ..., xn` are the elements of this traversable or iterator.
Returns `z` if this traversable or iterator is empty.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

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

Folds the elements of this traversable or iterator using the specified associative binary operator.

The order in which operations are performed on elements is unspecified and may be nondeterministic.

Note: will not terminate for infinite-sized collections.

  • A1
    • a type parameter for the binary operator, a supertype of A .
  • z
    • a neutral element for the fold operation; may be added to the result an arbitrary number of times, and must not change the result (e.g., Nil for list concatenation, 0 for addition, or 1 for multiplication).
  • op
    • a binary operator that must be associative.
  • returns
    • the result of applying the fold operator op between all the elements and z , or z if this traversable or iterator is empty.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

abstract def foreach[U](f: (A) ⇒ U): Unit

[use case]

Applies a function f to all elements of this traversable or iterator.

Note: this method underlies the implementation of most other bulk operations. It’s important to implement this method in an efficient way.

  • f
    • the function that is applied for its side-effect to every element. The result of function f is discarded.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def maxBy[B](f: (A) ⇒ B)(implicit cmp: Ordering[B]): A

[use case]

Finds the first element which yields the largest value measured by function f.

  • B
    • The result type of the function f.
  • f
    • The measuring function.
  • returns
    • the first element of this traversable or iterator with the largest value measured by function f.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def minBy[B](f: (A) ⇒ B)(implicit cmp: Ordering[B]): A

[use case]

Finds the first element which yields the smallest value measured by function f.

  • B
    • The result type of the function f.
  • f
    • The measuring function.
  • returns
    • the first element of this traversable or iterator with the smallest value measured by function f.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def mkString(sep: String): String

Displays all elements of this traversable or iterator in a string using a separator string.

  • sep
    • the separator string.
  • returns
    • a string representation of this traversable or iterator. In the resulting string the string representations (w.r.t. the method toString ) of all elements of this traversable or iterator are separated by the string sep .
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

Example:

List(1, 2, 3).mkString("|") = "1|2|3"

(defined at scala.collection.TraversableOnce)

def mkString(start: String, sep: String, end: String): String

Displays all elements of this traversable or iterator in a string using start, end, and separator strings.

  • start
    • the starting string.
  • sep
    • the separator string.
  • end
    • the ending string.
  • returns
    • a string representation of this traversable or iterator. The resulting string 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 traversable or iterator are separated by the string sep .
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

Example:

List(1, 2, 3).mkString("(", "; ", ")") = "(1; 2; 3)"

(defined at scala.collection.TraversableOnce)

def reduceLeftOption[B >: A](op: (B, A) ⇒ B): Option[B]

Optionally applies a binary operator to all elements of this traversable or iterator, going left to right.

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

  • B
    • the result type of the binary operator.
  • op
    • the binary operator.
  • returns
    • an option value containing the result of reduceLeft(op) if this traversable or iterator is nonempty, None otherwise.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def reduceLeft[B >: A](op: (B, A) ⇒ B): B

Applies a binary operator to all elements of this traversable or iterator, going left to right.

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

  • B
    • the result type of the binary operator.
  • op
    • the binary operator.
  • returns
    • the result of inserting op between consecutive elements of this traversable or iterator, going left to right:
    op( op( ... op(x_1, x_2) ..., x_{n-1}), x_n)
    
where `x1, ..., xn` are the elements of this traversable or iterator.
  • Exceptions thrown
    • UnsupportedOperationException if this traversable or iterator is empty.

(defined at scala.collection.TraversableOnce)

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

Reduces the elements of this traversable or iterator, if any, using the specified associative binary operator.

The order in which operations are performed on elements is unspecified and may be nondeterministic.

  • A1
    • A type parameter for the binary operator, a supertype of A .
  • op
    • A binary operator that must be associative.
  • returns
    • An option value containing result of applying reduce operator op between all the elements if the collection is nonempty, and None otherwise.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def reduceRightOption[B >: A](op: (A, B) ⇒ B): Option[B]

Optionally applies a binary operator to all elements of this traversable or iterator, going right to left.

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

  • B
    • the result type of the binary operator.
  • op
    • the binary operator.
  • returns
    • an option value containing the result of reduceRight(op) if this traversable or iterator is nonempty, None otherwise.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def reduceRight[B >: A](op: (A, B) ⇒ B): B

Applies a binary operator to all elements of this traversable or iterator, going right to left.

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

  • B
    • the result type of the binary operator.
  • op
    • the binary operator.
  • returns
    • the result of inserting op between consecutive elements of this traversable or iterator, going right to left:
    op(x_1, op(x_2, ..., op(x_{n-1}, x_n)...))
    
where `x1, ..., xn` are the elements of this traversable or iterator.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce
  • Exceptions thrown
    • UnsupportedOperationException if this traversable or iterator is empty.

(defined at scala.collection.TraversableOnce)

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

Reduces the elements of this traversable or iterator using the specified associative binary operator.

The order in which operations are performed on elements is unspecified and may be nondeterministic.

  • A1
    • A type parameter for the binary operator, a supertype of A .
  • op
    • A binary operator that must be associative.
  • returns
    • The result of applying reduce operator op between all the elements if the traversable or iterator is nonempty.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce
  • Exceptions thrown
    • UnsupportedOperationException if this traversable or iterator is empty.

(defined at scala.collection.TraversableOnce)

def toBuffer[B >: A]: Buffer[B]

Uses the contents of this traversable or iterator to create a new mutable buffer.

Note: will not terminate for infinite-sized collections.

  • returns
    • a buffer containing all elements of this traversable or iterator.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def toIndexedSeq: immutable.IndexedSeq[A]

Converts this traversable or iterator to an indexed sequence.

Note: will not terminate for infinite-sized collections.

  • returns
    • an indexed sequence containing all elements of this traversable or iterator.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def toMap[T, U](implicit ev: <:<[A, (T, U)]): immutable.Map[T, U]

[use case]

Converts this traversable or iterator to a map. This method is unavailable unless the elements are members of Tuple2, each ((T, U)) becoming a key-value pair in the map. Duplicate keys will be overwritten by later keys: if this is an unordered collection, which key is in the resulting map is undefined.

Note: will not terminate for infinite-sized collections.

  • returns
    • a map of type immutable.Map[T, U] containing all key/value pairs of type (T, U) of this traversable or iterator.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def toSet[B >: A]: immutable.Set[B]

Converts this traversable or iterator to a set.

Note: will not terminate for infinite-sized collections.

  • returns
    • a set containing all elements of this traversable or iterator.
  • Definition Classes
    • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

Concrete Value Members From Implicit scala.collection.TraversableOnce.MonadOps

def filter(p: (A) ⇒ Boolean): TraversableOnce[A]

  • Implicit information
    • This member is added by an implicit conversion from TraversableOnce [A] to MonadOps [A] performed by method MonadOps in scala.collection.TraversableOnce.
  • Definition Classes
    • MonadOps

(added by implicit convertion: scala.collection.TraversableOnce.MonadOps)

def flatMap[B](f: (A) ⇒ GenTraversableOnce[B]): TraversableOnce[B]

  • Implicit information
    • This member is added by an implicit conversion from TraversableOnce [A] to MonadOps [A] performed by method MonadOps in scala.collection.TraversableOnce.
  • Definition Classes
    • MonadOps

(added by implicit convertion: scala.collection.TraversableOnce.MonadOps)

def map[B](f: (A) ⇒ B): TraversableOnce[B]

  • Implicit information
    • This member is added by an implicit conversion from TraversableOnce [A] to MonadOps [A] performed by method MonadOps in scala.collection.TraversableOnce.
  • Definition Classes
    • MonadOps

(added by implicit convertion: scala.collection.TraversableOnce.MonadOps)

def withFilter(p: (A) ⇒ Boolean): Iterator[A]

  • Implicit information
    • This member is added by an implicit conversion from TraversableOnce [A] to MonadOps [A] performed by method MonadOps in scala.collection.TraversableOnce.
  • Definition Classes
    • MonadOps (added by implicit convertion: scala.collection.TraversableOnce.MonadOps)

Full Source:

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

package scala
package collection

import mutable.{ Buffer, Builder, 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] = {
    // TODO 2.12 -- move out alternate implementations into child classes
    val i: Iterator[A] = self match {
      case it: Iterator[A] => it
      case _: GenIterable[_] => self.toIterator   // If it might be parallel, be sure to .seq or use iterator!
      case _ =>                                   // Not parallel, not iterable--just traverse
        self.foreach(pf.runWith(b => return Some(b)))
        return None
    }
    // Presumably the fastest way to get in and out of a partial function is for a sentinel function to return itself
    // (Tested to be lower-overhead than runWith.  Would be better yet to not need to (formally) allocate it--change in 2.12.)
    val sentinel: Function1[A, Any] = new scala.runtime.AbstractFunction1[A, Any]{ def apply(a: A) = this }
    while (i.hasNext) {
      val x = pf.applyOrElse(i.next, sentinel)
      if (x.asInstanceOf[AnyRef] ne sentinel) return Some(x.asInstanceOf[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)
  }
}