scala.collection.immutable.Stream

abstract class Stream[+A] extends AbstractSeq[A] with LinearSeq[A] with GenericTraversableTemplate[A, Stream] with LinearSeqOptimized[A, Stream[A]] with Serializable

The class Stream implements lazy lists where elements are only evaluated when they are needed. Here is an example:

import scala.math.BigInt
object Main extends App {

  val fibs: Stream[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

The Stream class also employs memoization such that previously computed values are converted from Stream elements to concrete values of type A . 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: Stream[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 Stream requires us to have somewhere to store the information and a val allows us to do that.
• While the Stream 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 Stream 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 Stream ) 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 Stream , and a stream 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 Stream definition will be a val definition
val stream1: Stream[Int] = {
  def loop(v: Int): Stream[Int] = v #:: loop(v + 1)
  loop(0)
}

// Because stream1 is a val, everything that the iterator produces is held
// by virtue of the fact that the head of the Stream is held in stream1
val it1 = stream1.iterator
loop("Iterator1: ", it1.next, it1)

// We can redefine this Stream such that all we have is the Iterator left
// and allow the Stream to be garbage collected as required.  Using a def
// to provide the Stream ensures that no val is holding onto the head as
// is the case with stream1
def stream2: Stream[Int] = {
  def loop(v: Int): Stream[Int] = v #:: loop(v + 1)
  loop(0)
}
val it2 = stream2.iterator
loop("Iterator2: ", it2.next, it2)

// And, of course, we don't actually need a Stream at all for such a simple
// problem.  There's no reason to use a Stream 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, Stream(...)) 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...
val sov: Stream[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: Stream[Int] = {
  def loop(h: Int, n: Int): Stream[Int] = h #:: loop(n, h + n)
  loop(1, 1)
}

Note that mkString forces evaluation of a Stream , but addString does not. In both cases, a Stream that is or ends in a cycle (e.g. lazy val s: Stream[Int] = 0 #:: s ) will convert additional trips through the cycle to ... . Additionally, addString will display an un-memoized tail as ? .

• A
  • the type of the elements contained in this stream.
• Self Type
  • Stream [A]
• Annotations
  • @ deprecatedInheritance (message = “This class will be sealed.”, since = “2.11.0”)
• Source
  • Stream.scala - Blame
  • Stream.scala - History
• Version
  • 1.1 08/08/03
• Since
  • 2.8
• See also
  • “Scala’s Collection Library overview” section on Streams for more information.

Type Members

type Self = Stream[A]

The type implementing this traversable

• Attributes
  • protected[this]
• Definition Classes
  • TraversableLike

class WithFilter extends FilterMonadic[A, Repr]

A class supporting filtered operations. Instances of this class are returned by method withFilter .

• Definition Classes
  • TraversableLike

Concrete Value Members From scala.Function1

def compose[A](g: (A) ⇒ Int): (A) ⇒ A

Composes two instances of Function1 in a new Function1, with this function applied last.

• A
  • the type to which function g can be applied
• g
  • a function A => T1
• returns
  • a new function f such that f(x) == apply(g(x))
• Definition Classes
  • Function1
• Annotations
  • @ unspecialized ()

(defined at scala.Function1)

Concrete Value Members From scala.PartialFunction

def andThen[C](k: (A) ⇒ C): PartialFunction[Int, C]

Composes this partial function with a transformation function that gets applied to results of this partial function.

• C
  • the result type of the transformation function.
• k
  • the transformation function
• returns
  • a partial function with the same domain as this partial function, which maps arguments x to k(this(x)) .
• Definition Classes
  • PartialFunction → Function1

(defined at scala.PartialFunction)

def applyOrElse[A1 <: Int, B1 >: A](x: A1, default: (A1) ⇒ B1): B1

Applies this partial function to the given argument when it is contained in the function domain. Applies fallback function where this partial function is not defined.

Note that expression pf.applyOrElse(x, default) is equivalent to

if(pf isDefinedAt x) pf(x) else default(x)

except that applyOrElse method can be implemented more efficiently. For all partial function literals the compiler generates an applyOrElse implementation which avoids double evaluation of pattern matchers and guards. This makes applyOrElse the basis for the efficient implementation for many operations and scenarios, such as:

• combining partial functions into orElse / andThen chains does not lead to excessive apply / isDefinedAt evaluation
• lift and unlift do not evaluate source functions twice on each invocation
• runWith allows efficient imperative-style combining of partial functions with conditionally applied actions

For non-literal partial function classes with nontrivial isDefinedAt method it is recommended to override applyOrElse with custom implementation that avoids double isDefinedAt evaluation. This may result in better performance and more predictable behavior w.r.t. side effects.

• x
  • the function argument
• default
  • the fallback function
• returns
  • the result of this function or fallback function application.
• Definition Classes
  • PartialFunction
• Since
  • 2.10

(defined at scala.PartialFunction)

def lift: (Int) ⇒ Option[A]

Turns this partial function into a plain function returning an Option result.

• returns
  • a function that takes an argument x to Some(this(x)) if this is defined for x , and to None otherwise.
• Definition Classes
  • PartialFunction
• See also
  • Function.unlift

(defined at scala.PartialFunction)

def orElse[A1 <: Int, B1 >: A](that: PartialFunction[A1, B1]): PartialFunction[A1, B1]

Composes this partial function with a fallback partial function which gets applied where this partial function is not defined.

• A1
  • the argument type of the fallback function
• B1
  • the result type of the fallback function
• that
  • the fallback function
• returns
  • a partial function which has as domain the union of the domains of this partial function and that . The resulting partial function takes x to this(x) where this is defined, and to that(x) where it is not.
• Definition Classes
  • PartialFunction

(defined at scala.PartialFunction)

def runWith[U](action: (A) ⇒ U): (Int) ⇒ Boolean

Composes this partial function with an action function which gets applied to results of this partial function. The action function is invoked only for its side effects; its result is ignored.

Note that expression pf.runWith(action)(x) is equivalent to

if(pf isDefinedAt x) { action(pf(x)); true } else false

except that runWith is implemented via applyOrElse and thus potentially more efficient. Using runWith avoids double evaluation of pattern matchers and guards for partial function literals.

• action
  • the action function
• returns
  • a function which maps arguments x to isDefinedAt(x) . The resulting function runs action(this(x)) where this is defined.
• Definition Classes
  • PartialFunction
• Since
  • 2.10
• See also
  • applyOrElse .

(defined at scala.PartialFunction)

Concrete Value Members From scala.collection.GenSeqLike

def equals(that: Any): Boolean

The equals method for arbitrary sequences. Compares this sequence to some other object.

• that
  • The object to compare the sequence to
• returns
  • true if that is a sequence that has the same elements as this sequence in the same order, false otherwise
• Definition Classes
  • GenSeqLike → Equals → Any

(defined at scala.collection.GenSeqLike)

def indexOf[B >: A](elem: B): Int

[use case]

Finds index of first occurrence of some value in this stream.

Note: may not terminate for infinite-sized collections.

• elem
  • the element value to search for.
• returns
  • the index of the first element of this stream that is equal (as determined by == ) to elem , or -1 , if none exists.
• Definition Classes
  • GenSeqLike

(defined at scala.collection.GenSeqLike)

def indexOf[B >: A](elem: B, from: Int): Int

[use case]

Finds index of first occurrence of some value in this stream after or at some start index.

Note: may not terminate for infinite-sized collections.

• elem
  • the element value to search for.
• from
  • the start index
• returns
  • the index >= from of the first element of this stream that is equal (as determined by == ) to elem , or -1 , if none exists.
• Definition Classes
  • GenSeqLike

(defined at scala.collection.GenSeqLike)

def indexWhere(p: (A) ⇒ Boolean): Int

Finds index of first element satisfying some predicate.

Note: may not terminate for infinite-sized collections.

• p
  • the predicate used to test elements.
• returns
  • the index of the first element of this general sequence that satisfies the predicate p , or -1 , if none exists.
• Definition Classes
  • GenSeqLike

(defined at scala.collection.GenSeqLike)

def lastIndexOf[B >: A](elem: B): Int

[use case]

Finds index of last occurrence of some value in this stream.

Note: will not terminate for infinite-sized collections.

• elem
  • the element value to search for.
• returns
  • the index of the last element of this stream that is equal (as determined by == ) to elem , or -1 , if none exists.
• Definition Classes
  • GenSeqLike

(defined at scala.collection.GenSeqLike)

def lastIndexOf[B >: A](elem: B, end: Int): Int

[use case]

Finds index of last occurrence of some value in this stream before or at a given end index.

• elem
  • the element value to search for.
• end
  • the end index.
• returns
  • the index <= end of the last element of this stream that is equal (as determined by == ) to elem , or -1 , if none exists.
• Definition Classes
  • GenSeqLike

(defined at scala.collection.GenSeqLike)

def lastIndexWhere(p: (A) ⇒ Boolean): Int

Finds index of last element satisfying some predicate.

Note: will not terminate for infinite-sized collections.

• p
  • the predicate used to test elements.
• returns
  • the index of the last element of this general sequence that satisfies the predicate p , or -1 , if none exists.
• Definition Classes
  • GenSeqLike

(defined at scala.collection.GenSeqLike)

def prefixLength(p: (A) ⇒ Boolean): Int

Returns the length of the longest prefix whose elements all satisfy some predicate.

Note: may not terminate for infinite-sized collections.

• p
  • the predicate used to test elements.
• returns
  • the length of the longest prefix of this general sequence such that every element of the segment satisfies the predicate p .
• Definition Classes
  • GenSeqLike

(defined at scala.collection.GenSeqLike)

def startsWith[B](that: GenSeq[B]): Boolean

Tests whether this general sequence starts with the given sequence.

• that
  • the sequence to test
• returns
  • true if this collection has that as a prefix, false otherwise.
• Definition Classes
  • GenSeqLike

(defined at scala.collection.GenSeqLike)

Concrete Value Members From scala.collection.IterableLike

def canEqual(that: Any): Boolean

Method called from equality methods, so that user-defined subclasses can refuse to be equal to other collections of the same kind.

• that
  • The object with which this iterable collection should be compared
• returns
  • true , if this iterable collection can possibly equal that , false otherwise. The test takes into consideration only the run-time types of objects but ignores their elements.
• Definition Classes
  • IterableLike → Equals

(defined at scala.collection.IterableLike)

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

[use case]

Copies the elements of this stream to an array. Fills the given array xs with at most len elements of this stream, starting at position start . Copying will stop once either the end of the current stream 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
  • IterableLike → TraversableLike → TraversableOnce → GenTraversableOnce

(defined at scala.collection.IterableLike)

def grouped(size: Int): Iterator[Stream[A]]

Partitions elements in fixed size iterable collections.

• size
  • the number of elements per group
• returns
  • An iterator producing iterable collections of size size , except the last will be less than size size if the elements don’t divide evenly.
• Definition Classes
  • IterableLike
• See also
  • scala.collection.Iterator, method grouped

(defined at scala.collection.IterableLike)

def sliding(size: Int): Iterator[Stream[A]]

Groups elements in fixed size blocks by passing a “sliding window” over them (as opposed to partitioning them, as is done in grouped.) “Sliding window” step is 1 by default.

• size
  • the number of elements per group
• returns
  • An iterator producing iterable collections of size size , except the last and the only element will be truncated if there are fewer elements than size.
• Definition Classes
  • IterableLike
• See also
  • scala.collection.Iterator, method sliding

(defined at scala.collection.IterableLike)

def sliding(size: Int, step: Int): Iterator[Stream[A]]

Groups elements in fixed size blocks by passing a “sliding window” over them (as opposed to partitioning them, as is done in grouped.)

• size
  • the number of elements per group
• step
  • the distance between the first elements of successive groups
• returns
  • An iterator producing iterable collections of size size , except the last and the only element will be truncated if there are fewer elements than size.
• Definition Classes
  • IterableLike
• See also
  • scala.collection.Iterator, method sliding

(defined at scala.collection.IterableLike)

def toIterable: collection.Iterable[A]

Returns this iterable collection as an iterable collection.

A new collection will not be built; lazy collections will stay lazy.

Note: will not terminate for infinite-sized collections.

• returns
  • an Iterable containing all elements of this iterable collection.
• Definition Classes
  • IterableLike → TraversableOnce → GenTraversableOnce

(defined at scala.collection.IterableLike)

def zipAll[B, A1 >: A, That](that: GenIterable[B], thisElem: A1, thatElem: B)(implicit bf: CanBuildFrom[Stream[A], (A1, B), That]): That

[use case]

Returns a stream formed from this stream and another iterable collection by combining corresponding elements in pairs. If one of the two collections is shorter than the other, placeholder elements are used to extend the shorter collection to the length of the longer.

• B
  • the type of the second half of the returned pairs
• that
  • The iterable providing the second half of each result pair
• thisElem
  • the element to be used to fill up the result if this stream is shorter than that .
• thatElem
  • the element to be used to fill up the result if that is shorter than this stream.
• returns
  • a new stream containing pairs consisting of corresponding elements of this stream and that . The length of the returned collection is the maximum of the lengths of this stream and that . If this stream is shorter than that , thisElem values are used to pad the result. If that is shorter than this stream, thatElem values are used to pad the result.
• Definition Classes
  • IterableLike → GenIterableLike

(defined at scala.collection.IterableLike)

Concrete Value Members From scala.collection.LinearSeqLike

final def corresponds[B](that: GenSeq[B])(p: (A, B) ⇒ Boolean): Boolean

Tests whether every element of this sequence relates to the corresponding element of another sequence by satisfying a test predicate.

• B
  • the type of the elements of that
• that
  • the other sequence
• p
  • the test predicate, which relates elements from both sequences
• returns
  • true if both sequences have the same length and p(x, y) is true for all corresponding elements x of this sequence and y of that , otherwise false .
• Definition Classes
  • LinearSeqLike → SeqLike → GenSeqLike
• Annotations
  • @ tailrec ()

(defined at scala.collection.LinearSeqLike)

def thisCollection: collection.LinearSeq[A]

The underlying collection seen as an instance of Seq . By default this is implemented as the current collection object itself, but this can be overridden.

• Attributes
  • protected[this]
• Definition Classes
  • LinearSeqLike → SeqLike → IterableLike → TraversableLike

(defined at scala.collection.LinearSeqLike)

def toCollection(repr: Stream[A]): collection.LinearSeq[A]

A conversion from collections of type Repr to Seq objects. By default this is implemented as just a cast, but this can be overridden.

• Attributes
  • protected[this]
• Definition Classes
  • LinearSeqLike → SeqLike → IterableLike → TraversableLike

(defined at scala.collection.LinearSeqLike)

Concrete Value Members From scala.collection.LinearSeqOptimized

def apply(n: Int): A

Selects an element by its index in the sequence. Note: the execution of apply may take time proportional to the index value.

• returns
  • the element of this sequence at index idx , where 0 indicates the first element.
• Definition Classes
  • LinearSeqOptimized → SeqLike → GenSeqLike
• Exceptions thrown
  • IndexOutOfBoundsException if idx does not satisfy 0 <= idx < length .

(defined at scala.collection.LinearSeqOptimized)

def contains[A1 >: A](elem: A1): Boolean

Tests whether this sequence contains a given value as an element.

Note: may not terminate for infinite-sized collections.

• elem
  • the element to test.
• returns
  • true if this sequence has an element that is equal (as determined by == ) to elem , false otherwise.
• Definition Classes
  • LinearSeqOptimized → SeqLike

(defined at scala.collection.LinearSeqOptimized)

def exists(p: (A) ⇒ Boolean): Boolean

Tests whether a predicate holds for at least one element of this sequence.

Note: may not terminate for infinite-sized collections.

• p
  • the predicate used to test elements.
• returns
  • false if this sequence is empty, otherwise true if the given predicate p holds for some of the elements of this sequence, otherwise false
• Definition Classes
  • LinearSeqOptimized → IterableLike → TraversableLike → TraversableOnce → GenTraversableOnce

(defined at scala.collection.LinearSeqOptimized)

def find(p: (A) ⇒ Boolean): Option[A]

Finds the first element of the sequence satisfying a predicate, if any.

Note: may not terminate for infinite-sized collections.

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

(defined at scala.collection.LinearSeqOptimized)

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

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

Note: will not terminate for infinite-sized collections.

• 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 sequence, 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 sequence. Returns `z` if this
sequence is empty.

• Definition Classes
  • LinearSeqOptimized → IterableLike → TraversableOnce → GenTraversableOnce

(defined at scala.collection.LinearSeqOptimized)

def forall(p: (A) ⇒ Boolean): Boolean

Tests whether a predicate holds for all elements of this sequence.

Note: may not terminate for infinite-sized collections.

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

(defined at scala.collection.LinearSeqOptimized)

def indexWhere(p: (A) ⇒ Boolean, from: Int): Int

Finds index of the first element satisfying some predicate after or at some start index.

Note: may not terminate for infinite-sized collections.

• p
  • the predicate used to test elements.
• from
  • the start index
• returns
  • the index >= from of the first element of this sequence that satisfies the predicate p , or -1 , if none exists.
• Definition Classes
  • LinearSeqOptimized → SeqLike → GenSeqLike

(defined at scala.collection.LinearSeqOptimized)

def isDefinedAt(x: Int): Boolean

Tests whether this sequence contains given index.

The implementations of methods apply and isDefinedAt turn a Seq[A] into a PartialFunction[Int, A] .

• returns
  • true if this sequence contains an element at position idx , false otherwise.
• Definition Classes
  • LinearSeqOptimized → GenSeqLike

(defined at scala.collection.LinearSeqOptimized)

def lastIndexWhere(p: (A) ⇒ Boolean, end: Int): Int

Finds index of last element satisfying some predicate before or at given end index.

• p
  • the predicate used to test elements.
• returns
  • the index <= end of the last element of this sequence that satisfies the predicate p , or -1 , if none exists.
• Definition Classes
  • LinearSeqOptimized → SeqLike → GenSeqLike

(defined at scala.collection.LinearSeqOptimized)

def lengthCompare(len: Int): Int

Compares the length of this sequence to a test value.

• len
  • the test value that gets compared with the length.
• returns
  • A value x where

    x <  0       if this.length <  len
    x == 0       if this.length == len
    x >  0       if this.length >  len
    

The method as implemented here does not call `length` directly; its running
time is `O(length min len)` instead of `O(length)` . The method should be
overwritten if computing `length` is cheap.

• Definition Classes
  • LinearSeqOptimized → SeqLike

(defined at scala.collection.LinearSeqOptimized)

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

Applies a binary operator to all elements of this sequence, going right to left.

Note: will not terminate for infinite-sized collections.

• B
  • the result type of the binary operator.
• op
  • the binary operator.
• returns
  • the result of inserting op between consecutive elements of this sequence, going right to left:

    op(x_1, op(x_2, ..., op(x_{n-1}, x_n)...))
    

where `x1, ..., xn` are the elements of this sequence.

• Definition Classes
  • LinearSeqOptimized → IterableLike → TraversableOnce → GenTraversableOnce
• Exceptions thrown
  • UnsupportedOperationException if this sequence is empty.

(defined at scala.collection.LinearSeqOptimized)

def sameElements[B >: A](that: GenIterable[B]): Boolean

[use case]

Checks if the other iterable collection contains the same elements in the same order as this stream.

Note: will not terminate for infinite-sized collections.

• that
  • the collection to compare with.
• returns
  • true , if both collections contain the same elements in the same order, false otherwise.
• Definition Classes
  • LinearSeqOptimized → IterableLike → GenIterableLike

(defined at scala.collection.LinearSeqOptimized)

def segmentLength(p: (A) ⇒ Boolean, from: Int): Int

Computes length of longest segment whose elements all satisfy some predicate.

Note: may not terminate for infinite-sized collections.

• p
  • the predicate used to test elements.
• from
  • the index where the search starts.
• returns
  • the length of the longest segment of this sequence starting from index from such that every element of the segment satisfies the predicate p .
• Definition Classes
  • LinearSeqOptimized → SeqLike → GenSeqLike

(defined at scala.collection.LinearSeqOptimized)

def span(p: (A) ⇒ Boolean): (Stream[A], Stream[A])

Splits this sequence into a prefix/suffix pair according to a predicate.

Note: c span p is equivalent to (but possibly more efficient than) (c takeWhile p, c dropWhile p) , provided the evaluation of the predicate p does not cause any side-effects.

• returns
  • a pair consisting of the longest prefix of this sequence whose elements all satisfy p , and the rest of this sequence.
• Definition Classes
  • LinearSeqOptimized → TraversableLike → GenTraversableLike

(defined at scala.collection.LinearSeqOptimized)

Concrete Value Members From scala.collection.Parallelizable

def par: ParSeq[A]

Returns a parallel implementation of this collection.

For most collection types, this method creates a new parallel collection by copying all the elements. For these collection, par takes linear time. Mutable collections in this category do not produce a mutable parallel collection that has the same underlying dataset, so changes in one collection will not be reflected in the other one.

Specific collections (e.g. ParArray or mutable.ParHashMap ) override this default behaviour by creating a parallel collection which shares the same underlying dataset. For these collections, par takes constant or sublinear time.

All parallel collections return a reference to themselves.

• returns
  • a parallel implementation of this collection
• Definition Classes
  • Parallelizable

(defined at scala.collection.Parallelizable)

Concrete Value Members From scala.collection.SeqLike

def :+[B >: A, That](elem: B)(implicit bf: CanBuildFrom[Stream[A], B, That]): That

[use case]

A copy of this stream with an element appended.

A mnemonic for +: vs. :+ is: the COLon goes on the COLlection side.

Note: will not terminate for infinite-sized collections.

Example:

scala> val a = List(1)
a: List[Int] = List(1)

scala> val b = a :+ 2
b: List[Int] = List(1, 2)

scala> println(a)
List(1)

• elem
  • the appended element
• returns
  • a new stream consisting of all elements of this stream followed by elem .
• Definition Classes
  • SeqLike → GenSeqLike

(defined at scala.collection.SeqLike)

def combinations(n: Int): Iterator[Stream[A]]

Iterates over combinations. A combination of length n is a subsequence of the original sequence, with the elements taken in order. Thus, "xy" and "yy" are both length-2 combinations of "xyy" , but "yx" is not. If there is more than one way to generate the same subsequence, only one will be returned.

For example, "xyyy" has three different ways to generate "xy" depending on whether the first, second, or third "y" is selected. However, since all are identical, only one will be chosen. Which of the three will be taken is an implementation detail that is not defined.

• returns
  • An Iterator which traverses the possible n-element combinations of this sequence.
• Definition Classes
  • SeqLike

Example:

"abbbc".combinations(2) = Iterator(ab, ac, bb, bc)

(defined at scala.collection.SeqLike)

def containsSlice[B](that: GenSeq[B]): Boolean

Tests whether this sequence contains a given sequence as a slice.

Note: may not terminate for infinite-sized collections.

• that
  • the sequence to test
• returns
  • true if this sequence contains a slice with the same elements as that , otherwise false .
• Definition Classes
  • SeqLike

(defined at scala.collection.SeqLike)

def diff[B >: A](that: GenSeq[B]): Stream[A]

[use case]

Computes the multiset difference between this stream and another sequence.

Note: will not terminate for infinite-sized collections.

• that
  • the sequence of elements to remove
• returns
  • a new stream which contains all elements of this stream except some of occurrences of elements that also appear in that . If an element value x appears n times in that , then the first n occurrences of x will not form part of the result, but any following occurrences will.
• Definition Classes
  • SeqLike → GenSeqLike

(defined at scala.collection.SeqLike)

def endsWith[B](that: GenSeq[B]): Boolean

Tests whether this sequence ends with the given sequence.

Note: will not terminate for infinite-sized collections.

• that
  • the sequence to test
• returns
  • true if this sequence has that as a suffix, false otherwise.
• Definition Classes
  • SeqLike → GenSeqLike

(defined at scala.collection.SeqLike)

def indexOfSlice[B >: A](that: GenSeq[B]): Int

Finds first index where this sequence contains a given sequence as a slice.

Note: may not terminate for infinite-sized collections.

• that
  • the sequence to test
• returns
  • the first index such that the elements of this sequence starting at this index match the elements of sequence that , or -1 of no such subsequence exists.
• Definition Classes
  • SeqLike

(defined at scala.collection.SeqLike)

def indexOfSlice[B >: A](that: GenSeq[B], from: Int): Int

Finds first index after or at a start index where this sequence contains a given sequence as a slice.

Note: may not terminate for infinite-sized collections.

• that
  • the sequence to test
• from
  • the start index
• returns
  • the first index >= from such that the elements of this sequence starting at this index match the elements of sequence that , or -1 of no such subsequence exists.
• Definition Classes
  • SeqLike

(defined at scala.collection.SeqLike)

def indices: Range

Produces the range of all indices of this sequence.

• returns
  • a Range value from 0 to one less than the length of this sequence.
• Definition Classes
  • SeqLike

(defined at scala.collection.SeqLike)

def intersect[B >: A](that: GenSeq[B]): Stream[A]

[use case]

Computes the multiset intersection between this stream and another sequence.

Note: may not terminate for infinite-sized collections.

• that
  • the sequence of elements to intersect with.
• returns
  • a new stream which contains all elements of this stream which also appear in that . If an element value x appears n times in that , then the first n occurrences of x will be retained in the result, but any following occurrences will be omitted.
• Definition Classes
  • SeqLike → GenSeqLike

(defined at scala.collection.SeqLike)

def lastIndexOfSlice[B >: A](that: GenSeq[B]): Int

Finds last index where this sequence contains a given sequence as a slice.

Note: will not terminate for infinite-sized collections.

• that
  • the sequence to test
• returns
  • the last index such that the elements of this sequence starting a this index match the elements of sequence that , or -1 of no such subsequence exists.
• Definition Classes
  • SeqLike

(defined at scala.collection.SeqLike)

def lastIndexOfSlice[B >: A](that: GenSeq[B], end: Int): Int

Finds last index before or at a given end index where this sequence contains a given sequence as a slice.

• that
  • the sequence to test
• end
  • the end index
• returns
  • the last index <= end such that the elements of this sequence starting at this index match the elements of sequence that , or -1 of no such subsequence exists.
• Definition Classes
  • SeqLike

(defined at scala.collection.SeqLike)

def patch[B >: A, That](from: Int, patch: GenSeq[B], replaced: Int)(implicit bf: CanBuildFrom[Stream[A], B, That]): That

[use case]

Produces a new stream where a slice of elements in this stream is replaced by another sequence.

• from
  • the index of the first replaced element
• replaced
  • the number of elements to drop in the original stream
• returns
  • a new stream consisting of all elements of this stream except that replaced elements starting from from are replaced by patch .
• Definition Classes
  • SeqLike → GenSeqLike

(defined at scala.collection.SeqLike)

def permutations: Iterator[Stream[A]]

Iterates over distinct permutations.

• returns
  • An Iterator which traverses the distinct permutations of this sequence.
• Definition Classes
  • SeqLike

Example:

"abb".permutations = Iterator(abb, bab, bba)

(defined at scala.collection.SeqLike)

def reverseMap[B, That](f: (A) ⇒ B)(implicit bf: CanBuildFrom[Stream[A], B, That]): That

[use case]

Builds a new collection by applying a function to all elements of this stream and collecting the results in reversed order.

Note: will not terminate for infinite-sized collections.

Note: xs.reverseMap(f) is the same as xs.reverse.map(f) but might be more efficient.

• B
  • the element type of the returned collection.
• f
  • the function to apply to each element.
• returns
  • a new stream resulting from applying the given function f to each element of this stream and collecting the results in reversed order.
• Definition Classes
  • SeqLike → GenSeqLike

(defined at scala.collection.SeqLike)

def sortBy[B](f: (A) ⇒ B)(implicit ord: math.Ordering[B]): Stream[A]

Sorts this Seq according to the Ordering which results from transforming an implicitly given Ordering with a transformation function.

• B
  • the target type of the transformation f , and the type where the ordering ord is defined.
• f
  • the transformation function mapping elements to some other domain B .
• ord
  • the ordering assumed on domain B .
• returns
  • a sequence consisting of the elements of this sequence sorted according to the ordering where x < y if ord.lt(f(x), f(y)) .
• Definition Classes
  • SeqLike
• See also
  • scala.math.Ordering Note: will not terminate for infinite-sized collections.

Example:

val words = "The quick brown fox jumped over the lazy dog".split(' ')
// this works because scala.Ordering will implicitly provide an Ordering[Tuple2[Int, Char]]
words.sortBy(x => (x.length, x.head))
res0: Array[String] = Array(The, dog, fox, the, lazy, over, brown, quick, jumped)

(defined at scala.collection.SeqLike)

def sortWith(lt: (A, A) ⇒ Boolean): Stream[A]

Sorts this sequence according to a comparison function.

Note: will not terminate for infinite-sized collections.

The sort is stable. That is, elements that are equal (as determined by lt ) appear in the same order in the sorted sequence as in the original.

• lt
  • the comparison function which tests whether its first argument precedes its second argument in the desired ordering.
• returns
  • a sequence consisting of the elements of this sequence sorted according to the comparison function lt .
• Definition Classes
  • SeqLike

Example:

List("Steve", "Tom", "John", "Bob").sortWith(_.compareTo(_) < 0) =
List("Bob", "John", "Steve", "Tom")

(defined at scala.collection.SeqLike)

def sorted[B >: A](implicit ord: math.Ordering[B]): Stream[A]

Sorts this sequence according to an Ordering.

The sort is stable. That is, elements that are equal (as determined by lt ) appear in the same order in the sorted sequence as in the original.

• ord
  • the ordering to be used to compare elements.
• returns
  • a sequence consisting of the elements of this sequence sorted according to the ordering ord .
• Definition Classes
  • SeqLike
• See also
  • scala.math.Ordering

(defined at scala.collection.SeqLike)

def startsWith[B](that: GenSeq[B], offset: Int): Boolean

Tests whether this sequence contains the given sequence at a given index.

Note : If the both the receiver object this and the argument that are infinite sequences this method may not terminate.

• that
  • the sequence to test
• offset
  • the index where the sequence is searched.
• returns
  • true if the sequence that is contained in this sequence at index offset , otherwise false .
• Definition Classes
  • SeqLike → GenSeqLike

(defined at scala.collection.SeqLike)

def union[B >: A, That](that: GenSeq[B])(implicit bf: CanBuildFrom[Stream[A], B, That]): That

[use case]

Produces a new sequence which contains all elements of this stream and also all elements of a given sequence. xs union ys is equivalent to xs ++ ys .

Another way to express this is that xs union ys computes the order-preserving multi-set union of xs and ys . union is hence a counter-part of diff and intersect which also work on multi-sets.

Note: will not terminate for infinite-sized collections.

• that
  • the sequence to add.
• returns
  • a new stream which contains all elements of this stream followed by all elements of that .
• Definition Classes
  • SeqLike → GenSeqLike

(defined at scala.collection.SeqLike)

def updated[B >: A, That](index: Int, elem: B)(implicit bf: CanBuildFrom[Stream[A], B, That]): That

[use case]

A copy of this stream with one single replaced element.

• index
  • the position of the replacement
• elem
  • the replacing element
• returns
  • a copy of this stream with the element at position index replaced by elem .
• Definition Classes
  • SeqLike → GenSeqLike

(defined at scala.collection.SeqLike)

def view(from: Int, until: Int): SeqView[A, Stream[A]]

Creates a non-strict view of a slice of this sequence.

Note: the difference between view and slice is that view produces a view of the current sequence, whereas slice produces a new sequence.

Note: view(from, to) is equivalent to view.slice(from, to)

• from
  • the index of the first element of the view
• until
  • the index of the element following the view
• returns
  • a non-strict view of a slice of this sequence, starting at index from and extending up to (but not including) index until .
• Definition Classes
  • SeqLike → IterableLike → TraversableLike

(defined at scala.collection.SeqLike)

Concrete Value Members From scala.collection.TraversableLike

def ++:[B >: A, That](that: collection.Traversable[B])(implicit bf: CanBuildFrom[Stream[A], B, That]): That

As with ++ , returns a new collection containing the elements from the left operand followed by the elements from the right operand.

It differs from ++ in that the right operand determines the type of the resulting collection rather than the left one. Mnemonic: the COLon is on the side of the new COLlection type.

Example:

scala> val x = List(1)
x: List[Int] = List(1)

scala> val y = LinkedList(2)
y: scala.collection.mutable.LinkedList[Int] = LinkedList(2)

scala> val z = x ++: y
z: scala.collection.mutable.LinkedList[Int] = LinkedList(1, 2)

This overload exists because: for the implementation of ++: we should reuse that of ++ because many collections override it with more efficient versions.

Since TraversableOnce has no ++ method, we have to implement that directly, but Traversable and down can use the overload.

• B
  • the element type of the returned collection.
• That
  • the class of the returned collection. Where possible, That is the same class as the current collection class Repr , but this depends on the element type B being admissible for that class, which means that an implicit instance of type CanBuildFrom[Repr, B, That] is found.
• that
  • the traversable to append.
• bf
  • an implicit value of class CanBuildFrom which determines the result class That from the current representation type Repr and and the new element type B .
• returns
  • a new collection of type That which contains all elements of this traversable collection followed by all elements of that .
• Definition Classes
  • TraversableLike

(defined at scala.collection.TraversableLike)

def ++:[B >: A, That](that: TraversableOnce[B])(implicit bf: CanBuildFrom[Stream[A], B, That]): That

[use case]

As with ++ , returns a new collection containing the elements from the left operand followed by the elements from the right operand.

It differs from ++ in that the right operand determines the type of the resulting collection rather than the left one. Mnemonic: the COLon is on the side of the new COLlection type.

Example:

scala> val x = List(1)
x: List[Int] = List(1)

scala> val y = LinkedList(2)
y: scala.collection.mutable.LinkedList[Int] = LinkedList(2)

scala> val z = x ++: y
z: scala.collection.mutable.LinkedList[Int] = LinkedList(1, 2)

• B
  • the element type of the returned collection.
• that
  • the traversable to append.
• returns
  • a new stream which contains all elements of this stream followed by all elements of that .
• Definition Classes
  • TraversableLike

(defined at scala.collection.TraversableLike)

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

Selects all elements of this traversable collection which satisfy a predicate.

• p
  • the predicate used to test elements.
• returns
  • a new traversable collection consisting of all elements of this traversable collection that satisfy the given predicate p . The order of the elements is preserved.
• Definition Classes
  • TraversableLike → GenTraversableLike

(defined at scala.collection.TraversableLike)

def filterNot(p: (A) ⇒ Boolean): Stream[A]

Selects all elements of this traversable collection which do not satisfy a predicate.

• p
  • the predicate used to test elements.
• returns
  • a new traversable collection consisting of all elements of this traversable collection that do not satisfy the given predicate p . The order of the elements is preserved.
• Definition Classes
  • TraversableLike → GenTraversableLike

(defined at scala.collection.TraversableLike)

def groupBy[K](f: (A) ⇒ K): Map[K, Stream[A]]

Partitions this traversable collection into a map of traversable collections according to some discriminator function.

Note: this method is not re-implemented by views. This means when applied to a view it will always force the view and return a new traversable collection.

• K
  • the type of keys returned by the discriminator function.
• f
  • the discriminator function.
• returns
  • A map from keys to traversable collections such that the following invariant holds:

    (xs groupBy f)(k) = xs filter (x => f(x) == k)
    

That is, every key `k` is bound to a traversable collection of those
elements `x` for which `f(x)` equals `k` .

• Definition Classes
  • TraversableLike → GenTraversableLike

(defined at scala.collection.TraversableLike)

def inits: Iterator[Stream[A]]

Iterates over the inits of this traversable collection. The first value will be this traversable collection and the final one will be an empty traversable collection, with the intervening values the results of successive applications of init .

• returns
  • an iterator over all the inits of this traversable collection
• Definition Classes
  • TraversableLike

Example:

List(1,2,3).inits = Iterator(List(1,2,3), List(1,2), List(1), Nil)

(defined at scala.collection.TraversableLike)

def repr: Stream[A]

The collection of type traversable collection underlying this TraversableLike object. By default this is implemented as the TraversableLike object itself, but this can be overridden.

• Definition Classes
  • TraversableLike → GenTraversableLike

(defined at scala.collection.TraversableLike)

def scanRight[B, That](z: B)(op: (A, B) ⇒ B)(implicit bf: CanBuildFrom[Stream[A], B, That]): That

Produces a collection containing cumulative results of applying the operator going right to left. The head of the collection is the last cumulative result.

Note: will not terminate for infinite-sized collections.

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

Example:

List(1, 2, 3, 4).scanRight(0)(_ + _) == List(10, 9, 7, 4, 0)

• B
  • the type of the elements in the resulting collection
• That
  • the actual type of the resulting collection
• z
  • the initial value
• op
  • the binary operator applied to the intermediate result and the element
• bf
  • an implicit value of class CanBuildFrom which determines the result class That from the current representation type Repr and and the new element type B .
• returns
  • collection with intermediate results
• Definition Classes
  • TraversableLike → GenTraversableLike
• Annotations
  • @migration
• Migration
  • (Changed in version 2.9.0) The behavior of scanRight has changed. The previous behavior can be reproduced with scanRight.reverse.

(defined at scala.collection.TraversableLike)

def scan[B >: A, That](z: B)(op: (B, B) ⇒ B)(implicit cbf: CanBuildFrom[Stream[A], B, That]): That

Computes a prefix scan of the elements of the collection.

Note: The neutral element z may be applied more than once.

• B
  • element type of the resulting collection
• That
  • type of the resulting collection
• z
  • neutral element for the operator op
• op
  • the associative operator for the scan
• cbf
  • combiner factory which provides a combiner
• returns
  • a new traversable collection containing the prefix scan of the elements in this traversable collection
• Definition Classes
  • TraversableLike → GenTraversableLike

(defined at scala.collection.TraversableLike)

def tail: Stream[A]

Selects all elements except the first.

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

• returns
  • a traversable collection consisting of all elements of this traversable collection except the first one.
• Definition Classes
  • TraversableLike → GenTraversableLike
• Exceptions thrown
  • UnsupportedOperationException if the traversable collection is empty.

(defined at scala.collection.TraversableLike)

def tails: Iterator[Stream[A]]

Iterates over the tails of this traversable collection. The first value will be this traversable collection and the final one will be an empty traversable collection, with the intervening values the results of successive applications of tail .

• returns
  • an iterator over all the tails of this traversable collection
• Definition Classes
  • TraversableLike

Example:

List(1,2,3).tails = Iterator(List(1,2,3), List(2,3), List(3), Nil)

(defined at scala.collection.TraversableLike)

def toTraversable: collection.Traversable[A]

Converts this traversable collection to an unspecified Traversable. Will return the same collection if this instance is already Traversable.

Note: will not terminate for infinite-sized collections.

• returns
  • a Traversable containing all elements of this traversable collection.
• Definition Classes
  • TraversableLike → TraversableOnce → GenTraversableOnce
• Annotations
  • @ deprecatedOverriding (message =…, since = “2.11.0”)

(defined at scala.collection.TraversableLike)

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.
• Definition Classes
  • TraversableOnce

(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.
• Definition Classes
  • TraversableOnce

(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.
• Definition Classes
  • TraversableOnce

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 stream to an array. Fills the given array xs with values of this stream. Copying will stop once either the end of the current stream 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 stream to an array. Fills the given array xs with values of this stream, beginning at index start . Copying will stop once either the end of the current stream 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)

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.
• Definition Classes
  • TraversableOnce

(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 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)

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 stream 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 stream with the smallest value measured by function f.
• Definition Classes
  • TraversableOnce → GenTraversableOnce

(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 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 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 reversed: scala.List[A]

• Attributes
  • protected[this]
• Definition Classes
  • TraversableOnce

(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: 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 toList: scala.List[A]

Converts this traversable or iterator to a list.

Note: will not terminate for infinite-sized collections.

• returns
  • a list 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)]): Map[T, U]

[use case]

Converts this stream 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 stream.
• Definition Classes
  • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def toSet[B >: A]: 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)

def toVector: scala.Vector[A]

Converts this traversable or iterator to a Vector.

Note: will not terminate for infinite-sized collections.

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

(defined at scala.collection.TraversableOnce)

Concrete Value Members From scala.collection.generic.GenericTraversableTemplate ——————————————————————————–

def genericBuilder[B]: Builder[B, Stream[B]]

The generic builder that builds instances of Traversable at arbitrary element types.

• Definition Classes
  • GenericTraversableTemplate

(defined at scala.collection.generic.GenericTraversableTemplate)

def newBuilder: Builder[A, Stream[A]]

The builder that builds instances of type Traversable[A]

• Attributes
  • protected[this]
• Definition Classes
  • GenericTraversableTemplate → HasNewBuilder

(defined at scala.collection.generic.GenericTraversableTemplate)

def transpose[B](implicit asTraversable: (A) ⇒ GenTraversableOnce[B]): Stream[Stream[B]]

Transposes this collection of traversable collections into a collection of collections.

The resulting collection’s type will be guided by the static type of collection. For example:

val xs = List(
           Set(1, 2, 3),
           Set(4, 5, 6)).transpose
// xs == List(
//         List(1, 4),
//         List(2, 5),
//         List(3, 6))

val ys = Vector(
           List(1, 2, 3),
           List(4, 5, 6)).transpose
// ys == Vector(
//         Vector(1, 4),
//         Vector(2, 5),
//         Vector(3, 6))

• B
  • the type of the elements of each traversable collection.
• asTraversable
  • an implicit conversion which asserts that the element type of this collection is a Traversable .
• returns
  • a two-dimensional collection of collections which has as n th row the n th column of this collection.
• Definition Classes
  • GenericTraversableTemplate
• Annotations
  • @migration
• Migration
  • (Changed in version 2.9.0) transpose throws an IllegalArgumentException if collections are not uniformly sized.
• Exceptions thrown
  • IllegalArgumentException if all collections in this collection are not of the same size.

(defined at scala.collection.generic.GenericTraversableTemplate)

def unzip3[A1, A2, A3](implicit asTriple: (A) ⇒ (A1, A2, A3)): (Stream[A1], Stream[A2], Stream[A3])

Converts this collection of triples into three collections of the first, second, and third element of each triple.

val xs = Traversable(
           (1, "one", '1'),
           (2, "two", '2'),
           (3, "three", '3')).unzip3
// xs == (Traversable(1, 2, 3),
//        Traversable(one, two, three),
//        Traversable(1, 2, 3))

• A1
  • the type of the first member of the element triples
• A2
  • the type of the second member of the element triples
• A3
  • the type of the third member of the element triples
• asTriple
  • an implicit conversion which asserts that the element type of this collection is a triple.
• returns
  • a triple of collections, containing the first, second, respectively third member of each element triple of this collection.
• Definition Classes
  • GenericTraversableTemplate

(defined at scala.collection.generic.GenericTraversableTemplate)

def unzip[A1, A2](implicit asPair: (A) ⇒ (A1, A2)): (Stream[A1], Stream[A2])

Converts this collection of pairs into two collections of the first and second half of each pair.

val xs = Traversable(
           (1, "one"),
           (2, "two"),
           (3, "three")).unzip
// xs == (Traversable(1, 2, 3),
//        Traversable(one, two, three))

• A1
  • the type of the first half of the element pairs
• A2
  • the type of the second half of the element pairs
• asPair
  • an implicit conversion which asserts that the element type of this collection is a pair.
• returns
  • a pair of collections, containing the first, respectively second half of each element pair of this collection.
• Definition Classes
  • GenericTraversableTemplate

(defined at scala.collection.generic.GenericTraversableTemplate)

Concrete Value Members From scala.collection.immutable.LinearSeq

def seq: LinearSeq[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
  • LinearSeq → LinearSeq → LinearSeqLike → Seq → Seq → GenSeq → GenSeqLike → Iterable → Iterable → GenIterable → Traversable → Traversable → GenTraversable → Parallelizable → TraversableOnce → GenTraversableOnce

(defined at scala.collection.immutable.LinearSeq)

Concrete Value Members From scala.collection.immutable.Seq

def parCombiner: Combiner[A, ParSeq[A]]

The default par implementation uses the combiner provided by this method to create a new parallel collection.

• returns
  • a combiner for the parallel collection of type ParRepr
• Attributes
  • protected[this]
• Definition Classes
  • Seq → SeqLike → Iterable → TraversableLike → Parallelizable

(defined at scala.collection.immutable.Seq)

def toSeq: Seq[A]

Converts this immutable sequence to a sequence.

Note: will not terminate for infinite-sized collections.

A new collection will not be built; in particular, lazy sequences will stay lazy.

• returns
  • a sequence containing all elements of this immutable sequence.
• Definition Classes
  • Seq → SeqLike → GenSeqLike → TraversableOnce → GenTraversableOnce

(defined at scala.collection.immutable.Seq)

Concrete Value Members From scala.collection.immutable.Stream

abstract def tailDefined: Boolean

Is the tail of this stream defined?

• Attributes
  • protected

(defined at scala.collection.immutable.Stream)

def ++[B >: A, That](that: GenTraversableOnce[B])(implicit bf: CanBuildFrom[Stream[A], B, That]): That

Create a new stream which contains all elements of this stream followed by all elements of Traversable that .

• B
  • The element type of the returned collection. That
• That
  • the class of the returned collection. Where possible, That is the same class as the current collection class Repr , but this depends on the element type B being admissible for that class, which means that an implicit instance of type CanBuildFrom[Repr, B, That] is found.
• that
  • The scala.collection.GenTraversableOnce the be concatenated to this Stream .
• bf
  • an implicit value of class CanBuildFrom which determines the result class That from the current representation type Repr and the new element type B .
• returns
  • A new collection containing the result of concatenating this with that .
• Definition Classes
  • Stream → TraversableLike → GenTraversableLike
• Note
  • This method doesn’t cause the Stream to be fully realized but it should be noted that using the ++ operator from another collection type could cause infinite realization of a Stream . For example, referring to the definition of fibs in the preamble, the following would never return: List(BigInt(12)) ++ fibs ., It’s subtle why this works. We know that if the target type of the scala.collection.mutable.Builder That is either a Stream , or one of its supertypes, or undefined, then StreamBuilder will be chosen for the implicit. We recognize that fact and optimize to get more laziness.

(defined at scala.collection.immutable.Stream)

def +:[B >: A, That](elem: B)(implicit bf: CanBuildFrom[Stream[A], B, That]): That

[use case]

A copy of the stream with an element prepended.

Note that :-ending operators are right associative (see example). A mnemonic for +: vs. :+ is: the COLon goes on the COLlection side.

Also, the original stream is not modified, so you will want to capture the result.

Example:

scala> val x = List(1)
x: List[Int] = List(1)

scala> val y = 2 +: x
y: List[Int] = List(2, 1)

scala> println(x)
List(1)

• elem
  • the prepended element
• returns
  • a new stream consisting of elem followed by all elements of this stream.
• Definition Classes
  • Stream → SeqLike → GenSeqLike

(defined at scala.collection.immutable.Stream)

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

Write all defined elements of this iterable into given string builder. The written text begins with the string start and is finished by the string end . Inside, the string representations of defined elements (w.r.t. the method toString() ) are separated by the string sep . The method will not force evaluation of undefined elements. A tail of such elements will be represented by a "?" instead. A cyclic stream is represented by a "..." at the point where the cycle repeats.

• b
  • The collection.mutable.StringBuilder factory to which we need to add the string elements.
• start
  • The prefix of the resulting string (e.g. “Stream(“)
• sep
  • The separator between elements of the resulting string (e.g. “,”)
• end
  • The end of the resulting string (e.g. “)”)
• returns
  • The original collection.mutable.StringBuilder containing the resulting string.
• Definition Classes
  • Stream → TraversableOnce

(defined at scala.collection.immutable.Stream)

def append[B >: A](rest: ⇒ TraversableOnce[B]): Stream[B]

The stream resulting from the concatenation of this stream with the argument stream.

• rest
  • The stream that gets appended to this stream
• returns
  • The stream containing elements of this stream and the traversable object.

(defined at scala.collection.immutable.Stream)

final def collect[B, That](pf: PartialFunction[A, B])(implicit bf: CanBuildFrom[Stream[A], B, That]): That

[use case]

Builds a new collection by applying a partial function to all elements of this stream on which the function is defined.

• B
  • the element type of the returned collection.
• pf
  • the partial function which filters and maps the stream.
• returns
  • a new stream resulting from applying the given partial function pf to each element on which it is defined and collecting the results. The order of the elements is preserved.
• Definition Classes
  • Stream → TraversableLike → GenTraversableLike

(defined at scala.collection.immutable.Stream)

def companion: GenericCompanion[Stream]

The factory companion object that builds instances of class Stream . (or its Iterable superclass where class Stream is not a Seq .)

• Definition Classes
  • Stream → LinearSeq → LinearSeq → Seq → Iterable → Traversable → Seq → GenSeq → Iterable → GenIterable → Traversable → GenTraversable → GenericTraversableTemplate

(defined at scala.collection.immutable.Stream)

def distinct: Stream[A]

Builds a new stream from this stream in which any duplicates (as determined by == ) have been removed. Among duplicate elements, only the first one is retained in the resulting Stream .

• returns
  • A new Stream representing the result of applying distinctness to the original Stream .
• Definition Classes
  • Stream → SeqLike → GenSeqLike

Example:

// Creates a Stream where every element is duplicated
def naturalsFrom(i: Int): Stream[Int] = i #:: { i #:: naturalsFrom(i + 1) }
naturalsFrom(1) take 6 mkString ", "
// produces: "1, 1, 2, 2, 3, 3"
(naturalsFrom(1) distinct) take 6 mkString ", "
// produces: "1, 2, 3, 4, 5, 6"

(defined at scala.collection.immutable.Stream)

final def drop(n: Int): Stream[A]

Selects all elements except first n ones.

• n
  • the number of elements to drop from this stream.
• returns
  • a stream consisting of all elements of this stream except the first n ones, or else the empty stream, if this stream has less than n elements.
• Definition Classes
  • Stream → LinearSeqOptimized → IterableLike → TraversableLike → GenTraversableLike
• Annotations
  • @ tailrec ()

(defined at scala.collection.immutable.Stream)

def dropRight(n: Int): Stream[A]

Selects all elements except last n ones.

Note: lazily evaluated; will terminate for infinite-sized collections.

• n
  • The number of elements to take
• returns
  • a stream consisting of all elements of this stream except the last n ones, or else the empty stream, if this stream has less than n elements.
• Definition Classes
  • Stream → LinearSeqOptimized → IterableLike

(defined at scala.collection.immutable.Stream)

def dropWhile(p: (A) ⇒ Boolean): Stream[A]

Returns the a Stream representing the longest suffix of this iterable whose first element does not satisfy the predicate p .

• p
  • the test predicate.
• returns
  • A new Stream representing the results of applying p to the original Stream .
• Definition Classes
  • Stream → TraversableLike → GenTraversableLike
• Note
  • This method realizes the entire Stream beyond the truth value of the predicate p .

Example:

// Assume we have a Stream that takes the first 20 natural numbers
def naturalsLt50(i: Int): Stream[Int] = i #:: { if (i < 20) naturalsLt50(i * + 1) else Stream.Empty }
naturalsLt50(0) dropWhile { _ < 10 }
// produces: "10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20"

(defined at scala.collection.immutable.Stream)

final def flatMap[B, That](f: (A) ⇒ GenTraversableOnce[B])(implicit bf: CanBuildFrom[Stream[A], B, That]): That

Applies the given function f to each element of this stream, then concatenates the results. As with map this function does not need to realize the entire Stream but continues to keep it as a lazy Stream .

• B
  • The element type of the returned collection That .
• That
  • the class of the returned collection. Where possible, That is the same class as the current collection class Repr , but this depends on the element type B being admissible for that class, which means that an implicit instance of type CanBuildFrom[Repr, B, That] is found.
• f
  • the function to apply on each element.
• bf
  • an implicit value of class CanBuildFrom which determines the result class That from the current representation type Repr and the new element type B .
• returns
  • f(a0) ::: ... ::: f(an) if this stream is [a0, ..., an] .
• Definition Classes
  • Stream → TraversableLike → GenTraversableLike → FilterMonadic

Example:

flatMap

(defined at scala.collection.immutable.Stream)

def flatten[B](implicit asTraversable: (A) ⇒ GenTraversableOnce[B]): Stream[B]

Evaluates and concatenates all elements within the Stream into a new flattened Stream .

• B
  • The type of the elements of the resulting Stream .
• asTraversable
  • an implicit conversion which asserts that the element type of this stream is a GenTraversable .
• returns
  • A new Stream of type B of the flattened elements of this Stream .
• Definition Classes
  • Stream → GenericTraversableTemplate

Example:

val sov: Stream[Vector[Int]] = Vector(0) #:: Vector(0, 0) #:: sov.zip(sov.tail).map { n => n._1 ++ n._2 }
sov.flatten take 10 mkString ", "
// produces: "0, 0, 0, 0, 0, 0, 0, 0, 0, 0"

(defined at scala.collection.immutable.Stream)

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

Stream specialization of foldLeft which allows GC to collect along the way.

• B
  • The type of value being accumulated.
• z
  • The initial value seeded into the function op .
• op
  • The operation to perform on successive elements of the Stream .
• returns
  • The accumulated value from successive applications of op .
• Definition Classes
  • Stream → LinearSeqOptimized → TraversableOnce → GenTraversableOnce
• Annotations
  • @ tailrec ()

(defined at scala.collection.immutable.Stream)

def force: Stream[A]

Forces evaluation of the whole stream and returns it.

• returns
  • The fully realized Stream .
• Note
  • Often we use Stream s to represent an infinite set or series. If that’s the case for your particular Stream then this function will never return and will probably crash the VM with an OutOfMemory exception. This function will not hang on a finite cycle, however.

(defined at scala.collection.immutable.Stream)

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

Apply the given function f to each element of this linear sequence (while respecting the order of the elements).

• U
  • the type parameter describing the result of function f . This result will always be ignored. Typically U is Unit , but this is not necessary.
• f
  • The treatment to apply to each element.
• Definition Classes
  • Stream → LinearSeqOptimized → IterableLike → GenericTraversableTemplate → TraversableLike → GenTraversableLike → TraversableOnce → GenTraversableOnce → FilterMonadic
• Annotations
  • @ tailrec ()
• Note
  • This function will force the realization of the entire stream unless the f throws an exception., Overridden here as final to trigger tail-call optimization, which replaces ‘this’ with ‘tail’ at each iteration. This is absolutely necessary for allowing the GC to collect the underlying stream as elements are consumed.

(defined at scala.collection.immutable.Stream)

def init: Stream[A]

The stream without its last element.

• returns
  • A new Stream containing everything but the last element. If your Stream represents an infinite series, this method will not return.
• Definition Classes
  • Stream → TraversableLike → GenTraversableLike
• Exceptions thrown
  • UnsupportedOperationException if the stream is empty.

(defined at scala.collection.immutable.Stream)

final def map[B, That](f: (A) ⇒ B)(implicit bf: CanBuildFrom[Stream[A], B, That]): That

Returns the stream resulting from applying the given function f to each element of this stream. This returns a lazy Stream such that it does not need to be fully realized.

• B
  • The element type of the returned collection That .
• That
  • the class of the returned collection. Where possible, That is the same class as the current collection class Repr , but this depends on the element type B being admissible for that class, which means that an implicit instance of type CanBuildFrom[Repr, B, That] is found.
• f
  • function to apply to each element.
• bf
  • an implicit value of class CanBuildFrom which determines the result class That from the current representation type Repr and the new element type B .
• returns
  • f(a0), ..., f(an) if this sequence is a0, ..., an .
• Definition Classes
  • Stream → TraversableLike → GenTraversableLike → FilterMonadic

Example:

def naturalsFrom(i: Int): Stream[Int] = i #:: naturalsFrom(i + 1)
naturalsFrom(1).map(_ + 10) take 5 mkString(", ")
// produces: "11, 12, 13, 14, 15"

(defined at scala.collection.immutable.Stream)

def mkString(sep: String): String

Displays all elements of this stream in a string using a separator string.

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

Example:

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

(defined at scala.collection.immutable.Stream)

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

Displays all elements of this stream 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 stream. 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 stream are separated by the string sep .
• Definition Classes
  • Stream → TraversableOnce → GenTraversableOnce

Example:

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

(defined at scala.collection.immutable.Stream)

def padTo[B >: A, That](len: Int, elem: B)(implicit bf: CanBuildFrom[Stream[A], B, That]): That

Returns a new sequence of given length containing the elements of this sequence followed by zero or more occurrences of given elements.

• B
  • The type of the value to pad with.
• That
  • The type contained within the resulting Stream .
• len
  • The number of elements to pad into the Stream .
• elem
  • The value of the type B to use for padding.
• bf
  • an implicit value of class CanBuildFrom which determines the result class That from the current representation type Repr and the new element type B .
• returns
  • A new Stream representing the collection with values padding off to the end. If your Stream represents an infinite series, this method will not return.
• Definition Classes
  • Stream → SeqLike → GenSeqLike

Example:

def naturalsFrom(i: Int): Stream[Int] = i #:: { if (i < 5) naturalsFrom(i + 1) else Stream.Empty }
naturalsFrom(1) padTo(10, 0) foreach println
// prints
// 1
// 2
// 3
// 4
// 5
// 0
// 0
// 0
// 0
// 0

(defined at scala.collection.immutable.Stream)

def partition(p: (A) ⇒ Boolean): (Stream[A], Stream[A])

Returns all the elements of this stream that satisfy the predicate p returning of scala.Tuple2 of Stream s obeying the partition predicate p . The order of the elements is preserved.

• p
  • the predicate used to filter the stream.
• returns
  • the elements of this stream satisfying p .
• Definition Classes
  • Stream → TraversableLike → GenTraversableLike

Example:

def naturalsFrom(i: Int): Stream[Int] = i #:: naturalsFrom(i + 1)
val parts = naturalsFrom(1) partition { _ % 2 == 0 }
parts._1 take 10 mkString ", "
// produces: "2, 4, 6, 8, 10, 12, 14, 16, 18, 20"
parts._2 take 10 mkString ", "
// produces: "1, 3, 5, 7, 9, 11, 13, 15, 17, 19"

(defined at scala.collection.immutable.Stream)

def print(sep: String): Unit

Prints elements of this stream one by one, separated by sep .

• sep
  • The separator string printed between consecutive elements.

(defined at scala.collection.immutable.Stream)

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

Stream specialization of reduceLeft which allows GC to collect along the way.

• B
  • The type of value being accumulated.
• f
  • The operation to perform on successive elements of the Stream .
• returns
  • The accumulated value from successive applications of f .
• Definition Classes
  • Stream → LinearSeqOptimized → TraversableOnce

(defined at scala.collection.immutable.Stream)

def reverse: Stream[A]

A list consisting of all elements of this list in reverse order.

• returns
  • A new Stream containing the representing of the original Stream in reverse order.
• Definition Classes
  • Stream → SeqLike → GenSeqLike
• Note
  • This function must realize the entire Stream in order to perform this operation so if your Stream represents an infinite sequence then this function will never return.

Example:

def naturalsFrom(i: Int): Stream[Int] = i #:: { if (i < 5) naturalsFrom(i + 1) else Stream.Empty }
(naturalsFrom(1) reverse) foreach println
// prints
// 5
// 4
// 3
// 2
// 1

(defined at scala.collection.immutable.Stream)

final def scanLeft[B, That](z: B)(op: (B, A) ⇒ B)(implicit bf: CanBuildFrom[Stream[A], B, That]): That

Create a new stream which contains all intermediate results of applying the operator to subsequent elements left to right. scanLeft is analogous to foldLeft .

• B
  • the type of the elements in the resulting collection
• That
  • the actual type of the resulting collection
• z
  • The initial value for the scan.
• op
  • A function that will apply operations to successive values in the Stream against previous accumulated results.
• bf
  • an implicit value of class CanBuildFrom which determines the result class That from the current representation type Repr and the new element type B .
• returns
  • A new collection containing the modifications from the application of op .
• Definition Classes
  • Stream → TraversableLike → GenTraversableLike
• Note
  • This works because the target type of the scala.collection.mutable.Builder That is a Stream .

(defined at scala.collection.immutable.Stream)

def slice(from: Int, until: Int): Stream[A]

A substream starting at index from and extending up to (but not including) index until . This returns a Stream that is lazily evaluated.

• from
  • The index of the first element of the returned subsequence
• until
  • The index of the element following the returned subsequence
• returns
  • A new string containing the elements requested from start until end .
• Definition Classes
  • Stream → LinearSeqOptimized → IterableLike → TraversableLike → GenTraversableLike

Example:

naturalsFrom(0) slice(50, 60) mkString ", "
// produces: "50, 51, 52, 53, 54, 55, 56, 57, 58, 59"

(defined at scala.collection.immutable.Stream)

def splitAt(n: Int): (Stream[A], Stream[A])

Splits this stream into two at a given position. Note: c splitAt n is equivalent to (but possibly more efficient than) (c take n, c drop n) .

• n
  • the position at which to split.
• returns
  • a pair of streams consisting of the first n elements of this stream, and the other elements.
• Definition Classes
  • Stream → TraversableLike → GenTraversableLike

(defined at scala.collection.immutable.Stream)

def take(n: Int): Stream[A]

Returns the n first elements of this Stream as another Stream , or else the whole Stream , if it has less than n elements.

The result of take is, again, a Stream meaning that it also does not make any needless evaluations of the Stream itself, delaying that until the usage of the resulting Stream .

• n
  • the number of elements to take.
• returns
  • the n first elements of this stream.
• Definition Classes
  • Stream → LinearSeqOptimized → IterableLike → TraversableLike → GenTraversableLike

Example:

def naturalsFrom(i: Int): Stream[Int] = i #:: naturalsFrom(i + 1)
scala> naturalsFrom(5) take 5
res1: scala.collection.immutable.Stream[Int] = Stream(5, ?)
scala> naturalsFrom(5) take 5 mkString ", "
// produces: "5, 6, 7, 8, 9"

(defined at scala.collection.immutable.Stream)

def takeRight(n: Int): Stream[A]

Returns the rightmost n elements from this iterable.

• n
  • the number of elements to take
• returns
  • The last n elements from this Stream .
• Definition Classes
  • Stream → IterableLike
• Note
  • Take serious caution here. If the Stream represents an infinite series then this function will not return . The right most elements of an infinite series takes an infinite amount of time to produce.

(defined at scala.collection.immutable.Stream)

def takeWhile(p: (A) ⇒ Boolean): Stream[A]

Returns the longest prefix of this Stream whose elements satisfy the predicate p .

• p
  • the test predicate.
• returns
  • A new Stream representing the values that satisfy the predicate p .
• Definition Classes
  • Stream → LinearSeqOptimized → IterableLike → TraversableLike → GenTraversableLike

Example:

+ naturalsFrom(0) takeWhile { _ < 5 } mkString ", "
produces: "0, 1, 2, 3, 4"

(defined at scala.collection.immutable.Stream)

def toStream: Stream[A]

Converts this stream to a stream.

• returns
  • a stream containing all elements of this stream.
• Definition Classes
  • Stream → IterableLike → TraversableLike → GenTraversableOnce

(defined at scala.collection.immutable.Stream)

def view: StreamView[A, Stream[A]]

Creates a non-strict view of this stream.

• returns
  • a non-strict view of this stream.
• Definition Classes
  • Stream → SeqLike → IterableLike → TraversableLike

(defined at scala.collection.immutable.Stream)

final def withFilter(p: (A) ⇒ Boolean): FilterMonadic[A, Stream[A]]

A FilterMonadic which allows GC of the head of stream during processing

• p
  • the predicate used to test elements.
• returns
  • an object of class WithFilter , which supports map , flatMap , foreach , and withFilter operations. All these operations apply to those elements of this stream which satisfy the predicate p .
• Definition Classes
  • Stream → TraversableLike → FilterMonadic
• Annotations
  • @ noinline ()

(defined at scala.collection.immutable.Stream)

def zipWithIndex[A1 >: A, That](implicit bf: CanBuildFrom[Stream[A], (A1, Int), That]): That

Zips this iterable with its indices. s.zipWithIndex is equivalent to s zip s.indices .

This method is much like zip in that it returns a single lazy Stream of scala.Tuple2.

• A1
  • The type of the first element of the scala.Tuple2 in the resulting stream.
• That
  • The type of the resulting Stream .
• bf
  • an implicit value of class CanBuildFrom which determines the result class That from the current representation type Repr and the new element type (A1, Int) .
• returns
  • Stream({a0,0}, ..., {an,n)}
• Definition Classes
  • Stream → IterableLike → GenIterableLike

Example:

def naturalsFrom(i: Int): Stream[Int] = i #:: naturalsFrom(i + 1)
(naturalsFrom(1) zipWithIndex) take 5 foreach println
// prints
// (1,0)
// (2,1)
// (3,2)
// (4,3)
// (5,4)

(defined at scala.collection.immutable.Stream)

final def zip[A1 >: A, B, That](that: GenIterable[B])(implicit bf: CanBuildFrom[Stream[A], (A1, B), That]): That

Returns a stream formed from this stream and the specified stream that by associating each element of the former with the element at the same position in the latter.

If one of the two streams is longer than the other, its remaining elements are ignored.

The return type of this function may not be obvious. The lazy aspect of the returned value is different than that of partition . In partition we get back a scala.Tuple2 of two lazy Stream s whereas here we get back a single lazy Stream of scala.Tuple2 s where the scala.Tuple2 ‘s type signature is (A1, B) .

• A1
  • The type of the first parameter of the zipped tuple
• B
  • The type of the second parameter of the zipped tuple
• That
  • The type of the returned Stream .
• that
  • The iterable providing the second half of each result pair
• bf
  • an implicit value of class CanBuildFrom which determines the result class That from the current representation type Repr and the new element type (A1, B) .
• returns
  • Stream({a0,b0}, ..., {amin(m,n),bmin(m,n))} when Stream(a0, ..., am) zip Stream(b0, ..., bn) is invoked.
• Definition Classes
  • Stream → IterableLike → GenIterableLike

Example:

def naturalsFrom(i: Int): Stream[Int] = i #:: naturalsFrom(i + 1)
naturalsFrom(1) zip naturalsFrom(2) take 5 foreach println
// prints
// (1,2)
// (2,3)
// (3,4)
// (4,5)
// (5,6)

(defined at scala.collection.immutable.Stream)

Instance Constructors From scala.collection.immutable.Stream

new Stream()

(defined at scala.collection.immutable.Stream)

Concrete Value Members From Implicit scala.collection.immutable.Stream.consWrapper# ——————————————————————————–

def #::(hd: A): Stream[A]

Construct a stream consisting of a given first element followed by elements from a lazily evaluated Stream.

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

(added by implicit convertion: scala.collection.immutable.Stream.consWrapper#)

def #:::(prefix: Stream[A]): Stream[A]

Construct a stream consisting of the concatenation of the given stream and a lazily evaluated Stream.

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

(added by implicit convertion: scala.collection.immutable.Stream.consWrapper#)

Concrete Value Members From Implicit scala.collection.parallel.CollectionsHaveToParArray ——————————————————————————–

def toParArray: ParArray[T]

• Implicit information
  • This member is added by an implicit conversion from Stream [A] to CollectionsHaveToParArray [Stream [A], T] performed by method CollectionsHaveToParArray in scala.collection.parallel. This conversion will take place only if an implicit value of type (Stream [A]) ⇒ GenTraversableOnce [T] is in scope.
• Definition Classes
  • CollectionsHaveToParArray (added by implicit convertion: scala.collection.parallel.CollectionsHaveToParArray)

Full Source:

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

package scala
package collection
package immutable

import generic._
import mutable.{Builder, StringBuilder, LazyBuilder}
import scala.annotation.tailrec
import Stream.cons
import scala.language.implicitConversions

/** The class `Stream` implements lazy lists where elements
 *  are only evaluated when they are needed. Here is an example:
 *
 *  {{{
 *  import scala.math.BigInt
 *  object Main extends App {
 *
 *    val fibs: Stream[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
 *  }}}
 *
 *  The `Stream` class also employs memoization such that previously computed
 *  values are converted from `Stream` elements to concrete values of type `A`.
 *  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: Stream[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
 *  `Stream` requires us to have somewhere to store the information and a `val`
 *  allows us to do that.
 *
 *  - While the `Stream` 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 `Stream` 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 `Stream`) 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 `Stream`, and a stream 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 Stream definition will be a val definition
 *  val stream1: Stream[Int] = {
 *    def loop(v: Int): Stream[Int] = v #:: loop(v + 1)
 *    loop(0)
 *  }
 *
 *  // Because stream1 is a val, everything that the iterator produces is held
 *  // by virtue of the fact that the head of the Stream is held in stream1
 *  val it1 = stream1.iterator
 *  loop("Iterator1: ", it1.next, it1)
 *
 *  // We can redefine this Stream such that all we have is the Iterator left
 *  // and allow the Stream to be garbage collected as required.  Using a def
 *  // to provide the Stream ensures that no val is holding onto the head as
 *  // is the case with stream1
 *  def stream2: Stream[Int] = {
 *    def loop(v: Int): Stream[Int] = v #:: loop(v + 1)
 *    loop(0)
 *  }
 *  val it2 = stream2.iterator
 *  loop("Iterator2: ", it2.next, it2)
 *
 *  // And, of course, we don't actually need a Stream at all for such a simple
 *  // problem.  There's no reason to use a Stream 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, Stream(...))` 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...
 *  val sov: Stream[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: Stream[Int] = {
 *    def loop(h: Int, n: Int): Stream[Int] = h #:: loop(n, h + n)
 *    loop(1, 1)
 *  }
 *  }}}
 *
 *  Note that `mkString` forces evaluation of a `Stream`, but `addString` does
 *  not.  In both cases, a `Stream` that is or ends in a cycle
 *  (e.g. `lazy val s: Stream[Int] = 0 #:: s`) will convert additional trips
 *  through the cycle to `...`.  Additionally, `addString` will display an
 *  un-memoized tail as `?`.
 *
 *  @tparam A    the type of the elements contained in this stream.
 *
 *  @author Martin Odersky, Matthias Zenger
 *  @version 1.1 08/08/03
 *  @since   2.8
 *  @see [[http://docs.scala-lang.org/overviews/collections/concrete-immutable-collection-classes.html#streams "Scala's Collection Library overview"]]
 *  section on `Streams` for more information.

 *  @define naturalsEx def naturalsFrom(i: Int): Stream[Int] = i #:: naturalsFrom(i + 1)
 *  @define Coll `Stream`
 *  @define coll stream
 *  @define orderDependent
 *  @define orderDependentFold
 *  @define willTerminateInf Note: lazily evaluated; will terminate for infinite-sized collections.
 */
@deprecatedInheritance("This class will be sealed.", "2.11.0")
abstract class Stream[+A] extends AbstractSeq[A]
                             with LinearSeq[A]
                             with GenericTraversableTemplate[A, Stream]
                             with LinearSeqOptimized[A, Stream[A]]
                             with Serializable { self =>

  override def companion: GenericCompanion[Stream] = Stream

  /** Indicates whether or not the `Stream` is empty.
   *
   * @return `true` if the `Stream` is empty and `false` otherwise.
   */
  def isEmpty: Boolean

  /** Gives constant time access to the first element of this `Stream`.  Using
   * the `fibs` example from earlier:
   *
   * {{{
   * println(fibs head)
   * // prints
   * // 0
   * }}}
   *
   *  @return The first element of the `Stream`.
   *  @throws java.util.NoSuchElementException if the stream is empty.
   */
  def head: A

  /** A stream consisting of the remaining elements of this stream after the
   *  first one.
   *
   *  Note that this method does not force evaluation of the `Stream` but merely
   *  returns the lazy result.
   *
   *  @return The tail of the `Stream`.
   *  @throws UnsupportedOperationException if the stream is empty.
   */
  def tail: Stream[A]

  /** Is the tail of this stream defined? */
  protected def tailDefined: Boolean

  // Implementation of abstract method in Traversable

  // New methods in Stream

  /** The stream resulting from the concatenation of this stream with the argument stream.
   *  @param rest   The stream that gets appended to this stream
   *  @return       The stream containing elements of this stream and the traversable object.
   */
  def append[B >: A](rest: => TraversableOnce[B]): Stream[B] =
    if (isEmpty) rest.toStream else cons(head, tail append rest)

  /** Forces evaluation of the whole stream and returns it.
   *
   * @note Often we use `Stream`s to represent an infinite set or series.  If
   * that's the case for your particular `Stream` then this function will never
   * return and will probably crash the VM with an `OutOfMemory` exception.
   * This function will not hang on a finite cycle, however.
   *
   *  @return The fully realized `Stream`.
   */
  def force: Stream[A] = {
    // Use standard 2x 1x iterator trick for cycle detection ("those" is slow one)
    var these, those = this
    if (!these.isEmpty) these = these.tail
    while (those ne these) {
      if (these.isEmpty) return this
      these = these.tail
      if (these.isEmpty) return this
      these = these.tail
      if (these eq those) return this
      those = those.tail
    }
    this
  }

  /** Prints elements of this stream one by one, separated by commas. */
  def print() { print(", ") }

  /** Prints elements of this stream one by one, separated by `sep`.
   *  @param sep   The separator string printed between consecutive elements.
   */
  def print(sep: String) {
    def loop(these: Stream[A], start: String) {
      Console.print(start)
      if (these.isEmpty) Console.print("empty")
      else {
        Console.print(these.head)
        loop(these.tail, sep)
      }
    }
    loop(this, "")
  }

  /** Returns the length of this `Stream`.
   *
   * @note In order to compute the length of the `Stream`, it must first be
   * fully realized, which could cause the complete evaluation of an infinite
   * series, assuming that's what your `Stream` represents.
   *
   * @return The length of this `Stream`.
   */
  override def length: Int = {
    var len = 0
    var left = this
    while (!left.isEmpty) {
      len += 1
      left = left.tail
    }
    len
  }

  // It's an imperfect world, but at least we can bottle up the
  // imperfection in a capsule.
  @inline private def asThat[That](x: AnyRef): That     = x.asInstanceOf[That]
  @inline private def asStream[B](x: AnyRef): Stream[B] = x.asInstanceOf[Stream[B]]
  @inline private def isStreamBuilder[B, That](bf: CanBuildFrom[Stream[A], B, That]) =
    bf(repr).isInstanceOf[Stream.StreamBuilder[_]]

  // Overridden methods from Traversable

  override def toStream: Stream[A] = this

  override def hasDefiniteSize: Boolean = isEmpty || {
    if (!tailDefined) false
    else {
      // Two-iterator trick (2x & 1x speed) for cycle detection.
      var those = this
      var these = tail
      while (those ne these) {
        if (these.isEmpty) return true
        if (!these.tailDefined) return false
        these = these.tail
        if (these.isEmpty) return true
        if (!these.tailDefined) return false
        these = these.tail
        if (those eq these) return false
        those = those.tail
      }
      false  // Cycle detected
    }
  }

  /** Create a new stream which contains all elements of this stream followed by
   * all elements of Traversable `that`.
   *
   * @note It's subtle why this works. We know that if the target type of the
   * [[scala.collection.mutable.Builder]] `That` is either a `Stream`, or one of
   * its supertypes, or undefined, then `StreamBuilder` will be chosen for the
   * implicit.  We recognize that fact and optimize to get more laziness.
   *
   * @note This method doesn't cause the `Stream` to be fully realized but it
   * should be noted that using the `++` operator from another collection type
   * could cause infinite realization of a `Stream`.  For example, referring to
   * the definition of `fibs` in the preamble, the following would never return:
   * `List(BigInt(12)) ++ fibs`.
   *
   * @tparam B The element type of the returned collection.'''That'''
   * @param that The [[scala.collection.GenTraversableOnce]] the be concatenated
   * to this `Stream`.
   * @return A new collection containing the result of concatenating `this` with
   * `that`.
   */
  override def ++[B >: A, That](that: GenTraversableOnce[B])(implicit bf: CanBuildFrom[Stream[A], B, That]): That =
    // we assume there is no other builder factory on streams and therefore know that That = Stream[A]
    if (isStreamBuilder(bf)) asThat(
      if (isEmpty) that.toStream
      else cons(head, asStream[A](tail ++ that))
    )
    else super.++(that)(bf)

  override def +:[B >: A, That](elem: B)(implicit bf: CanBuildFrom[Stream[A], B, That]): That =
    if (isStreamBuilder(bf)) asThat(cons(elem, this))
    else super.+:(elem)(bf)

  /**
   * Create a new stream which contains all intermediate results of applying the
   * operator to subsequent elements left to right.  `scanLeft` is analogous to
   * `foldLeft`.
   *
   * @note This works because the target type of the
   * [[scala.collection.mutable.Builder]] `That` is a `Stream`.
   *
   * @param z The initial value for the scan.
   * @param op A function that will apply operations to successive values in the
   * `Stream` against previous accumulated results.
   * @return A new collection containing the modifications from the application
   * of `op`.
   */
  override final def scanLeft[B, That](z: B)(op: (B, A) => B)(implicit bf: CanBuildFrom[Stream[A], B, That]): That =
    if (isStreamBuilder(bf)) asThat(
      if (isEmpty) Stream(z)
      else cons(z, asStream[B](tail.scanLeft(op(z, head))(op)))
    )
    else super.scanLeft(z)(op)(bf)

  /** Returns the stream resulting from applying the given function `f` to each
   * element of this stream.  This returns a lazy `Stream` such that it does not
   * need to be fully realized.
   *
   * @example {{{
   * $naturalsEx
   * naturalsFrom(1).map(_ + 10) take 5 mkString(", ")
   * // produces: "11, 12, 13, 14, 15"
   * }}}
   *
   * @tparam B The element type of the returned collection '''That'''.
   * @param f function to apply to each element.
   * @return  `f(a,,0,,), ..., f(a,,n,,)` if this sequence is `a,,0,,, ..., a,,n,,`.
   */
  override final def map[B, That](f: A => B)(implicit bf: CanBuildFrom[Stream[A], B, That]): That = {
    if (isStreamBuilder(bf)) asThat(
      if (isEmpty) Stream.Empty
      else cons(f(head), asStream[B](tail map f))
    )
    else super.map(f)(bf)
  }

  override final def collect[B, That](pf: PartialFunction[A, B])(implicit bf: CanBuildFrom[Stream[A], B, That]): That = {
    if (!isStreamBuilder(bf)) super.collect(pf)(bf)
    else {
      // this implementation avoids:
      // 1) stackoverflows (could be achieved with tailrec, too)
      // 2) out of memory errors for big streams (`this` reference can be eliminated from the stack)
      var rest: Stream[A] = this

      // Avoids calling both `pf.isDefined` and `pf.apply`.
      var newHead: B = null.asInstanceOf[B]
      val runWith = pf.runWith((b: B) => newHead = b)

      while (rest.nonEmpty && !runWith(rest.head)) rest = rest.tail

      //  without the call to the companion object, a thunk is created for the tail of the new stream,
      //  and the closure of the thunk will reference `this`
      if (rest.isEmpty) Stream.Empty.asInstanceOf[That]
      else Stream.collectedTail(newHead, rest, pf, bf).asInstanceOf[That]
    }
  }

  /** Applies the given function `f` to each element of this stream, then
   * concatenates the results.  As with `map` this function does not need to
   * realize the entire `Stream` but continues to keep it as a lazy `Stream`.
   *
   * @example {{{
   * // Let's create a Stream of Vectors, each of which contains the
   * // collection of Fibonacci numbers up to the current value.  We
   * // can then 'flatMap' that Stream.
   *
   * val fibVec: Stream[Vector[Int]] = Vector(0) #:: Vector(0, 1) #:: fibVec.zip(fibVec.tail).map(n => {
   *   n._2 ++ Vector(n._1.last + n._2.last)
   * })
   *
   * fibVec take 5 foreach println
   * // prints
   * // Vector(0)
   * // Vector(0, 1)
   * // Vector(0, 1, 1)
   * // Vector(0, 1, 1, 2)
   * // Vector(0, 1, 1, 2, 3)
   *
   * // If we now want to `flatMap` across that stream by adding 10
   * // we can see what the series turns into:
   *
   * fibVec.flatMap(_.map(_ + 10)) take 15 mkString(", ")
   * // produces: 10, 10, 11, 10, 11, 11, 10, 11, 11, 12, 10, 11, 11, 12, 13
   * }}}
   *
   * ''Note:''  Currently `flatMap` will evaluate as much of the Stream as needed
   * until it finds a non-empty element for the head, which is non-lazy.
   *
   * @tparam B The element type of the returned collection '''That'''.
   * @param f  the function to apply on each element.
   * @return  `f(a,,0,,) ::: ... ::: f(a,,n,,)` if
   *           this stream is `[a,,0,,, ..., a,,n,,]`.
   */
  override final def flatMap[B, That](f: A => GenTraversableOnce[B])(implicit bf: CanBuildFrom[Stream[A], B, That]): That =
    // we assume there is no other builder factory on streams and therefore know that That = Stream[B]
    // optimisations are not for speed, but for functionality
    // see tickets #153, #498, #2147, and corresponding tests in run/ (as well as run/stream_flatmap_odds.scala)
    if (isStreamBuilder(bf)) asThat(
      if (isEmpty) Stream.Empty
      else {
        // establish !prefix.isEmpty || nonEmptyPrefix.isEmpty
        var nonEmptyPrefix = this
        var prefix = f(nonEmptyPrefix.head).toStream
        while (!nonEmptyPrefix.isEmpty && prefix.isEmpty) {
          nonEmptyPrefix = nonEmptyPrefix.tail
          if(!nonEmptyPrefix.isEmpty)
            prefix = f(nonEmptyPrefix.head).toStream
        }

        if (nonEmptyPrefix.isEmpty) Stream.empty
        else prefix append asStream[B](nonEmptyPrefix.tail flatMap f)
      }
    )
    else super.flatMap(f)(bf)

  override private[scala] def filterImpl(p: A => Boolean, isFlipped: Boolean): Stream[A] = {
    // optimization: drop leading prefix of elems for which f returns false
    // var rest = this dropWhile (!p(_)) - forget DRY principle - GC can't collect otherwise
    var rest = this
    while (!rest.isEmpty && p(rest.head) == isFlipped) rest = rest.tail
    // private utility func to avoid `this` on stack (would be needed for the lazy arg)
    if (rest.nonEmpty) Stream.filteredTail(rest, p, isFlipped)
    else Stream.Empty
  }

  /** A FilterMonadic which allows GC of the head of stream during processing */
  @noinline // Workaround SI-9137, see https://github.com/scala/scala/pull/4284#issuecomment-73180791
  override final def withFilter(p: A => Boolean): FilterMonadic[A, Stream[A]] = new Stream.StreamWithFilter(this, p)

  /** A lazier Iterator than LinearSeqLike's. */
  override def iterator: Iterator[A] = new StreamIterator(self)

  /** Apply the given function `f` to each element of this linear sequence
   * (while respecting the order of the elements).
   *
   *  @param f The treatment to apply to each element.
   *  @note  Overridden here as final to trigger tail-call optimization, which
   *  replaces 'this' with 'tail' at each iteration. This is absolutely
   *  necessary for allowing the GC to collect the underlying stream as elements
   *  are consumed.
   *  @note  This function will force the realization of the entire stream
   *  unless the `f` throws an exception.
   */
  @tailrec
  override final def foreach[U](f: A => U) {
    if (!this.isEmpty) {
      f(head)
      tail.foreach(f)
    }
  }

  /** Stream specialization of foldLeft which allows GC to collect along the
   * way.
   *
   * @tparam B The type of value being accumulated.
   * @param z The initial value seeded into the function `op`.
   * @param op The operation to perform on successive elements of the `Stream`.
   * @return The accumulated value from successive applications of `op`.
   */
  @tailrec
  override final def foldLeft[B](z: B)(op: (B, A) => B): B = {
    if (this.isEmpty) z
    else tail.foldLeft(op(z, head))(op)
  }

  /** Stream specialization of reduceLeft which allows GC to collect
   *  along the way.
   *
   * @tparam B The type of value being accumulated.
   * @param f The operation to perform on successive elements of the `Stream`.
   * @return The accumulated value from successive applications of `f`.
   */
  override final def reduceLeft[B >: A](f: (B, A) => B): B = {
    if (this.isEmpty) throw new UnsupportedOperationException("empty.reduceLeft")
    else {
      var reducedRes: B = this.head
      var left = this.tail
      while (!left.isEmpty) {
        reducedRes = f(reducedRes, left.head)
        left = left.tail
      }
      reducedRes
    }
  }

  /** Returns all the elements of this stream that satisfy the predicate `p`
   * returning of [[scala.Tuple2]] of `Stream`s obeying the partition predicate
   * `p`. The order of the elements is preserved.
   *
   * @param p the predicate used to filter the stream.
   * @return the elements of this stream satisfying `p`.
   *
   * @example {{{
   * $naturalsEx
   * val parts = naturalsFrom(1) partition { _ % 2 == 0 }
   * parts._1 take 10 mkString ", "
   * // produces: "2, 4, 6, 8, 10, 12, 14, 16, 18, 20"
   * parts._2 take 10 mkString ", "
   * // produces: "1, 3, 5, 7, 9, 11, 13, 15, 17, 19"
   * }}}
   *
   */
  override def partition(p: A => Boolean): (Stream[A], Stream[A]) = (filter(p(_)), filterNot(p(_)))

  /** Returns a stream formed from this stream and the specified stream `that`
   * by associating each element of the former with the element at the same
   * position in the latter.
   *
   * If one of the two streams is longer than the other, its remaining elements
   * are ignored.
   *
   * The return type of this function may not be obvious.  The lazy aspect of
   * the returned value is different than that of `partition`.  In `partition`
   * we get back a [[scala.Tuple2]] of two lazy `Stream`s whereas here we get
   * back a single lazy `Stream` of [[scala.Tuple2]]s where the
   * [[scala.Tuple2]]'s type signature is `(A1, B)`.
   *
   * @tparam A1 The type of the first parameter of the zipped tuple
   * @tparam B The type of the second parameter of the zipped tuple
   * @tparam That The type of the returned `Stream`.
   * @return `Stream({a,,0,,,b,,0,,}, ...,
   *         {a,,min(m,n),,,b,,min(m,n),,)}` when
   *         `Stream(a,,0,,, ..., a,,m,,)
   *         zip Stream(b,,0,,, ..., b,,n,,)` is invoked.
   *
   * @example {{{
   * $naturalsEx
   * naturalsFrom(1) zip naturalsFrom(2) take 5 foreach println
   * // prints
   * // (1,2)
   * // (2,3)
   * // (3,4)
   * // (4,5)
   * // (5,6)
   * }}}
   */
  override final def zip[A1 >: A, B, That](that: scala.collection.GenIterable[B])(implicit bf: CanBuildFrom[Stream[A], (A1, B), That]): That =
    // we assume there is no other builder factory on streams and therefore know that That = Stream[(A1, B)]
    if (isStreamBuilder(bf)) asThat(
      if (this.isEmpty || that.isEmpty) Stream.Empty
      else cons((this.head, that.head), asStream[(A1, B)](this.tail zip that.tail))
    )
    else super.zip(that)(bf)

  /** Zips this iterable with its indices. `s.zipWithIndex` is equivalent to `s
   * zip s.indices`.
   *
   * This method is much like `zip` in that it returns a single lazy `Stream` of
   * [[scala.Tuple2]].
   *
   * @tparam A1 The type of the first element of the [[scala.Tuple2]] in the
   * resulting stream.
   * @tparam That The type of the resulting `Stream`.
   * @return `Stream({a,,0,,,0}, ..., {a,,n,,,n)}`
   *
   * @example {{{
   * $naturalsEx
   * (naturalsFrom(1) zipWithIndex) take 5 foreach println
   * // prints
   * // (1,0)
   * // (2,1)
   * // (3,2)
   * // (4,3)
   * // (5,4)
   * }}}
   */
  override def zipWithIndex[A1 >: A, That](implicit bf: CanBuildFrom[Stream[A], (A1, Int), That]): That =
    this.zip[A1, Int, That](Stream.from(0))

  /** Write all defined elements of this iterable into given string builder.
   *  The written text begins with the string `start` and is finished by the string
   *  `end`. Inside, the string representations of defined elements (w.r.t.
   *  the method `toString()`) are separated by the string `sep`. The method will
   *  not force evaluation of undefined elements. A tail of such elements will be
   * represented by a `"?"` instead.  A cyclic stream is represented by a `"..."`
   * at the point where the cycle repeats.
   *
   * @param b The [[collection.mutable.StringBuilder]] factory to which we need
   * to add the string elements.
   * @param start The prefix of the resulting string (e.g. "Stream(")
   * @param sep The separator between elements of the resulting string (e.g. ",")
   * @param end The end of the resulting string (e.g. ")")
   * @return The original [[collection.mutable.StringBuilder]] containing the
   * resulting string.
   */
  override def addString(b: StringBuilder, start: String, sep: String, end: String): StringBuilder = {
    b append start
    if (!isEmpty) {
      b append head
      var cursor = this
      var n = 1
      if (cursor.tailDefined) {  // If tailDefined, also !isEmpty
        var scout = tail
        if (scout.isEmpty) {
          // Single element.  Bail out early.
          b append end
          return b
        }
        if (cursor ne scout) {
          cursor = scout
          if (scout.tailDefined) {
            scout = scout.tail
            // Use 2x 1x iterator trick for cycle detection; slow iterator can add strings
            while ((cursor ne scout) && scout.tailDefined) {
              b append sep append cursor.head
              n += 1
              cursor = cursor.tail
              scout = scout.tail
              if (scout.tailDefined) scout = scout.tail
            }
          }
        }
        if (!scout.tailDefined) {  // Not a cycle, scout hit an end
          while (cursor ne scout) {
            b append sep append cursor.head
            n += 1
            cursor = cursor.tail
          }
          if (cursor.nonEmpty) {
            b append sep append cursor.head
          }
        }
        else {
          // Cycle.
          // If we have a prefix of length P followed by a cycle of length C,
          // the scout will be at position (P%C) in the cycle when the cursor
          // enters it at P.  They'll then collide when the scout advances another
          // C - (P%C) ahead of the cursor.
          // If we run the scout P farther, then it will be at the start of
          // the cycle: (C - (P%C) + (P%C)) == C == 0.  So if another runner
          // starts at the beginning of the prefix, they'll collide exactly at
          // the start of the loop.
          var runner = this
          var k = 0
          while (runner ne scout) {
            runner = runner.tail
            scout = scout.tail
            k += 1
          }
          // Now runner and scout are at the beginning of the cycle.  Advance
          // cursor, adding to string, until it hits; then we'll have covered
          // everything once.  If cursor is already at beginning, we'd better
          // advance one first unless runner didn't go anywhere (in which case
          // we've already looped once).
          if ((cursor eq scout) && (k > 0)) {
            b append sep append cursor.head
            n += 1
            cursor = cursor.tail
          }
          while (cursor ne scout) {
            b append sep append cursor.head
            n += 1
            cursor = cursor.tail
          }
          // Subtract prefix length from total length for cycle reporting.
          // (Not currently used, but probably a good idea for the future.)
          n -= k
        }
      }
      if (!cursor.isEmpty) {
        // Either undefined or cyclic; we can check with tailDefined
        if (!cursor.tailDefined) b append sep append "?"
        else b append sep append "..."
      }
    }
    b append end
    b
  }

  override def mkString(sep: String): String = mkString("", sep, "")
  override def mkString: String = mkString("")
  override def mkString(start: String, sep: String, end: String): String = {
    this.force
    super.mkString(start, sep, end)
  }
  override def toString = super.mkString(stringPrefix + "(", ", ", ")")

  override def splitAt(n: Int): (Stream[A], Stream[A]) = (take(n), drop(n))

  /** Returns the `n` first elements of this `Stream` as another `Stream`, or
   * else the whole `Stream`, if it has less than `n` elements.
   *
   * The result of `take` is, again, a `Stream` meaning that it also does not
   * make any needless evaluations of the `Stream` itself, delaying that until
   * the usage of the resulting `Stream`.
   *
   * @param n the number of elements to take.
   * @return the `n` first elements of this stream.
   *
   * @example {{{
   * $naturalsEx
   * scala> naturalsFrom(5) take 5
   * res1: scala.collection.immutable.Stream[Int] = Stream(5, ?)
   *
   * scala> naturalsFrom(5) take 5 mkString ", "
   * // produces: "5, 6, 7, 8, 9"
   * }}}
   */
  override def take(n: Int): Stream[A] = (
    // Note that the n == 1 condition appears redundant but is not.
    // It prevents "tail" from being referenced (and its head being evaluated)
    // when obtaining the last element of the result. Such are the challenges
    // of working with a lazy-but-not-really sequence.
    if (n <= 0 || isEmpty) Stream.empty
    else if (n == 1) cons(head, Stream.empty)
    else cons(head, tail take n-1)
  )

  @tailrec final override def drop(n: Int): Stream[A] =
    if (n <= 0 || isEmpty) this
    else tail drop n-1

  /** A substream starting at index `from` and extending up to (but not including)
   *  index `until`.  This returns a `Stream` that is lazily evaluated.
   *
   * @param from    The index of the first element of the returned subsequence
   * @param until   The index of the element following the returned subsequence
   * @return A new string containing the elements requested from `start` until
   * `end`.
   *
   * @example {{{
   * naturalsFrom(0) slice(50, 60) mkString ", "
   * // produces: "50, 51, 52, 53, 54, 55, 56, 57, 58, 59"
   * }}}
   */
  override def slice(from: Int, until: Int): Stream[A] = {
    val lo = from max 0
    if (until <= lo || isEmpty) Stream.empty
    else this drop lo take (until - lo)
  }

  /** The stream without its last element.
   *
   * @return A new `Stream` containing everything but the last element.  If your
   * `Stream` represents an infinite series, this method will not return.
   *
   *  @throws UnsupportedOperationException if the stream is empty.
   */
  override def init: Stream[A] =
    if (isEmpty) super.init
    else if (tail.isEmpty) Stream.Empty
    else cons(head, tail.init)

  /** Returns the rightmost `n` elements from this iterable.
   *
   * @note Take serious caution here.  If the `Stream` represents an infinite
   * series then this function ''will not return''.  The right most elements of
   * an infinite series takes an infinite amount of time to produce.
   *
   *  @param n the number of elements to take
   *  @return The last `n` elements from this `Stream`.
   */
  override def takeRight(n: Int): Stream[A] = {
    var these: Stream[A] = this
    var lead = this drop n
    while (!lead.isEmpty) {
      these = these.tail
      lead = lead.tail
    }
    these
  }

  /**
   * @inheritdoc
   * $willTerminateInf
   */
  override def dropRight(n: Int): Stream[A] = {
    // We make dropRight work for possibly infinite streams by carrying
    // a buffer of the dropped size. As long as the buffer is full and the
    // rest is non-empty, we can feed elements off the buffer head.  When
    // the rest becomes empty, the full buffer is the dropped elements.
    def advance(stub0: List[A], stub1: List[A], rest: Stream[A]): Stream[A] = {
      if (rest.isEmpty) Stream.empty
      else if (stub0.isEmpty) advance(stub1.reverse, Nil, rest)
      else cons(stub0.head, advance(stub0.tail, rest.head :: stub1, rest.tail))
    }
    if (n <= 0) this
    else advance((this take n).toList, Nil, this drop n)
  }

  /** Returns the longest prefix of this `Stream` whose elements satisfy the
   * predicate `p`.
   *
   * @param p the test predicate.
   * @return A new `Stream` representing the values that satisfy the predicate
   * `p`.
   *
   * @example {{{
   + naturalsFrom(0) takeWhile { _ < 5 } mkString ", "
   * produces: "0, 1, 2, 3, 4"
   * }}}
   */
  override def takeWhile(p: A => Boolean): Stream[A] =
    if (!isEmpty && p(head)) cons(head, tail takeWhile p)
    else Stream.Empty

  /** Returns the a `Stream` representing the longest suffix of this iterable
   * whose first element does not satisfy the predicate `p`.
   *
   * @note This method realizes the entire `Stream` beyond the truth value of
   * the predicate `p`.
   *
   * @param p the test predicate.
   * @return A new `Stream` representing the results of applying `p` to the
   * original `Stream`.
   *
   * @example {{{
   * // Assume we have a Stream that takes the first 20 natural numbers
   * def naturalsLt50(i: Int): Stream[Int] = i #:: { if (i < 20) naturalsLt50(i * + 1) else Stream.Empty }
   * naturalsLt50(0) dropWhile { _ < 10 }
   * // produces: "10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20"
   * }}}
   */
  override def dropWhile(p: A => Boolean): Stream[A] = {
    var these: Stream[A] = this
    while (!these.isEmpty && p(these.head)) these = these.tail
    these
  }

  /** Builds a new stream from this stream in which any duplicates (as
   * determined by `==`) have been removed. Among duplicate elements, only the
   * first one is retained in the resulting `Stream`.
   *
   * @return A new `Stream` representing the result of applying distinctness to
   * the original `Stream`.
   * @example {{{
   * // Creates a Stream where every element is duplicated
   * def naturalsFrom(i: Int): Stream[Int] = i #:: { i #:: naturalsFrom(i + 1) }
   * naturalsFrom(1) take 6 mkString ", "
   * // produces: "1, 1, 2, 2, 3, 3"
   * (naturalsFrom(1) distinct) take 6 mkString ", "
   * // produces: "1, 2, 3, 4, 5, 6"
   * }}}
   */
  override def distinct: Stream[A] = {
    // This should use max memory proportional to N, whereas
    // recursively calling distinct on the tail is N^2.
    def loop(seen: Set[A], rest: Stream[A]): Stream[A] = {
      if (rest.isEmpty) rest
      else if (seen(rest.head)) loop(seen, rest.tail)
      else cons(rest.head, loop(seen + rest.head, rest.tail))
    }
    loop(Set(), this)
  }

  /** Returns a new sequence of given length containing the elements of this
   * sequence followed by zero or more occurrences of given elements.
   *
   * @tparam B The type of the value to pad with.
   * @tparam That The type contained within the resulting `Stream`.
   * @param len The number of elements to pad into the `Stream`.
   * @param elem The value of the type `B` to use for padding.
   * @return A new `Stream` representing the collection with values padding off
   * to the end. If your `Stream` represents an infinite series, this method will
   * not return.
   * @example {{{
   * def naturalsFrom(i: Int): Stream[Int] = i #:: { if (i < 5) naturalsFrom(i + 1) else Stream.Empty }
   * naturalsFrom(1) padTo(10, 0) foreach println
   * // prints
   * // 1
   * // 2
   * // 3
   * // 4
   * // 5
   * // 0
   * // 0
   * // 0
   * // 0
   * // 0
   * }}}
   */
  override def padTo[B >: A, That](len: Int, elem: B)(implicit bf: CanBuildFrom[Stream[A], B, That]): That = {
    def loop(len: Int, these: Stream[A]): Stream[B] =
      if (these.isEmpty) Stream.fill(len)(elem)
      else cons(these.head, loop(len - 1, these.tail))

    if (isStreamBuilder(bf)) asThat(loop(len, this))
    else super.padTo(len, elem)(bf)
  }

  /** A list consisting of all elements of this list in reverse order.
   *
   * @note This function must realize the entire `Stream` in order to perform
   * this operation so if your `Stream` represents an infinite sequence then
   * this function will never return.
   *
   * @return A new `Stream` containing the representing of the original `Stream`
   * in reverse order.
   *
   * @example {{{
   * def naturalsFrom(i: Int): Stream[Int] = i #:: { if (i < 5) naturalsFrom(i + 1) else Stream.Empty }
   * (naturalsFrom(1) reverse) foreach println
   * // prints
   * // 5
   * // 4
   * // 3
   * // 2
   * // 1
   * }}}
   */
  override def reverse: Stream[A] = {
    var result: Stream[A] = Stream.Empty
    var these = this
    while (!these.isEmpty) {
      val r = Stream.consWrapper(result).#::(these.head)
      r.tail // force it!
      result = r
      these = these.tail
    }
    result
  }

  /** Evaluates and concatenates all elements within the `Stream` into a new
   * flattened `Stream`.
   *
   * @tparam B The type of the elements of the resulting `Stream`.
   * @return A new `Stream` of type `B` of the flattened elements of `this`
   * `Stream`.
   * @example {{{
   * val sov: Stream[Vector[Int]] = Vector(0) #:: Vector(0, 0) #:: sov.zip(sov.tail).map { n => n._1 ++ n._2 }
   * sov.flatten take 10 mkString ", "
   * // produces: "0, 0, 0, 0, 0, 0, 0, 0, 0, 0"
   * }}}
   */
  override def flatten[B](implicit asTraversable: A => /*<:<!!!*/ GenTraversableOnce[B]): Stream[B] = {
    var st: Stream[A] = this
    while (st.nonEmpty) {
      val h = asTraversable(st.head)
      if (h.isEmpty) {
        st = st.tail
      } else {
        return h.toStream #::: st.tail.flatten
      }
    }
    Stream.empty
  }

  override def view = new StreamView[A, Stream[A]] {
    protected lazy val underlying = self.repr
    override def iterator = self.iterator
    override def length = self.length
    override def apply(idx: Int) = self.apply(idx)
  }

  /** Defines the prefix of this object's `toString` representation as `Stream`.
   */
  override def stringPrefix = "Stream"

}

/** A specialized, extra-lazy implementation of a stream iterator, so it can
 *  iterate as lazily as it traverses the tail.
 */
final class StreamIterator[+A] private() extends AbstractIterator[A] with Iterator[A] {
  def this(self: Stream[A]) {
    this()
    these = new LazyCell(self)
  }

  // A call-by-need cell.
  class LazyCell(st: => Stream[A]) {
    lazy val v = st
  }

  private var these: LazyCell = _

  def hasNext: Boolean = these.v.nonEmpty
  def next(): A =
    if (isEmpty) Iterator.empty.next()
    else {
      val cur    = these.v
      val result = cur.head
      these = new LazyCell(cur.tail)
      result
    }
  override def toStream = {
    val result = these.v
    these = new LazyCell(Stream.empty)
    result
  }
  override def toList   = toStream.toList
}

/**
 * The object `Stream` provides helper functions to manipulate streams.
 *
 * @author Martin Odersky, Matthias Zenger
 * @version 1.1 08/08/03
 * @since   2.8
 */
object Stream extends SeqFactory[Stream] {

  /** The factory for streams.
   *  @note Methods such as map/flatMap will not invoke the `Builder` factory,
   *        but will return a new stream directly, to preserve laziness.
   *        The new stream is then cast to the factory's result type.
   *        This means that every CanBuildFrom that takes a
   *        Stream as its From type parameter must yield a stream as its result parameter.
   *        If that assumption is broken, cast errors might result.
   */
  class StreamCanBuildFrom[A] extends GenericCanBuildFrom[A]

  implicit def canBuildFrom[A]: CanBuildFrom[Coll, A, Stream[A]] = new StreamCanBuildFrom[A]

  /** Creates a new builder for a stream */
  def newBuilder[A]: Builder[A, Stream[A]] = new StreamBuilder[A]

  /** A builder for streams
   *  @note This builder is lazy only in the sense that it does not go downs the spine
   *        of traversables that are added as a whole. If more laziness can be achieved,
   *        this builder should be bypassed.
   */
  class StreamBuilder[A] extends LazyBuilder[A, Stream[A]] {
    def result: Stream[A] = parts.toStream flatMap (_.toStream)
  }

  object Empty extends Stream[Nothing] {
    override def isEmpty = true
    override def head = throw new NoSuchElementException("head of empty stream")
    override def tail = throw new UnsupportedOperationException("tail of empty stream")
    def tailDefined = false
  }

  /** The empty stream */
  override def empty[A]: Stream[A] = Empty

  /** A stream consisting of given elements */
  override def apply[A](xs: A*): Stream[A] = xs.toStream

  /** A wrapper class that adds `#::` for cons and `#:::` for concat as operations
   *  to streams.
   */
  class ConsWrapper[A](tl: => Stream[A]) {
    /** Construct a stream consisting of a given first element followed by elements
     *  from a lazily evaluated Stream.
     */
    def #::(hd: A): Stream[A] = cons(hd, tl)
    /** Construct a stream consisting of the concatenation of the given stream and
     *  a lazily evaluated Stream.
     */
    def #:::(prefix: Stream[A]): Stream[A] = prefix append tl
  }

  /** A wrapper method that adds `#::` for cons and `#:::` for concat as operations
   *  to streams.
   */
  implicit def consWrapper[A](stream: => Stream[A]): ConsWrapper[A] =
    new ConsWrapper[A](stream)

  /** An extractor that allows to pattern match streams with `#::`.
   */
  object #:: {
    def unapply[A](xs: Stream[A]): Option[(A, Stream[A])] =
      if (xs.isEmpty) None
      else Some((xs.head, xs.tail))
  }

  /** An alternative way of building and matching Streams using Stream.cons(hd, tl).
   */
  object cons {

    /** A stream consisting of a given first element and remaining elements
     *  @param hd   The first element of the result stream
     *  @param tl   The remaining elements of the result stream
     */
    def apply[A](hd: A, tl: => Stream[A]) = new Cons(hd, tl)

    /** Maps a stream to its head and tail */
    def unapply[A](xs: Stream[A]): Option[(A, Stream[A])] = #::.unapply(xs)
  }

  /** A lazy cons cell, from which streams are built. */
  @SerialVersionUID(-602202424901551803L)
  final class Cons[+A](hd: A, tl: => Stream[A]) extends Stream[A] {
    override def isEmpty = false
    override def head = hd
    @volatile private[this] var tlVal: Stream[A] = _
    @volatile private[this] var tlGen = tl _
    def tailDefined: Boolean = tlGen eq null
    override def tail: Stream[A] = {
      if (!tailDefined)
        synchronized {
          if (!tailDefined) {
            tlVal = tlGen()
            tlGen = null
          }
        }

      tlVal
    }
  }

  /** An infinite stream that repeatedly applies a given function to a start value.
   *
   *  @param start the start value of the stream
   *  @param f     the function that's repeatedly applied
   *  @return      the stream returning the infinite sequence of values `start, f(start), f(f(start)), ...`
   */
  def iterate[A](start: A)(f: A => A): Stream[A] = cons(start, iterate(f(start))(f))

  override def iterate[A](start: A, len: Int)(f: A => A): Stream[A] =
    iterate(start)(f) take len

  /**
   * Create an infinite stream starting at `start` and incrementing by
   * step `step`.
   *
   * @param start the start value of the stream
   * @param step the increment value of the stream
   * @return the stream starting at value `start`.
   */
  def from(start: Int, step: Int): Stream[Int] =
    cons(start, from(start+step, step))

  /**
   * Create an infinite stream starting at `start` and incrementing by `1`.
   *
   * @param start the start value of the stream
   * @return the stream starting at value `start`.
   */
  def from(start: Int): Stream[Int] = from(start, 1)

  /**
   * Create an infinite stream containing the given element expression (which
   * is computed for each occurrence).
   *
   * @param elem the element composing the resulting stream
   * @return the stream containing an infinite number of elem
   */
  def continually[A](elem: => A): Stream[A] = cons(elem, continually(elem))

  override def fill[A](n: Int)(elem: => A): Stream[A] =
    if (n <= 0) Empty else cons(elem, fill(n-1)(elem))

  override def tabulate[A](n: Int)(f: Int => A): Stream[A] = {
    def loop(i: Int): Stream[A] =
      if (i >= n) Empty else cons(f(i), loop(i+1))
    loop(0)
  }

  override def range[T: Integral](start: T, end: T, step: T): Stream[T] = {
    val num = implicitly[Integral[T]]
    import num._

    if (if (step < zero) start <= end else end <= start) Empty
    else cons(start, range(start + step, end, step))
  }

  private[immutable] def filteredTail[A](stream: Stream[A], p: A => Boolean, isFlipped: Boolean) = {
    cons(stream.head, stream.tail.filterImpl(p, isFlipped))
  }

  private[immutable] def collectedTail[A, B, That](head: B, stream: Stream[A], pf: PartialFunction[A, B], bf: CanBuildFrom[Stream[A], B, That]) = {
    cons(head, stream.tail.collect(pf)(bf).asInstanceOf[Stream[B]])
  }

  /** An implementation of `FilterMonadic` allowing GC of the filtered-out elements of
    * the `Stream` as it is processed.
    *
    * Because this is not an inner class of `Stream` with a reference to the original
    * head, it is now possible for GC to collect any leading and filtered-out elements
    * which do not satisfy the filter, while the tail is still processing (see SI-8990).
    */
  private[immutable] final class StreamWithFilter[A](sl: => Stream[A], p: A => Boolean) extends FilterMonadic[A, Stream[A]] {
    private var s = sl                                              // set to null to allow GC after filtered
    private lazy val filtered = { val f = s filter p; s = null; f } // don't set to null if throw during filter

    def map[B, That](f: A => B)(implicit bf: CanBuildFrom[Stream[A], B, That]): That =
      filtered map f

    def flatMap[B, That](f: A => scala.collection.GenTraversableOnce[B])(implicit bf: CanBuildFrom[Stream[A], B, That]): That =
      filtered flatMap f

    def foreach[U](f: A => U): Unit =
      filtered foreach f

    def withFilter(q: A => Boolean): FilterMonadic[A, Stream[A]] =
      new StreamWithFilter[A](filtered, q)
  }

}