Get the element at the specified index. This operation is provided for convenience in Seq. It should not be assumed to be efficient unless you have an IndexedSeq.

Iterator can be used only once

The length (number of elements) of the sequence. size is an alias for length in Seq collections.

Test two objects for inequality.

Equivalent to x.hashCode except for boxed numeric types and null. For numerics, it returns a hash value which is consistent with value equality: if two value type instances compare as true, then ## will produce the same hash value for each of them. For null returns a hashcode where null.hashCode throws a NullPointerException.

Alias for concat

Alias for prependedAll

Alias for prepended.

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

Alias for appended

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

Alias for appendedAll

The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).

Appends all elements of this sequence to a string builder. The written text consists of the string representations (w.r.t. the method toString) of all elements of this sequence 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

Appends all elements of this sequence 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 sequence, 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

Appends all elements of this sequence to a string builder using start, end, and separator strings. The written text begins with the string start and ends with the string end. Inside, the string representations (w.r.t. the method toString) of all elements of this sequence are separated by the string sep.

Example:

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

scala> val b = new StringBuilder()
b: StringBuilder =

scala> a.addString(b , "List(" , ", " , ")")
res5: StringBuilder = List(1, 2, 3, 4)

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

Note that calling isDefinedAt on the resulting partial function may apply the first partial function and execute its side effect. It is highly recommended to call applyOrElse instead of isDefinedAt / apply for efficiency.

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

If the runtime type of the function is a PartialFunction then the other andThen method is used (note its cautions).

A copy of this sequence with an element appended.

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)

Returns a new sequence containing the elements from the left hand operand followed by the elements from the right hand operand. The element type of the sequence is the most specific superclass encompassing the element types of the two operands.

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.

Cast the receiver object to be of type T0.

Note that the success of a cast at runtime is modulo Scala's erasure semantics. Therefore the expression 1.asInstanceOf[String] will throw a ClassCastException at runtime, while the expression List(1).asInstanceOf[List[String]] will not. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the requested type.

A method that should be called from every well-designed equals method that is open to be overridden in a subclass. See Programming in Scala, Chapter 28 for discussion and design.

Defines the prefix of this object's toString representation.

It is recommended to return the name of the concrete collection type, but not implementation subclasses. For example, for ListMap this method should return "ListMap", not "Map" (the supertype) or "Node" (an implementation subclass).

The default implementation returns "Iterable". It is overridden for the basic collection kinds "Seq", "IndexedSeq", "LinearSeq", "Buffer", "Set", "Map", "SortedSet", "SortedMap" and "View".

Create a copy of the receiver object.

The default implementation of the clone method is platform dependent.

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

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

Note: may not terminate for infinite-sized collections.

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.

Note: Even when applied to a view or a lazy collection it will always force the elements.

Composes another partial function k with this partial function so that this partial function gets applied to results of k.

Note that calling isDefinedAt on the resulting partial function may apply the first partial function and execute its side effect. It is highly recommended to call applyOrElse instead of isDefinedAt / apply for efficiency.

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

Returns a new sequence containing the elements from the left hand operand followed by the elements from the right hand operand. The element type of the sequence is the most specific superclass encompassing the element types of the two operands.

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

Note: may not terminate for infinite-sized collections.

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

Note: may not terminate for infinite-sized collections.

Copy elements to an array, returning the number of elements written.

Fills the given array xs starting at index start with at most len elements of this sequence.

Copying will stop once either all the elements of this sequence have been copied, or the end of the array is reached, or len elements have been copied.

Copy elements to an array, returning the number of elements written.

Fills the given array xs starting at index start with values of this sequence.

Copying will stop once either all the elements of this sequence have been copied, or the end of the array is reached.

Copy elements to an array, returning the number of elements written.

Fills the given array xs starting at index start with values of this sequence.

Copying will stop once either all the elements of this sequence have been copied, or the end of the array is reached.

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

Tests whether every element of this collection's iterator relates to the corresponding element of another collection by satisfying a test predicate.

Note: will not terminate for infinite-sized collections.

Counts the number of elements in the sequence which satisfy a predicate.

Note: will not terminate for infinite-sized collections.

Computes the multiset difference between this sequence and another sequence.

Selects all the elements of this sequence ignoring the duplicates.

Selects all the elements of this sequence ignoring the duplicates as determined by == after applying the transforming function f.

Selects all elements except first n ones.

Selects all elements except last n ones.

Drops longest prefix of elements that satisfy a predicate.

Returns an extractor object with a unapplySeq method, which extracts each element of a sequence data.

The empty iterable of the same type as this iterable

Tests whether this sequence ends with the given sequence.

Note: will not terminate for infinite-sized collections.

Tests whether the argument (that) is a reference to the receiver object (this).

The eq method implements an equivalence relation on non-null instances of AnyRef, and has three additional properties:

• It is consistent: for any non-null instances x and y of type AnyRef, multiple invocations of x.eq(y) consistently returns true or consistently returns false.
• For any non-null instance x of type AnyRef, x.eq(null) and null.eq(x) returns false.
• null.eq(null) returns true.

When overriding the equals or hashCode methods, it is important to ensure that their behavior is consistent with reference equality. Therefore, if two objects are references to each other (o1 eq o2), they should be equal to each other (o1 == o2) and they should hash to the same value (o1.hashCode == o2.hashCode).

The universal equality method defined in AnyRef.

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

Note: may not terminate for infinite-sized collections.

Selects all elements of this sequence which satisfy a predicate.

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

Called by the garbage collector on the receiver object when there are no more references to the object.

The details of when and if the finalize method is invoked, as well as the interaction between finalize and non-local returns and exceptions, are all platform dependent.

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

Note: may not terminate for infinite-sized collections.

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

Note: will not terminate for infinite-sized collections.

Builds a new sequence by applying a function to all elements of this sequence and using the elements of the resulting collections.

For example:

def getWords(lines: Seq[String]): Seq[String] = lines flatMap (line => line split "\\W+")

The type of the resulting collection is guided by the static type of sequence. This might cause unexpected results sometimes. For example:

// lettersOf will return a Seq[Char] of likely repeated letters, instead of a Set
def lettersOf(words: Seq[String]) = words flatMap (word => word.toSet)

// lettersOf will return a Set[Char], not a Seq
def lettersOf(words: Seq[String]) = words.toSet flatMap ((word: String) => word.toSeq)

// xs will be an Iterable[Int]
val xs = Map("a" -> List(11,111), "b" -> List(22,222)).flatMap(_._2)

// ys will be a Map[Int, Int]
val ys = Map("a" -> List(1 -> 11,1 -> 111), "b" -> List(2 -> 22,2 -> 222)).flatMap(_._2)

Converts this sequence of traversable collections into a sequence formed by the elements of these traversable collections.

The resulting collection's type will be guided by the type of sequence. For example:

val xs = List(
           Set(1, 2, 3),
           Set(1, 2, 3)
         ).flatten
// xs == List(1, 2, 3, 1, 2, 3)

val ys = Set(
           List(1, 2, 3),
           List(3, 2, 1)
         ).flatten
// ys == Set(1, 2, 3)

Folds the elements of this sequence using the specified associative binary operator. The default implementation in IterableOnce is equivalent to foldLeft but may be overridden for more efficient traversal orders.

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

Note: will not terminate for infinite-sized collections.

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

Note: will not terminate for infinite-sized collections.

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.

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

Note: may not terminate for infinite-sized collections.

Apply f to each element for its side effects Note: [U] parameter needed to help scalac's type inference.

Returns string formatted according to given format string. Format strings are as for String.format (@see java.lang.String.format).

Defines how to turn a given Iterable[A] into a collection of type C.

This process can be done in a strict way or a non-strict way (ie. without evaluating the elements of the resulting collections). In other words, this methods defines the evaluation model of the collection.

Returns the runtime class representation of the object.

Partitions this sequence into a map of sequences according to some discriminator function.

Note: Even when applied to a view or a lazy collection it will always force the elements.

Partitions this sequence into a map of sequences according to a discriminator function key. Each element in a group is transformed into a value of type B using the value function.

It is equivalent to groupBy(key).mapValues(_.map(f)), but more efficient.

case class User(name: String, age: Int)

def namesByAge(users: Seq[User]): Map[Int, Seq[String]] =
  users.groupMap(_.age)(_.name)

Note: Even when applied to a view or a lazy collection it will always force the elements.

Partitions this sequence into a map according to a discriminator function key. All the values that have the same discriminator are then transformed by the f function and then reduced into a single value with the reduce function.

It is equivalent to groupBy(key).mapValues(_.map(f).reduce(reduce)), but more efficient.

def occurrences[A](as: Seq[A]): Map[A, Int] =
  as.groupMapReduce(identity)(_ => 1)(_ + _)

Note: Even when applied to a view or a lazy collection it will always force the elements.

Partitions elements in fixed size sequences.

The hashCode method for reference types. See hashCode in scala.Any.

Selects the first element of this sequence.

Optionally selects the first element.

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

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

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

Note: may not terminate for infinite-sized collections.

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.

Finds index of the first element satisfying some predicate.

Note: may not terminate for infinite-sized collections.

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

Note: may not terminate for infinite-sized collections.

Produces the range of all indices of this sequence.

Note: Even when applied to a view or a lazy collection it will always force the elements.

The initial part of the collection without its last element.

Note: Even when applied to a view or a lazy collection it will always force the elements.

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

Note: Even when applied to a view or a lazy collection it will always force the elements.

Computes the multiset intersection between this sequence and another sequence.

Tests whether this sequence contains given index.

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

Tests whether the sequence is empty.

Note: Implementations in subclasses that are not repeatedly traversable must take care not to consume any elements when isEmpty is called.

Test whether the dynamic type of the receiver object is T0.

Note that the result of the test is modulo Scala's erasure semantics. Therefore the expression 1.isInstanceOf[String] will return false, while the expression List(1).isInstanceOf[List[String]] will return true. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the specified type.

Tests whether this sequence can be repeatedly traversed. Always true for Iterables and false for Iterators unless overridden.

The companion object of this sequence, providing various factory methods.

Selects the last element.

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

Note: will not terminate for infinite-sized collections.

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

Note: will not terminate for infinite-sized collections.

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

Note: will not terminate for infinite-sized collections.

Finds index of last element satisfying some predicate.

Note: will not terminate for infinite-sized collections.

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

Note: will not terminate for infinite-sized collections.

Optionally selects the last element.

Analogous to zip except that the elements in each collection are not consumed until a strict operation is invoked on the returned LazyZip2 decorator.

Calls to lazyZip can be chained to support higher arities (up to 4) without incurring the expense of constructing and deconstructing intermediary tuples.

val xs = List(1, 2, 3)
val res = (xs lazyZip xs lazyZip xs lazyZip xs).map((a, b, c, d) => a + b + c + d)
// res == List(4, 8, 12)

Compares the length of this sequence to the size of another Iterable.

Compares the length of this sequence to a test value.

Returns a value class containing operations for comparing the length of this sequence to a test value.

These operations are implemented in terms of lengthCompare(Int), and allow the following more readable usages:

this.lengthIs < len     // this.lengthCompare(len) < 0
this.lengthIs <= len    // this.lengthCompare(len) <= 0
this.lengthIs == len    // this.lengthCompare(len) == 0
this.lengthIs != len    // this.lengthCompare(len) != 0
this.lengthIs >= len    // this.lengthCompare(len) >= 0
this.lengthIs > len     // this.lengthCompare(len) > 0

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

Builds a new sequence by applying a function to all elements of this sequence.

Finds the largest element.

Note: will not terminate for infinite-sized collections.

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

Note: will not terminate for infinite-sized collections.

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

Note: will not terminate for infinite-sized collections.

Finds the largest element.

Note: will not terminate for infinite-sized collections.

Finds the smallest element.

Note: will not terminate for infinite-sized collections.

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

Note: will not terminate for infinite-sized collections.

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

Note: will not terminate for infinite-sized collections.

Finds the smallest element.

Note: will not terminate for infinite-sized collections.

Displays all elements of this sequence in a string.

Delegates to addString, which can be overridden.

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

Delegates to addString, which can be overridden.

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

Delegates to addString, which can be overridden.

Equivalent to !(this eq that).

Tests whether the sequence is not empty.

Wakes up a single thread that is waiting on the receiver object's monitor.

Wakes up all threads that are waiting on the receiver object's monitor.

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

A copy of this sequence with an element value appended until a given target length is reached.

A pair of, first, all elements that satisfy predicate p and, second, all elements that do not. Interesting because it splits a collection in two.

The default implementation provided here needs to traverse the collection twice. Strict collections have an overridden version of partition in StrictOptimizedIterableOps, which requires only a single traversal.

Applies a function f to each element of the sequence and returns a pair of sequences: the first one made of those values returned by f that were wrapped in scala.util.Left, and the second one made of those wrapped in scala.util.Right.

Example:

val xs = `Seq`(1, "one", 2, "two", 3, "three") partitionMap {
 case i: Int => Left(i)
 case s: String => Right(s)
}
// xs == (`Seq`(1, 2, 3),
//        `Seq`(one, two, three))

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

Patching at negative indices is the same as patching starting at 0. Patching at indices at or larger than the length of the original sequence appends the patch to the end. If more values are replaced than actually exist, the excess is ignored.

Iterates over distinct permutations.

Note: Even when applied to a view or a lazy collection it will always force the elements.

A copy of the sequence with an element prepended.

Also, the original sequence 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)

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.

Multiplies up the elements of this collection.

Note: will not terminate for infinite-sized collections.

Reduces the elements of this sequence using the specified associative binary operator.

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

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

Note: will not terminate for infinite-sized collections.

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

Note: will not terminate for infinite-sized collections.

Reduces the elements of this sequence, if any, using the specified associative binary operator.

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

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

Note: will not terminate for infinite-sized collections.

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

Note: will not terminate for infinite-sized collections.

Returns new sequence with elements in reversed order.

Note: will not terminate for infinite-sized collections.

Note: Even when applied to a view or a lazy collection it will always force the elements.

An iterator yielding elements in reversed order.

Note: will not terminate for infinite-sized collections.

Note: xs.reverseIterator is the same as xs.reverse.iterator but might be more efficient.

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.

Are the elements of this collection the same (and in the same order) as those of that?

Computes a prefix scan of the elements of the collection.

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

Produces a sequence containing cumulative results of applying the operator going left to right, including the initial value.

Note: will not terminate for infinite-sized collections.

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: Even when applied to a view or a lazy collection it will always force the elements.

Example:

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

Search within an interval in this sorted sequence for a specific element. If this sequence is an IndexedSeq, a binary search is used. Otherwise, a linear search is used.

The sequence should be sorted with the same Ordering before calling; otherwise, the results are undefined.

Search this sorted sequence for a specific element. If the sequence is an IndexedSeq, a binary search is used. Otherwise, a linear search is used.

The sequence should be sorted with the same Ordering before calling; otherwise, the results are undefined.

Computes the length of the longest segment that starts from some index and whose elements all satisfy some predicate.

Note: may not terminate for infinite-sized collections.

Computes the length of the longest segment that starts from the first element and whose elements all satisfy some predicate.

Note: may not terminate for infinite-sized collections.

The size of this sequence.

Note: will not terminate for infinite-sized collections.

Compares the size of this sequence to the size of another Iterable.

Compares the size of this sequence to a test value.

Returns a value class containing operations for comparing the size of this sequence to a test value.

These operations are implemented in terms of sizeCompare(Int), and allow the following more readable usages:

this.sizeIs < size     // this.sizeCompare(size) < 0
this.sizeIs <= size    // this.sizeCompare(size) <= 0
this.sizeIs == size    // this.sizeCompare(size) == 0
this.sizeIs != size    // this.sizeCompare(size) != 0
this.sizeIs >= size    // this.sizeCompare(size) >= 0
this.sizeIs > size     // this.sizeCompare(size) > 0

Selects an interval of elements. The returned sequence is made up of all elements x which satisfy the invariant:

from <= indexOf(x) < until

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

The returned iterator will be empty when called on an empty collection. The last element the iterator produces may be smaller than the window size when the original collection isn't exhausted by the window before it and its last element isn't skipped by the step before it.

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

An empty collection returns an empty iterator, and a non-empty collection containing fewer elements than the window size returns an iterator that will produce the original collection as its only element.

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

Note: will not terminate for infinite-sized collections.

Note: Even when applied to a view or a lazy collection it will always force the elements.

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

Sorts this sequence according to a comparison function.

Note: will not terminate for infinite-sized collections.

Note: Even when applied to a view or a lazy collection it will always force the elements.

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.

Sorts this sequence according to an Ordering.

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

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.

Splits this sequence into a prefix/suffix pair at a given position.

Note: c splitAt n is equivalent to (but possibly more efficient than) (c take n, c drop n).

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.

Returns a scala.collection.Stepper for the elements of this collection.

The Stepper enables creating a Java stream to operate on the collection, see scala.jdk.StreamConverters. For collections holding primitive values, the Stepper can be used as an iterator which doesn't box the elements.

The implicit scala.collection.StepperShape parameter defines the resulting Stepper type according to the element type of this collection.

• For collections of Int, Short, Byte or Char, an scala.collection.IntStepper is returned
• For collections of Double or Float, a scala.collection.DoubleStepper is returned
• For collections of Long a scala.collection.LongStepper is returned
• For any other element type, an scala.collection.AnyStepper is returned

Note that this method is overridden in subclasses and the return type is refined to S with EfficientSplit, for example scala.collection.IndexedSeqOps.stepper. For Steppers marked with scala.collection.Stepper.EfficientSplit, the converters in scala.jdk.StreamConverters allow creating parallel streams, whereas bare Steppers can be converted only to sequential streams.

Sums up the elements of this collection.

Note: will not terminate for infinite-sized collections.