Packages

trait Foldable[F[_]] extends UnorderedFoldable[F] with FoldableNFunctions[F]

Data structures that can be folded to a summary value.

In the case of a collection (such as List or Vector), these methods will fold together (combine) the values contained in the collection to produce a single result. Most collection types have foldLeft methods, which will usually be used by the associated Foldable[_] instance.

Instances of Foldable should be ordered collections to allow for consistent folding. Use the UnorderedFoldable type class if you want to fold over unordered collections.

Foldable[F] is implemented in terms of two basic methods:

  • foldLeft(fa, b)(f) eagerly folds fa from left-to-right.
  • foldRight(fa, b)(f) lazily folds fa from right-to-left.

Beyond these it provides many other useful methods related to folding over F[A] values.

See: A tutorial on the universality and expressiveness of fold

Self Type
Foldable[F]
Annotations
@implicitNotFound( ... ) @typeclass( List("FoldableNFunctions") , ... )
Source
Foldable.scala
Linear Supertypes
FoldableNFunctions[F], UnorderedFoldable[F], Serializable, Serializable, AnyRef, Any
Known Subclasses
NonEmptyReducible, NonEmptyTraverse, Reducible, Traverse
Ordering
  1. Grouped
  2. Alphabetic
  3. By Inheritance
Inherited
  1. Foldable
  2. FoldableNFunctions
  3. UnorderedFoldable
  4. Serializable
  5. Serializable
  6. AnyRef
  7. Any
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Visibility
  1. Public
  2. All

Abstract Value Members

  1. abstract def foldLeft[A, B](fa: F[A], b: B)(f: (B, A) ⇒ B): B

    Left associative fold on 'F' using the function 'f'.

    Left associative fold on 'F' using the function 'f'.

    Example:

    scala> import cats.Foldable, cats.implicits._
scala> val fa = Option(1)

Folding by addition to zero:
scala> Foldable[Option].foldLeft(fa, Option(0))((a, n) => a.map(_ + n))
res0: Option[Int] = Some(1)

    With syntax extensions, foldLeft can be used like:

    Folding `Option` with addition from zero:
scala> fa.foldLeft(Option(0))((a, n) => a.map(_ + n))
res1: Option[Int] = Some(1)

There's also an alias `foldl` which is equivalent:
scala> fa.foldl(Option(0))((a, n) => a.map(_ + n))
res2: Option[Int] = Some(1)
  2. abstract def foldRight[A, B](fa: F[A], lb: Eval[B])(f: (A, Eval[B]) ⇒ Eval[B]): Eval[B]

    Right associative lazy fold on F using the folding function 'f'.

    Right associative lazy fold on F using the folding function 'f'.

    This method evaluates lb lazily (in some cases it will not be needed), and returns a lazy value. We are using (A, Eval[B]) => Eval[B] to support laziness in a stack-safe way. Chained computation should be performed via .map and .flatMap.

    For more detailed information about how this method works see the documentation for Eval[_].

    Example:

    scala> import cats.Foldable, cats.Eval, cats.implicits._
scala> val fa = Option(1)

Folding by addition to zero:
scala> val folded1 = Foldable[Option].foldRight(fa, Eval.now(0))((n, a) => a.map(_ + n))
Since `foldRight` yields a lazy computation, we need to force it to inspect the result:
scala> folded1.value
res0: Int = 1

With syntax extensions, we can write the same thing like this:
scala> val folded2 = fa.foldRight(Eval.now(0))((n, a) => a.map(_ + n))
scala> folded2.value
res1: Int = 1

Unfortunately, since `foldRight` is defined on many collections - this
extension clashes with the operation defined in `Foldable`.

To get past this and make sure you're getting the lazy `foldRight` defined
in `Foldable`, there's an alias `foldr`:
scala> val folded3 = fa.foldr(Eval.now(0))((n, a) => a.map(_ + n))
scala> folded3.value
res1: Int = 1

Concrete Value Members

  1. final def !=(arg0: Any): Boolean
    Definition Classes
    AnyRef → Any
  2. final def ##(): Int
    Definition Classes
    AnyRef → Any
  3. final def ==(arg0: Any): Boolean
    Definition Classes
    AnyRef → Any
  4. final def asInstanceOf[T0]: T0
    Definition Classes
    Any
  5. def clone(): AnyRef
    Attributes
    protected[lang]
    Definition Classes
    AnyRef
    Annotations
    @throws( ... ) @native() @IntrinsicCandidate()
  6. def collectFirst[A, B](fa: F[A])(pf: PartialFunction[A, B]): Option[B]
  7. def collectFirstSome[A, B](fa: F[A])(f: (A) ⇒ Option[B]): Option[B]

    Like collectFirst from scala.collection.Traversable but takes A => Option[B] instead of PartialFunctions.

    Like collectFirst from scala.collection.Traversable but takes A => Option[B] instead of PartialFunctions.

    scala> import cats.implicits._
scala> val keys = List(1, 2, 4, 5)
scala> val map = Map(4 -> "Four", 5 -> "Five")
scala> keys.collectFirstSome(map.get)
res0: Option[String] = Some(Four)
scala> val map2 = Map(6 -> "Six", 7 -> "Seven")
scala> keys.collectFirstSome(map2.get)
res1: Option[String] = None
  8. def collectFirstSomeM[G[_], A, B](fa: F[A])(f: (A) ⇒ G[Option[B]])(implicit G: Monad[G]): G[Option[B]]

    Monadic version of collectFirstSome.

    Monadic version of collectFirstSome.

    If there are no elements, the result is None. collectFirstSomeM short-circuits, i.e. once a Some element is found, no further effects are produced.

    For example:

    scala> import cats.implicits._
scala> def parseInt(s: String): Either[String, Int] = Either.catchOnly[NumberFormatException](s.toInt).leftMap(_.getMessage)
scala> val keys1 = List("1", "2", "4", "5")
scala> val map1 = Map(4 -> "Four", 5 -> "Five")
scala> Foldable[List].collectFirstSomeM(keys1)(parseInt(_) map map1.get)
res0: scala.util.Either[String,Option[String]] = Right(Some(Four))

scala> val map2 = Map(6 -> "Six", 7 -> "Seven")
scala> Foldable[List].collectFirstSomeM(keys1)(parseInt(_) map map2.get)
res1: scala.util.Either[String,Option[String]] = Right(None)

scala> val keys2 = List("1", "x", "4", "5")
scala> Foldable[List].collectFirstSomeM(keys2)(parseInt(_) map map1.get)
res2: scala.util.Either[String,Option[String]] = Left(For input string: "x")

scala> val keys3 = List("1", "2", "4", "x")
scala> Foldable[List].collectFirstSomeM(keys3)(parseInt(_) map map1.get)
res3: scala.util.Either[String,Option[String]] = Right(Some(Four))
    Annotations
    @noop()
  9. def collectFold[A, B](fa: F[A])(f: PartialFunction[A, B])(implicit B: Monoid[B]): B

    Tear down a subset of this structure using a PartialFunction.

    Tear down a subset of this structure using a PartialFunction.

    scala> import cats.implicits._
scala> val xs = List(1, 2, 3, 4)
scala> Foldable[List].collectFold(xs) { case n if n % 2 == 0 => n }
res0: Int = 6
    Annotations
    @noop()
  10. def collectFoldSome[A, B](fa: F[A])(f: (A) ⇒ Option[B])(implicit B: Monoid[B]): B

    Tear down a subset of this structure using a A => Option[M].

    Tear down a subset of this structure using a A => Option[M].

    scala> import cats.implicits._
scala> val xs = List(1, 2, 3, 4)
scala> def f(n: Int): Option[Int] = if (n % 2 == 0) Some(n) else None
scala> Foldable[List].collectFoldSome(xs)(f)
res0: Int = 6
  11. def combineAll[A](fa: F[A])(implicit arg0: Monoid[A]): A

    Alias for fold.

  12. def combineAllOption[A](fa: F[A])(implicit ev: Semigroup[A]): Option[A]
  13. def compose[G[_]](implicit arg0: Foldable[G]): Foldable[[α]F[G[α]]]
  14. def count[A](fa: F[A])(p: (A) ⇒ Boolean): Long

    Count the number of elements in the structure that satisfy the given predicate.

    Count the number of elements in the structure that satisfy the given predicate.

    For example:

    scala> import cats.implicits._
scala> val map1 = Map[Int, String]()
scala> val p1: String => Boolean = _.length > 0
scala> UnorderedFoldable[Map[Int, *]].count(map1)(p1)
res0: Long = 0

scala> val map2 = Map(1 -> "hello", 2 -> "world", 3 -> "!")
scala> val p2: String => Boolean = _.length > 1
scala> UnorderedFoldable[Map[Int, *]].count(map2)(p2)
res1: Long = 2
    Definition Classes
    UnorderedFoldable
    Annotations
    @noop()
  15. def dropWhile_[A](fa: F[A])(p: (A) ⇒ Boolean): List[A]

    Convert F[A] to a List[A], dropping all initial elements which match p.

  16. final def eq(arg0: AnyRef): Boolean
    Definition Classes
    AnyRef
  17. def equals(arg0: Any): Boolean
    Definition Classes
    AnyRef → Any
  18. def exists[A](fa: F[A])(p: (A) ⇒ Boolean): Boolean

    Check whether at least one element satisfies the predicate.

    Check whether at least one element satisfies the predicate.

    If there are no elements, the result is false.

    Definition Classes
    Foldable → UnorderedFoldable
  19. def existsM[G[_], A](fa: F[A])(p: (A) ⇒ G[Boolean])(implicit G: Monad[G]): G[Boolean]

    Check whether at least one element satisfies the effectful predicate.

    Check whether at least one element satisfies the effectful predicate.

    If there are no elements, the result is false. existsM short-circuits, i.e. once a true result is encountered, no further effects are produced.

    For example:

    scala> import cats.implicits._
scala> val F = Foldable[List]
scala> F.existsM(List(1,2,3,4))(n => Option(n <= 4))
res0: Option[Boolean] = Some(true)

scala> F.existsM(List(1,2,3,4))(n => Option(n > 4))
res1: Option[Boolean] = Some(false)

scala> F.existsM(List(1,2,3,4))(n => if (n <= 2) Option(true) else Option(false))
res2: Option[Boolean] = Some(true)

scala> F.existsM(List(1,2,3,4))(n => if (n <= 2) Option(true) else None)
res3: Option[Boolean] = Some(true)

scala> F.existsM(List(1,2,3,4))(n => if (n <= 2) None else Option(true))
res4: Option[Boolean] = None
  20. def filter_[A](fa: F[A])(p: (A) ⇒ Boolean): List[A]

    Convert F[A] to a List[A], only including elements which match p.

  21. def find[A](fa: F[A])(f: (A) ⇒ Boolean): Option[A]

    Find the first element matching the predicate, if one exists.

  22. def findM[G[_], A](fa: F[A])(p: (A) ⇒ G[Boolean])(implicit G: Monad[G]): G[Option[A]]

    Find the first element matching the effectful predicate, if one exists.

    Find the first element matching the effectful predicate, if one exists.

    If there are no elements, the result is None. findM short-circuits, i.e. once an element is found, no further effects are produced.

    For example:

    scala> import cats.implicits._
scala> val list = List(1,2,3,4)
scala> Foldable[List].findM(list)(n => (n >= 2).asRight[String])
res0: Either[String,Option[Int]] = Right(Some(2))

scala> Foldable[List].findM(list)(n => (n > 4).asRight[String])
res1: Either[String,Option[Int]] = Right(None)

scala> Foldable[List].findM(list)(n => Either.cond(n < 3, n >= 2, "error"))
res2: Either[String,Option[Int]] = Right(Some(2))

scala> Foldable[List].findM(list)(n => Either.cond(n < 3, false, "error"))
res3: Either[String,Option[Int]] = Left(error)
    Annotations
    @noop()
  23. def fold[A](fa: F[A])(implicit A: Monoid[A]): A

    Fold implemented using the given Monoid[A] instance.

  24. def foldA[G[_], A](fga: F[G[A]])(implicit G: Applicative[G], A: Monoid[A]): G[A]

    Fold implemented using the given Applicative[G] and Monoid[A] instance.

    Fold implemented using the given Applicative[G] and Monoid[A] instance.

    This method is similar to fold, but may short-circuit.

    For example:

    scala> import cats.implicits._
scala> val F = Foldable[List]
scala> F.foldA(List(Either.right[String, Int](1), Either.right[String, Int](2)))
res0: Either[String, Int] = Right(3)

    See this issue for an explanation of @noop usage.

    Annotations
    @noop()
  25. def foldK[G[_], A](fga: F[G[A]])(implicit G: MonoidK[G]): G[A]

    Fold implemented using the given MonoidK[G] instance.

    Fold implemented using the given MonoidK[G] instance.

    This method is identical to fold, except that we use the universal monoid (MonoidK[G]) to get a Monoid[G[A]] instance.

    For example:

    scala> import cats.implicits._
scala> val F = Foldable[List]
scala> F.foldK(List(1 :: 2 :: Nil, 3 :: 4 :: 5 :: Nil))
res0: List[Int] = List(1, 2, 3, 4, 5)
  26. final def foldLeftM[G[_], A, B](fa: F[A], z: B)(f: (B, A) ⇒ G[B])(implicit G: Monad[G]): G[B]

    Alias for foldM.

  27. def foldM[G[_], A, B](fa: F[A], z: B)(f: (B, A) ⇒ G[B])(implicit G: Monad[G]): G[B]

    Perform a stack-safe monadic left fold from the source context F into the target monad G.

    Perform a stack-safe monadic left fold from the source context F into the target monad G.

    This method can express short-circuiting semantics. Even when fa is an infinite structure, this method can potentially terminate if the foldRight implementation for F and the tailRecM implementation for G are sufficiently lazy.

    Instances for concrete structures (e.g. List) will often have a more efficient implementation than the default one in terms of foldRight.

  28. def foldMap[A, B](fa: F[A])(f: (A) ⇒ B)(implicit B: Monoid[B]): B

    Fold implemented by mapping A values into B and then combining them using the given Monoid[B] instance.

  29. def foldMapA[G[_], A, B](fa: F[A])(f: (A) ⇒ G[B])(implicit G: Applicative[G], B: Monoid[B]): G[B]

    Fold in an Applicative context by mapping the A values to G[B].

    Fold in an Applicative context by mapping the A values to G[B]. combining the B values using the given Monoid[B] instance.

    Similar to foldMapM, but will typically be less efficient.

    scala> import cats.Foldable
scala> import cats.implicits._
scala> val evenNumbers = List(2,4,6,8,10)
scala> val evenOpt: Int => Option[Int] =
     |   i => if (i % 2 == 0) Some(i) else None
scala> Foldable[List].foldMapA(evenNumbers)(evenOpt)
res0: Option[Int] = Some(30)
scala> Foldable[List].foldMapA(evenNumbers :+ 11)(evenOpt)
res1: Option[Int] = None
  30. def foldMapK[G[_], A, B](fa: F[A])(f: (A) ⇒ G[B])(implicit G: MonoidK[G]): G[B]

    Fold implemented by mapping A values into B in a context G and then combining them using the MonoidK[G] instance.

    Fold implemented by mapping A values into B in a context G and then combining them using the MonoidK[G] instance.

    scala> import cats._, cats.implicits._
scala> val f: Int => Endo[String] = i => (s => s + i)
scala> val x: Endo[String] = Foldable[List].foldMapK(List(1, 2, 3))(f)
scala> val a = x("foo")
a: String = "foo321"
    Annotations
    @noop()
  31. def foldMapM[G[_], A, B](fa: F[A])(f: (A) ⇒ G[B])(implicit G: Monad[G], B: Monoid[B]): G[B]

    Monadic folding on F by mapping A values to G[B], combining the B values using the given Monoid[B] instance.

    Monadic folding on F by mapping A values to G[B], combining the B values using the given Monoid[B] instance.

    Similar to foldM, but using a Monoid[B]. Will typically be more efficient than foldMapA.

    scala> import cats.Foldable
scala> import cats.implicits._
scala> val evenNumbers = List(2,4,6,8,10)
scala> val evenOpt: Int => Option[Int] =
     |   i => if (i % 2 == 0) Some(i) else None
scala> Foldable[List].foldMapM(evenNumbers)(evenOpt)
res0: Option[Int] = Some(30)
scala> Foldable[List].foldMapM(evenNumbers :+ 11)(evenOpt)
res1: Option[Int] = None
  32. def foldRightDefer[G[_], A, B](fa: F[A], gb: G[B])(fn: (A, G[B]) ⇒ G[B])(implicit arg0: Defer[G]): G[B]
  33. def forall[A](fa: F[A])(p: (A) ⇒ Boolean): Boolean

    Check whether all elements satisfy the predicate.

    Check whether all elements satisfy the predicate.

    If there are no elements, the result is true.

    Definition Classes
    Foldable → UnorderedFoldable
  34. def forallM[G[_], A](fa: F[A])(p: (A) ⇒ G[Boolean])(implicit G: Monad[G]): G[Boolean]

    Check whether all elements satisfy the effectful predicate.

    Check whether all elements satisfy the effectful predicate.

    If there are no elements, the result is true. forallM short-circuits, i.e. once a false result is encountered, no further effects are produced.

    For example:

    scala> import cats.implicits._
scala> val F = Foldable[List]
scala> F.forallM(List(1,2,3,4))(n => Option(n <= 4))
res0: Option[Boolean] = Some(true)

scala> F.forallM(List(1,2,3,4))(n => Option(n <= 1))
res1: Option[Boolean] = Some(false)

scala> F.forallM(List(1,2,3,4))(n => if (n <= 2) Option(true) else Option(false))
res2: Option[Boolean] = Some(false)

scala> F.forallM(List(1,2,3,4))(n => if (n <= 2) Option(false) else None)
res3: Option[Boolean] = Some(false)

scala> F.forallM(List(1,2,3,4))(n => if (n <= 2) None else Option(false))
res4: Option[Boolean] = None
  35. def get[A](fa: F[A])(idx: Long): Option[A]

    Get the element at the index of the Foldable.

  36. final def getClass(): Class[_]
    Definition Classes
    AnyRef → Any
    Annotations
    @native() @IntrinsicCandidate()
  37. def hashCode(): Int
    Definition Classes
    AnyRef → Any
    Annotations
    @native() @IntrinsicCandidate()
  38. def intercalate[A](fa: F[A], a: A)(implicit A: Monoid[A]): A

    Intercalate/insert an element between the existing elements while folding.

    Intercalate/insert an element between the existing elements while folding.

    scala> import cats.implicits._
scala> Foldable[List].intercalate(List("a","b","c"), "-")
res0: String = a-b-c
scala> Foldable[List].intercalate(List("a"), "-")
res1: String = a
scala> Foldable[List].intercalate(List.empty[String], "-")
res2: String = ""
scala> Foldable[Vector].intercalate(Vector(1,2,3), 1)
res3: Int = 8
  39. def intersperseList[A](xs: List[A], x: A): List[A]
    Attributes
    protected
  40. def isEmpty[A](fa: F[A]): Boolean

    Returns true if there are no elements.

    Returns true if there are no elements. Otherwise false.

    Definition Classes
    Foldable → UnorderedFoldable
  41. final def isInstanceOf[T0]: Boolean
    Definition Classes
    Any
  42. def maximumByList[A, B](fa: F[A])(f: (A) ⇒ B)(implicit arg0: Order[B]): List[A]

    Find all the maximum A items in this structure according to an Order.by(f).

    Find all the maximum A items in this structure according to an Order.by(f). For all elements in the result Order.eqv(x, y) is true. Preserves order.

    See also

    Reducible#maximumByNel for a version that doesn't need to return an Option for structures that are guaranteed to be non-empty.

    minimumByList for minimum instead of maximum.

  43. def maximumByOption[A, B](fa: F[A])(f: (A) ⇒ B)(implicit arg0: Order[B]): Option[A]

    Find the maximum A item in this structure according to an Order.by(f).

    Find the maximum A item in this structure according to an Order.by(f).

    returns

    None if the structure is empty, otherwise the maximum element wrapped in a Some.

    See also

    Reducible#maximumBy for a version that doesn't need to return an Option for structures that are guaranteed to be non-empty.

    minimumByOption for minimum instead of maximum.

  44. def maximumList[A](fa: F[A])(implicit A: Order[A]): List[A]

    Find all the maximum A items in this structure.

    Find all the maximum A items in this structure. For all elements in the result Order.eqv(x, y) is true. Preserves order.

    See also

    Reducible#maximumNel for a version that doesn't need to return an Option for structures that are guaranteed to be non-empty.

    minimumList for minimum instead of maximum.

  45. def maximumOption[A](fa: F[A])(implicit A: Order[A]): Option[A]

    Find the maximum A item in this structure according to the Order[A].

    Find the maximum A item in this structure according to the Order[A].

    returns

    None if the structure is empty, otherwise the maximum element wrapped in a Some.

    See also

    Reducible#maximum for a version that doesn't need to return an Option for structures that are guaranteed to be non-empty.

    minimumOption for minimum instead of maximum.

  46. def minimumByList[A, B](fa: F[A])(f: (A) ⇒ B)(implicit arg0: Order[B]): List[A]

    Find all the minimum A items in this structure according to an Order.by(f).

    Find all the minimum A items in this structure according to an Order.by(f). For all elements in the result Order.eqv(x, y) is true. Preserves order.

    See also

    Reducible#minimumByNel for a version that doesn't need to return an Option for structures that are guaranteed to be non-empty.

    maximumByList for maximum instead of minimum.

  47. def minimumByOption[A, B](fa: F[A])(f: (A) ⇒ B)(implicit arg0: Order[B]): Option[A]

    Find the minimum A item in this structure according to an Order.by(f).

    Find the minimum A item in this structure according to an Order.by(f).

    returns

    None if the structure is empty, otherwise the minimum element wrapped in a Some.

    See also

    Reducible#minimumBy for a version that doesn't need to return an Option for structures that are guaranteed to be non-empty.

    maximumByOption for maximum instead of minimum.

  48. def minimumList[A](fa: F[A])(implicit A: Order[A]): List[A]

    Find all the minimum A items in this structure.

    Find all the minimum A items in this structure. For all elements in the result Order.eqv(x, y) is true. Preserves order.

    See also

    Reducible#minimumNel for a version that doesn't need to return an Option for structures that are guaranteed to be non-empty.

    maximumList for maximum instead of minimum.

  49. def minimumOption[A](fa: F[A])(implicit A: Order[A]): Option[A]

    Find the minimum A item in this structure according to the Order[A].

    Find the minimum A item in this structure according to the Order[A].

    returns

    None if the structure is empty, otherwise the minimum element wrapped in a Some.

    See also

    Reducible#minimum for a version that doesn't need to return an Option for structures that are guaranteed to be non-empty.

    maximumOption for maximum instead of minimum.

  50. final def ne(arg0: AnyRef): Boolean
    Definition Classes
    AnyRef
  51. def nonEmpty[A](fa: F[A]): Boolean
    Definition Classes
    Foldable → UnorderedFoldable
  52. final def notify(): Unit
    Definition Classes
    AnyRef
    Annotations
    @native() @IntrinsicCandidate()
  53. final def notifyAll(): Unit
    Definition Classes
    AnyRef
    Annotations
    @native() @IntrinsicCandidate()
  54. def partitionBifold[H[_, _], A, B, C](fa: F[A])(f: (A) ⇒ H[B, C])(implicit A: Alternative[F], H: Bifoldable[H]): (F[B], F[C])

    Separate this Foldable into a Tuple by a separating function A => H[B, C] for some Bifoldable[H] Equivalent to Functor#map and then Alternative#separate.

    Separate this Foldable into a Tuple by a separating function A => H[B, C] for some Bifoldable[H] Equivalent to Functor#map and then Alternative#separate.

    scala> import cats.implicits._, cats.Foldable, cats.data.Const
scala> val list = List(1,2,3,4)
scala> Foldable[List].partitionBifold(list)(a => ("value " + a.toString(), if (a % 2 == 0) -a else a))
res0: (List[String], List[Int]) = (List(value 1, value 2, value 3, value 4),List(1, -2, 3, -4))
scala> Foldable[List].partitionBifold(list)(a => Const[Int, Nothing with Any](a))
res1: (List[Int], List[Nothing with Any]) = (List(1, 2, 3, 4),List())
    Annotations
    @noop()
  55. def partitionBifoldM[G[_], H[_, _], A, B, C](fa: F[A])(f: (A) ⇒ G[H[B, C]])(implicit A: Alternative[F], M: Monad[G], H: Bifoldable[H]): G[(F[B], F[C])]

    Separate this Foldable into a Tuple by an effectful separating function A => G[H[B, C]] for some Bifoldable[H] Equivalent to Traverse#traverse over Alternative#separate

    Separate this Foldable into a Tuple by an effectful separating function A => G[H[B, C]] for some Bifoldable[H] Equivalent to Traverse#traverse over Alternative#separate

    scala> import cats.implicits._, cats.Foldable, cats.data.Const
scala> val list = List(1,2,3,4)
`Const`'s second parameter is never instantiated, so we can use an impossible type:
scala> Foldable[List].partitionBifoldM(list)(a => Option(Const[Int, Nothing with Any](a)))
res0: Option[(List[Int], List[Nothing with Any])] = Some((List(1, 2, 3, 4),List()))
    Annotations
    @noop()
  56. def partitionEither[A, B, C](fa: F[A])(f: (A) ⇒ Either[B, C])(implicit A: Alternative[F]): (F[B], F[C])

    Separate this Foldable into a Tuple by a separating function A => Either[B, C] Equivalent to Functor#map and then Alternative#separate.

    Separate this Foldable into a Tuple by a separating function A => Either[B, C] Equivalent to Functor#map and then Alternative#separate.

    scala> import cats.implicits._
scala> val list = List(1,2,3,4)
scala> Foldable[List].partitionEither(list)(a => if (a % 2 == 0) Left(a.toString) else Right(a))
res0: (List[String], List[Int]) = (List(2, 4),List(1, 3))
scala> Foldable[List].partitionEither(list)(a => Right(a * 4))
res1: (List[Nothing], List[Int]) = (List(),List(4, 8, 12, 16))
  57. def partitionEitherM[G[_], A, B, C](fa: F[A])(f: (A) ⇒ G[Either[B, C]])(implicit A: Alternative[F], M: Monad[G]): G[(F[B], F[C])]

    Separate this Foldable into a Tuple by an effectful separating function A => G[Either[B, C]] Equivalent to Traverse#traverse over Alternative#separate

    Separate this Foldable into a Tuple by an effectful separating function A => G[Either[B, C]] Equivalent to Traverse#traverse over Alternative#separate

    scala> import cats.implicits._, cats.Foldable, cats.Eval
scala> val list = List(1,2,3,4)
scala> val partitioned1 = Foldable[List].partitionEitherM(list)(a => if (a % 2 == 0) Eval.now(Either.left[String, Int](a.toString)) else Eval.now(Either.right[String, Int](a)))
Since `Eval.now` yields a lazy computation, we need to force it to inspect the result:
scala> partitioned1.value
res0: (List[String], List[Int]) = (List(2, 4),List(1, 3))
scala> val partitioned2 = Foldable[List].partitionEitherM(list)(a => Eval.later(Either.right(a * 4)))
scala> partitioned2.value
res1: (List[Nothing], List[Int]) = (List(),List(4, 8, 12, 16))
    Annotations
    @noop()
  58. def productAll[A](fa: F[A])(implicit A: Numeric[A]): A
  59. def reduceLeftOption[A](fa: F[A])(f: (A, A) ⇒ A): Option[A]

    Reduce the elements of this structure down to a single value by applying the provided aggregation function in a left-associative manner.

    Reduce the elements of this structure down to a single value by applying the provided aggregation function in a left-associative manner.

    returns

    None if the structure is empty, otherwise the result of combining the cumulative left-associative result of the f operation over all of the elements.

    See also

    reduceRightOption for a right-associative alternative.

    Reducible#reduceLeft for a version that doesn't need to return an Option for structures that are guaranteed to be non-empty. Example:

    scala> import cats.implicits._
scala> val l = List(6, 3, 2)
This is equivalent to (6 - 3) - 2
scala> Foldable[List].reduceLeftOption(l)(_ - _)
res0: Option[Int] = Some(1)

scala> Foldable[List].reduceLeftOption(List.empty[Int])(_ - _)
res1: Option[Int] = None
  60. def reduceLeftToOption[A, B](fa: F[A])(f: (A) ⇒ B)(g: (B, A) ⇒ B): Option[B]
  61. def reduceRightOption[A](fa: F[A])(f: (A, Eval[A]) ⇒ Eval[A]): Eval[Option[A]]

    Reduce the elements of this structure down to a single value by applying the provided aggregation function in a right-associative manner.

    Reduce the elements of this structure down to a single value by applying the provided aggregation function in a right-associative manner.

    returns

    None if the structure is empty, otherwise the result of combining the cumulative right-associative result of the f operation over the A elements.

    See also

    reduceLeftOption for a left-associative alternative

    Reducible#reduceRight for a version that doesn't need to return an Option for structures that are guaranteed to be non-empty. Example:

    scala> import cats.implicits._
scala> val l = List(6, 3, 2)
This is equivalent to 6 - (3 - 2)
scala> Foldable[List].reduceRightOption(l)((current, rest) => rest.map(current - _)).value
res0: Option[Int] = Some(5)

scala> Foldable[List].reduceRightOption(List.empty[Int])((current, rest) => rest.map(current - _)).value
res1: Option[Int] = None
  62. def reduceRightToOption[A, B](fa: F[A])(f: (A) ⇒ B)(g: (A, Eval[B]) ⇒ Eval[B]): Eval[Option[B]]
  63. def sequence_[G[_], A](fga: F[G[A]])(implicit arg0: Applicative[G]): G[Unit]

    Sequence F[G[A]] using Applicative[G].

    Sequence F[G[A]] using Applicative[G].

    This is similar to traverse_ except it operates on F[G[A]] values, so no additional functions are needed.

    For example:

    scala> import cats.implicits._
scala> val F = Foldable[List]
scala> F.sequence_(List(Option(1), Option(2), Option(3)))
res0: Option[Unit] = Some(())
scala> F.sequence_(List(Option(1), None, Option(3)))
res1: Option[Unit] = None
  64. def size[A](fa: F[A]): Long

    The size of this UnorderedFoldable.

    The size of this UnorderedFoldable.

    This is overridden in structures that have more efficient size implementations (e.g. Vector, Set, Map).

    Note: will not terminate for infinite-sized collections.

    Definition Classes
    UnorderedFoldable
  65. def sliding10[A](fa: F[A]): List[(A, A, A, A, A, A, A, A, A, A)]

    Definition Classes
    FoldableNFunctions
  66. def sliding11[A](fa: F[A]): List[(A, A, A, A, A, A, A, A, A, A, A)]

    Definition Classes
    FoldableNFunctions
  67. def sliding12[A](fa: F[A]): List[(A, A, A, A, A, A, A, A, A, A, A, A)]

    Definition Classes
    FoldableNFunctions
  68. def sliding13[A](fa: F[A]): List[(A, A, A, A, A, A, A, A, A, A, A, A, A)]

    Definition Classes
    FoldableNFunctions
  69. def sliding14[A](fa: F[A]): List[(A, A, A, A, A, A, A, A, A, A, A, A, A, A)]

    Definition Classes
    FoldableNFunctions
  70. def sliding15[A](fa: F[A]): List[(A, A, A, A, A, A, A, A, A, A, A, A, A, A, A)]

    Definition Classes
    FoldableNFunctions
  71. def sliding16[A](fa: F[A]): List[(A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A)]

    Definition Classes
    FoldableNFunctions
  72. def sliding17[A](fa: F[A]): List[(A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A)]

    Definition Classes
    FoldableNFunctions
  73. def sliding18[A](fa: F[A]): List[(A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A)]

    Definition Classes
    FoldableNFunctions
  74. def sliding19[A](fa: F[A]): List[(A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A)]

    Definition Classes
    FoldableNFunctions
  75. def sliding2[A](fa: F[A]): List[(A, A)]

    Definition Classes
    FoldableNFunctions
  76. def sliding20[A](fa: F[A]): List[(A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A)]

    Definition Classes
    FoldableNFunctions
  77. def sliding21[A](fa: F[A]): List[(A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A)]

    Definition Classes
    FoldableNFunctions
  78. def sliding22[A](fa: F[A]): List[(A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A, A)]

    Definition Classes
    FoldableNFunctions
  79. def sliding3[A](fa: F[A]): List[(A, A, A)]

    Definition Classes
    FoldableNFunctions
  80. def sliding4[A](fa: F[A]): List[(A, A, A, A)]

    Definition Classes
    FoldableNFunctions
  81. def sliding5[A](fa: F[A]): List[(A, A, A, A, A)]

    Definition Classes
    FoldableNFunctions
  82. def sliding6[A](fa: F[A]): List[(A, A, A, A, A, A)]

    Definition Classes
    FoldableNFunctions
  83. def sliding7[A](fa: F[A]): List[(A, A, A, A, A, A, A)]

    Definition Classes
    FoldableNFunctions
  84. def sliding8[A](fa: F[A]): List[(A, A, A, A, A, A, A, A)]

    Definition Classes
    FoldableNFunctions
  85. def sliding9[A](fa: F[A]): List[(A, A, A, A, A, A, A, A, A)]

    Definition Classes
    FoldableNFunctions
  86. def sumAll[A](fa: F[A])(implicit A: Numeric[A]): A
  87. final def synchronized[T0](arg0: ⇒ T0): T0
    Definition Classes
    AnyRef
  88. def takeWhile_[A](fa: F[A])(p: (A) ⇒ Boolean): List[A]

    Convert F[A] to a List[A], retaining only initial elements which match p.

  89. def toIterable[A](fa: F[A]): Iterable[A]

    Convert F[A] to an Iterable[A].

    Convert F[A] to an Iterable[A].

    This method may be overridden for the sake of performance, but implementers should take care not to force a full materialization of the collection.

  90. def toList[A](fa: F[A]): List[A]

    Convert F[A] to a List[A].

  91. def toString(): String
    Definition Classes
    AnyRef → Any
  92. def traverse_[G[_], A, B](fa: F[A])(f: (A) ⇒ G[B])(implicit G: Applicative[G]): G[Unit]

    Traverse F[A] using Applicative[G].

    Traverse F[A] using Applicative[G].

    A values will be mapped into G[B] and combined using Applicative#map2.

    For example:

    scala> import cats.implicits._
scala> def parseInt(s: String): Option[Int] = Either.catchOnly[NumberFormatException](s.toInt).toOption
scala> val F = Foldable[List]
scala> F.traverse_(List("333", "444"))(parseInt)
res0: Option[Unit] = Some(())
scala> F.traverse_(List("333", "zzz"))(parseInt)
res1: Option[Unit] = None

    This method is primarily useful when G[_] represents an action or effect, and the specific A aspect of G[A] is not otherwise needed.

  93. def unorderedFold[A](fa: F[A])(implicit arg0: CommutativeMonoid[A]): A
    Definition Classes
    Foldable → UnorderedFoldable
  94. def unorderedFoldMap[A, B](fa: F[A])(f: (A) ⇒ B)(implicit arg0: CommutativeMonoid[B]): B
    Definition Classes
    Foldable → UnorderedFoldable
  95. final def wait(arg0: Long, arg1: Int): Unit
    Definition Classes
    AnyRef
    Annotations
    @throws( ... )
  96. final def wait(arg0: Long): Unit
    Definition Classes
    AnyRef
    Annotations
    @throws( ... ) @native()
  97. final def wait(): Unit
    Definition Classes
    AnyRef
    Annotations
    @throws( ... )

Deprecated Value Members

  1. def finalize(): Unit
    Attributes
    protected[lang]
    Definition Classes
    AnyRef
    Annotations
    @throws( classOf[java.lang.Throwable] ) @Deprecated
    Deprecated

Inherited from FoldableNFunctions[F]

Inherited from UnorderedFoldable[F]

Inherited from Serializable

Inherited from Serializable

Inherited from AnyRef

Inherited from Any

Ungrouped

foldable arity

Group sequential elements into fixed sized tuples by passing a "sliding window" over them. A foldable with fewer elements than the window size will return an empty list unlike Iterable#sliding(size: Int). Example:

import cats.Foldable
scala> Foldable[List].sliding2((1 to 10).toList)
val res0: List[(Int, Int)] = List((1,2), (2,3), (3,4), (4,5), (5,6), (6,7), (7,8), (8,9), (9,10))

scala> Foldable[List].sliding4((1 to 10).toList)
val res1: List[(Int, Int, Int, Int)] = List((1,2,3,4), (2,3,4,5), (3,4,5,6), (4,5,6,7), (5,6,7,8), (6,7,8,9), (7,8,9,10))

scala> Foldable[List].sliding4((1 to 2).toList)
val res2: List[(Int, Int, Int, Int)] = List()