Scalaz provides foldable to define all fold operations. It has Foldable[F[_]] trait with two foldMap and foldRight abstract methods:

def foldMap[A,B](fa: F[A])(f: A ⇒ B)(implicit F: Monoid[B]): B
def foldRight[A, B](fa: F[A], z: ⇒ B)(f: (A, => B) => B): B

foldMap says having a foldable A, a mapping function from A to B and B is monoid it will fold F[A] into B.

foldRight says having a foldable A, initial value z of type B, and a mapping function from (A, B) combination to B, it will fold fa int B. By providing implementations for those two methods we’ll get following operations on foldable for free.

  • foldMap. As already mentioned having B be a monoid and a mapping from A to B it will fold into B. Here are some examples:
    • List(1, 2, 3).foldMap() = 6 (which is same as List(1, 2, 3).foldMap((a: Int) ⇒ a) since identity is the default value for mapping function). Not that here Monoid for integer is used on zero and plus options so we get back the sum by doing foldMap. Another example with int to string mapping:
    • List(1, 2, 3).foldMap((a: Int) ⇒ a.toString) = "123", and here monoid for string is used and we get back a concatenated string.
    • nil[Int].foldMap() = 0 since B is monoid (int in this instance) when foldable is empty it will fold it into zero.
  • foldMap1Opt. Where foldMap requires B to be monoid, foldMap1Opt releives it a bit by requiring B to be a semigroup instead of monid (meaning it doesn’t have zero) and returnign Option[B]. Examples:
    • List(1, 2, 3).foldMap1Opt() = Some(6) here we get the sum back since the plus is the append operator of int semigroup we imported.
    • nil[Int].foldMap1Opt() = None. Here we get None since your List is empty.
  • foldRight. As already mentioned is one of the abstract methods that need to be implemented. Let’s look at it for list of integeres. List(1, 2, 3, 4).foldRight(1)(_ * _) = 24 here we’re getting product back as I defined my inital value 1 and my mapping function is product( of course there is an easier way to do this in scalaz, which I’ll conver when I get to Traverse but this will do good for the demo.)
  • foldMapRight1Opt. Similar to foldRight but instead of providing starting value we provide a function that returns starting value. But here if it’s empty, starting value won’t be there, so it returns optional. Examples:
    • List(1, 2, 3).foldMapRight1Opt(a ⇒ a + 1)(_ + _) = Some(7)
    • nil[Int].foldMapRight1Opt(a ⇒ a + 1)(_ + _) = None
  • foldRight1Opt is similar to foldMapRight1Opt but intead of taking a function for starting value it defaults to identity function.
    • List(1, 2, 3).foldRight1Opt(_ + _) = Some(6)
    • nil[Int].foldRight1Opt(_ + _) = None
  • foldLeft. Nothing fancy here just a regular fold left. Instard of folding from right it folds from left. List(1, 2, 3).foldLeft(0)(_ - _) = -6 where fold right will give 2.
  • foldMapLeft1Opt and foldLeft1Opt. Similar to their right version except it’s fold left.
  • foldRightM simpilar to foldRight but mapping function maps (a, b) compbination to G[B] provided G monad and returns G[B]. Example: Option is a monad List(1, 2, 3).foldRightM(0)((a, b) ⇒ Option(a + b)) = Some(6)
  • foldLeftM similar to foldRightM but does fold left.
  • foldMapM similar to foldRightM where B is a monoid and it can do fold map on monoid. List(1, 2, 3).foldMapM(a ⇒ Option(a.toString)) = Some(123)
  • fold or concatenate do what foldMap does except here A and B are the same and mapping function is the identity, so it will fold to a value of same type using the monoid.
  • foldr and foldr1Opt are curied versions of foldRight and foldRight1Opt
  • foldl and foldl1Opt are curied versions of foldLeft and foldLeft1Opt
  • foldrM and foldlM are curied versions of foldRightM and foldLeftM
  • length or count calculates lenght using foldLeft. List(1, 2, 3).length = 3
  • index find item at given index using foldLeft and returns optional.
    • List(1, 2, 3).index(0) = Some(1)
    • List(1, 2, 3).index(3) = None
  • indexOr similar to index but instead of returning optional it takes default value.
    • List(1, 2, 3).indexOr(-1, 0) = 1
    • List(1, 2, 3).indexOr(-1, 3) = -1
  • sumr and suml are calculateing sum, one using foldRight other using foldLeft
    • List(1, 2, 3).sumr = 6
    • List(1, 2, 3).suml = 6
  • toList uses foldLeft and list’s Nil and :: cons operators to build list.
  • toIndexedSeq uses foldLeft and :+ cons operators to build indexed sequence.
  • toSet uses foldLeft and + cons operators to build set.
  • toStream uses foldRight and Stream’s cons operators to build Stream.
  • toIList uses foldLeft and :: cons operators to scalaz IList.
  • toEphemeralStream uses foldRight and EphemeralStream’s cons operators to build scalaz EphemeralStream.
  • ∀ or all use foldRight to determine if predicate is true for all elements List(1, 1, 1) ∀ (a ⇒ (a % 2) ≠ 0) = true
  • ∃ or any use foldRight to determine if predicate is true for at least one element List(1, 2, 3) ∃ (a ⇒ (a % 2) ≟ 0) = true
  • allM and anyM are monadic verions of all and any.
  • maximum gets the maxinum of the values List(1, 2, 3).maximum = Some(3)
  • maximumOf gets the maxinum value maping function generates. List(0D, math.Pi).maximumOf(math.cos) = Some(1.0)
  • maximumBy gets the element on which maping function generates the greates value. List(0D, math.Pi).maximumBy(math.cos) = Some(0.0)
  • minimum gets the minimum of the values List(1, 2, 3).minimum = Some(1)
  • minimumOf gets the minimum value maping function generates. List(0D, math.Pi).minimumOf(math.cos) = Some(-1.0)
  • minimumBy gets the element on which maping function generates the lowest value. List(0D, math.Pi).minimumBy(math.cos) = Some(3.141592653589793)
  • emtpy tells if foldable is empty. Examples:
    • List(1, 2, 3).empty = false
    • nil[Int].empty = true
  • element tells if element is in foldable. Examples:
    • List(1, 2, 3).element(1) = true
    • List(1, 2, 3).element(3) = false
  • splitWith splits the elements into groups that alternatively satisfy and don’t satisfy the predicate. List(2, 4, 1, 3, 5).splitWith(a ⇒ (a % 2) ≟ 0) = List(NonEmptyList(2, 4), NonEmptyList(1, 3, 5))
  • selectSplit selects only the elements that satisfy the predicate: List(1, 2, 3).selectSplit(a ⇒ (a % 2) ≟ 0) = List(NonEmptyList(2))
  • intercalate inserts specified element between every elements and folds using the monoid. List(1, 2, 3).intercalate(10) = 26

Source

import scalaz.std.anyVal.doubleInstance
import scalaz.std.anyVal.intInstance
import scalaz.std.string._
import scalaz.std.option._
import scalaz.std.list._
import scalaz.syntax.equal._
import scalaz.syntax.foldable._

display(List(1, 2, 3).foldMap(), """ List(1, 2, 3).foldMap() """)
display(List(1, 2, 3).foldMap((a: Int) ⇒ a.toString), """ List(1, 2, 3).foldMap((a: Int) => a.toString) """)
display(nil[Int].foldMap(), """ nil[Int].foldMap() """)

display(List(1, 2, 3).foldMap1Opt(), """ List(1, 2, 3).foldMap1Opt() """)
display(nil[Int].foldMap1Opt(), """ nil[Int].foldMap1Opt() """)

display(List(1, 2, 3, 4).foldRight(1)(_ * _), """ List(1, 2, 3, 4).foldRight(1)(_ * _) """)

display(List(1, 2, 3).foldMapRight1Opt(a ⇒ a + 1)(_ + _), """ List(1, 2, 3).foldMapRight1Opt(a => a + 1)(_ + _) """)
display(nil[Int].foldMapRight1Opt(a ⇒ a + 1)(_ + _), """ nil[Int].foldMapRight1Opt(a => a + 1)(_ + _) """)

display(List(1, 2, 3).foldRight1Opt(_ + _), """ List(1, 2, 3).foldRight1Opt(_ + _) """)
display(nil[Int].foldRight1Opt(_ + _), """ nil[Int].foldRight1Opt(_ + _) """)

display(List(1, 2, 3).foldRight(0)(_ - _), """ List(1, 2, 3, 4).foldRight(1)(_ * _) """)
display(List(1, 2, 3).foldLeft(0)(_ - _), """ List(1, 2, 3, 4).foldRight(1)(_ * _) """)

display(List(1, 2, 3).foldRightM(0)((a, b) ⇒ Option(a + b)), """ List(1, 2, 3).foldRightM(0)((a, b) ⇒ Option(a + b)) """)
display(List(1, 2, 3).foldMapM(a ⇒ Option(a.toString)), """ List(1, 2, 3).foldRightM(0)((a, b) ⇒ Option(a + b)) """)
display(List(1, 2, 3).concatenate, """ List(1, 2, 3).concatenate """)

display(List(1, 2, 3).length, """ List(1, 2, 3).length """)
display(List(1, 2, 3).index(0), """ List(1, 2, 3).index(0) """)
display(List(1, 2, 3).index(3), """ List(1, 2, 3).index(3) """)
display(List(1, 2, 3).indexOr(-1, 0), """ List(1, 2, 3).indexOr(-1, 0) """)
display(List(1, 2, 3).indexOr(-1, 3), """ List(1, 2, 3).indexOr(-1, 3) """)
display(List(1, 2, 3).sumr, """ List(1, 2, 3).sumr """)
display(List(1, 2, 3).suml, """ List(1, 2, 3).suml """)

display(List(1, 1, 1) ∀ (a ⇒ (a % 2) ≠ 0), """ List(1, 1, 1) ∀ (a ⇒ (a % 2) ≠ 0) """)
display(List(1, 2, 3) ∃ (a ⇒ (a % 2) ≟ 0), """ List(1, 2, 3) ∃ (a ⇒ (a % 2) ≟ 0) """)

display(List(1, 2, 3).maximum, """ List(1, 2, 3).maximum """)
display(List(0D, math.Pi).maximumOf(math.cos), """ List(0D, math.Pi).maximumOf(math.cos) """)
display(List(0D, math.Pi).maximumBy(math.cos), """ List(0D, math.Pi).maximumBy(math.cos) """)

display(List(1, 2, 3).minimum, """ List(1, 2, 3).minimum """)
display(List(0D, math.Pi).minimumOf(math.cos), """ List(0D, math.Pi).minimumOf(math.cos) """)
display(List(0D, math.Pi).minimumBy(math.cos), """ List(0D, math.Pi).minimumBy(math.cos) """)

display(List(1, 2, 3).empty, """ List(1, 2, 3).empty """)
display(nil[Int].empty, """ nil[Int].empty """)

display(List(1, 2, 3).element(1), """ List(1, 2, 3).element(1) """)
display(List(1, 2, 3).element(4), """ List(1, 2, 3).element(4) """)

display(List(2, 4, 1, 3, 5).splitWith(a ⇒ (a % 2) ≟ 0), """ List(1, 2, 3).element(4) """)
display(List(1, 2, 3).selectSplit(a ⇒ (a % 2) ≟ 0), """ List(1, 2, 3).selectSplit(a ⇒ (a % 2) ≟ 0) """)
display(List(1, 2, 3).intercalate(10), """ List(1, 2, 3).intercalate(10) """)

Output

List(1, 2, 3).foldMap()                                    6
List(1, 2, 3).foldMap((a: Int) => a.toString)              123
nil[Int].foldMap()                                         0
List(1, 2, 3).foldMap1Opt()                                Some(6)
nil[Int].foldMap1Opt()                                     None
List(1, 2, 3, 4).foldRight(1)(_ * _)                       24
List(1, 2, 3).foldMapRight1Opt(a => a + 1)(_ + _)          Some(7)
nil[Int].foldMapRight1Opt(a => a + 1)(_ + _)               None
List(1, 2, 3).foldRight1Opt(_ + _)                         Some(6)
nil[Int].foldRight1Opt(_ + _)                              None
List(1, 2, 3, 4).foldRight(1)(_ * _)                       2
List(1, 2, 3, 4).foldRight(1)(_ * _)                       -6
List(1, 2, 3).foldRightM(0)((a, b) ⇒ Option(a + b))        Some(6)
List(1, 2, 3).foldRightM(0)((a, b) ⇒ Option(a + b))        Some(123)
List(1, 2, 3).concatenate                                  6
List(1, 2, 3).length                                       3
List(1, 2, 3).index(0)                                     Some(1)
List(1, 2, 3).index(3)                                     None
List(1, 2, 3).indexOr(-1, 0)                               1
List(1, 2, 3).indexOr(-1, 3)                               -1
List(1, 2, 3).sumr                                         6
List(1, 2, 3).suml                                         6
List(1, 1, 1) ∀ (a ⇒ (a % 2) ≠ 0)                          true
List(1, 2, 3) ∃ (a ⇒ (a % 2) ≟ 0)                          true
List(1, 2, 3).maximum                                      Some(3)
List(0D, math.Pi).maximumOf(math.cos)                      Some(1.0)
List(0D, math.Pi).maximumBy(math.cos)                      Some(0.0)
List(1, 2, 3).minimum                                      Some(1)
List(0D, math.Pi).minimumOf(math.cos)                      Some(-1.0)
List(0D, math.Pi).minimumBy(math.cos)                      Some(3.141592653589793)
List(1, 2, 3).empty                                        false
nil[Int].empty                                             true
List(1, 2, 3).element(1)                                   true
List(1, 2, 3).element(4)                                   false
List(1, 2, 3).element(4)                                   List(NonEmptyList(2, 4), NonEmptyList(1, 3, 5))
List(1, 2, 3).selectSplit(a ⇒ (a % 2) ≟ 0)                 List(NonEmptyList(2))
List(1, 2, 3).intercalate(10)                              26