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Scala example source code file (FoldableUsage.scala)

This example Scala source code file (FoldableUsage.scala) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Scala by Example" TM.

Learn more about this Scala project at its project page.

Java - Scala tags/keywords

app, boolean, foldable, foldableusage, foldlistofoptions, ilist, int, int\,int, list, none, option, some, stream

The FoldableUsage.scala Scala example source code

package scalaz.example

object FoldableUsage extends App {
  import scalaz._
  import scalaz.std.stream._
  import scalaz.std.list._
  import scalaz.std.string._
  import scalaz.std.option._
  import scalaz.std.anyVal._
  import scalaz.std.vector._
  import scalaz.syntax.std.option._
  import scalaz.syntax.equal._

  // a continous stream of true, true, true, true, true, …
  val trues: Stream[Boolean] = Stream.continually(true)

  // we cannot use foldRight from the standard library with an
  // infinite stream, as it attempts to reverse the stream, causing a
  // Stack Overflow
  val so = \/.fromTryCatchThrowable[Boolean,Throwable](trues.foldRight(false)(_ || _))
  assert(so.fold(_.toString, _.toString) === "java.lang.StackOverflowError")

  // however, the Foldable typeclass has an implementation named foldr
  // which traverses the stream from left to right lazily, so, given a
  // function which is lazy in the right argument, we can use a foldr
  // on an infinite stream.
  def lazyOr(x: Boolean)(y: => Boolean) = x || y
  assert(Foldable[Stream].foldr(trues, false)(lazyOr))

  // when we have an available Monoid for the parameterized type, we
  // can collapse the Foldable using the monoid:
  val digits = List(0,1,2,3,4,5,6,7,8,9)
  assert(Foldable[List].fold(digits) === 45)

  // we can also map the structure to values for which we have a
  // monoid, we can collapse the list, as we can see, this one is also
  // properly lazy, allowing us to collapse our infinite stream again
  assert(Tag.unwrap(Foldable[Stream].foldMap(trues)((b: Boolean) => Tags.Disjunction(b))))

  // We can import syntax for foldable, allowing us to "enhance" the foldable with the new methods:
  import scalaz.syntax.foldable._
  assert(trues.foldr(false)(lazyOr))
  assert(Tag.unwrap(trues.foldMap((b: Boolean) => Tags.Disjunction(b))))
  assert(digits.map(_.toString).intercalate(",") === "0,1,2,3,4,5,6,7,8,9")
  assert(digits.maximum === Some(9))
  assert(digits.minimum === Some(0))

  // Foldables can be composed:
  val FoldListOfOptions = Foldable[List] compose Foldable[Option]

  val listOfOptions: List[Option[Int]] = List(1.some, 2.some, none[Int], 3.some, 4.some)
  assert(FoldListOfOptions.fold(listOfOptions) === 10)

  // with this you get a collapse function which is perhaps like the
  // flatten method in the standard library, however it is more versatile
  assert(FoldListOfOptions.collapse[List, Int](listOfOptions) === listOfOptions.flatten)

  // we can accumulate into any type for which we have an
  // ApplicativePlus instance, so here we can collapse our List of
  // Options into a Vector
  assert(FoldListOfOptions.collapse[Vector, Int](listOfOptions) === Vector(listOfOptions.flatten: _*))

  // we can go deeeeep:
  val deepFolder = Foldable[List] compose Foldable[Vector] compose Foldable[Stream] compose Foldable[Option]
  val deep: List[Vector[Stream[Option[Int]]]] = List(Vector(Stream(1.some, none[Int]), Stream(2.some)), Vector(Stream(3.some)))
  assert(deepFolder.fold(deep) === 6)
  assert(deepFolder.collapse[IList, Int](deep) === IList(1,2,3))
  assert(deepFolder.foldLeft(deep, "")(_ + _.toString) === "123")

  // Monadic Folds: we can fold over a structure with a function
  // which returns its value in a Monad,
  val sumEvens: (Int,Int) => Option[Int] = { (x, y) =>
    // if the right int is even, add it to the left
    // otherwise return None
    if((y % 2) == 0) Some(x+y) else None
  }

  // all numbers are even, so we end up with Some
  val allEvens = List(2,4,6,8,10)
  assert(allEvens.foldLeftM[Option,Int](0)(sumEvens) === Some(30))

  // when the 7 is encountered, the entire computation results in None
  val notAllEvens = List(2,4,7,8,10)
  assert(notAllEvens.foldLeftM[Option,Int](0)(sumEvens) === None)
}

Other Scala examples (source code examples)

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