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Scala example source code file (FunctorUsage.scala)

This example Scala source code file (FunctorUsage.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, functor, functorusage, int, list, map, none, option, some, string, task

The FunctorUsage.scala Scala example source code

package scalaz
package example

import std.option._
import std.list._
import std.map._
import std.anyVal._
import std.string._
import std.tuple._
import syntax.equal._
import scalaz.concurrent.Task
import syntax.functor._

/**
  * A Functor is a ubiquitous typeclass involving type constructors of
  * kind * → *, which is another way of saying types that have a
  * single type variable. Examples might be Option, List, Future.
  *
  * The Functor category involves a single operation, named `map`:
  *
  * def map[A, B](fa: F[A])(f: A => B): F[B]
  *
  * This method takes a Function from A => B and turns an F[A] into an F[B]
  */
object FunctorUsage extends App {

  val len: String => Int = _.length

  //
  // map
  //

  // Option is a functor which always returns a Some with the function
  // applied when the Option value is a Some.
  assert(Functor[Option].map(Some("adsf"))(len) === Some(4))
  // When the Option is a None, it always returns None
  assert(Functor[Option].map(None)(len) === None)

  // List is a functor which applies the function to each element of
  // the list.
  assert(Functor[List].map(List("qwer", "adsfg"))(len) === List(4,5))

  //
  // lift
  //

  // We can use the Funtor to "lift" a function to operate on the Functor type:
  val lenOption: Option[String] => Option[Int] = Functor[Option].lift(len)
  assert(lenOption(Some("abcd")) === Some(4))

  //
  // strength
  //

  // Functors in scalaz all come equipped with tensorial strenth! does
  // that sound exciting? It's not that exciting, it means that we get
  // two additional derived functions which allow us to turn the
  // contained values into tuples:
  assert(Functor[List].strengthL("a", List(1,2,3)) === List("a" -> 1, "a" -> 2, "a" -> 3))
  assert(Functor[List].strengthR(List(1,2,3), "a") === List(1 -> "a", 2 -> "a", 3 -> "a"))

  // there is syntax for the strength functions
  assert(List(1,2,3).strengthL("a") === List("a" -> 1, "a" -> 2, "a" -> 3))
  assert(List(1,2,3).strengthR("a") === List(1 -> "a", 2 -> "a", 3 -> "a"))

  //
  // fproduct
  //

  // Functor provides a fproduct function which pairs a value with the
  // result of applying a function to that value.
  val source = List("a", "aa", "b", "ccccc")
  val result = Map("a" -> 1, "aa" -> 2, "b" ->  1, "ccccc" -> 5)

  assert(source.fproduct(len).toMap === result)


  //
  // void
  //

  // We can "void" a functor, which will change any F[A] into a F[Unit]
  assert(Functor[Option].void(Some(1)) === Some(()))

  // You might wonder why such a thing would ever be useful, it will
  // become useful when we have functors that control side-effects
  // here's a bit of a contrived example to show where we might want
  // to void a functor.

  // pretend this is our database
  var database = Map("abc" → 1,
                     "aaa" → 2,
                     "qqq" → 3)

  // Return a Task which removes items from our database and returns the number of items deleted
  def del(f: String => Boolean): Task[Int] = Task.delay {
    val (count, db) = database.foldRight(0 → List.empty[(String,Int)]) {
      case ((k,_),(d,r)) if f(k) => (d+1, r)
      case (i,(d,r)) => (d, i::r)
    }
    database = db.toMap
    count
  }

  // This is a task which will delete two of the three items in our database,
  val delTask = del(_.startsWith("a"))

  // it hasn't run yet
  assert(database.size === 3)

  // but perhaps we don't care about the number of items that were
  // deleted, we really just want to execute the side-effects, and get
  // a Task[Unit]
  val voidTask: Task[Unit] = Functor[Task].void(delTask)

  // There is syntax for void.
  val voidTask2: Task[Unit] = delTask.void

  // Running the task returns a Unit.
  assert(voidTask.unsafePerformSync === (()))

  // And now our database is smaller
  assert(database.size === 1)

  //
  // Composition
  //

  // Functors compose! Given any Functor F[_] and any Functor G[_] we
  // can compose the two Functors to create a new Functor on F[G[_]]:
  val listOpt = Functor[List] compose Functor[Option]
  assert(listOpt.map(List(Some(1), None, Some(3)))(_ + 1) === List(Some(2), None, Some(4)))
}

Other Scala examples (source code examples)

Here is a short list of links related to this Scala FunctorUsage.scala source code file:



my book on functional programming

 

new blog posts

 

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