Merge pull request #89 from scala-exercises/enrique-2-13-1-update

Update to Scala 2.13.1

Author: kiroco12

.travis.yml

@@ -1,6 +1,6 @@
 language: scala
 scala:
-- 2.12.10
+- 2.13.1
 jdk:
 - openjdk8
 script:

build.sbt

@@ -1,6 +1,6 @@
 import ProjectPlugin.autoImport._
 
-val scalaExercisesV = "0.5.0-SNAPSHOT"
+val scalaExercisesV = "0.6.0-SNAPSHOT"
 
 def dep(artifactId: String) = "org.scala-exercises" %% artifactId % scalaExercisesV
 

project/ProjectPlugin.scala

@@ -15,7 +15,7 @@ object ProjectPlugin extends AutoPlugin {
   object autoImport {
 
     lazy val V = new {
-      val scala212: String            = "2.12.10"
+      val scala213: String            = "2.13.1"
       val shapeless: String           = "2.3.3"
       val scalatest: String           = "3.1.0"
       val scalatestplusScheck: String = "3.1.0.0-RC2"
@@ -39,7 +39,7 @@ object ProjectPlugin extends AutoPlugin {
         organizationEmail = "hello@47deg.com"
       ),
       orgLicenseSetting := ApacheLicense,
-      scalaVersion := V.scala212,
+      scalaVersion := V.scala213,
       scalaOrganization := "org.scala-lang",
       resolvers ++= Seq(
         Resolver.mavenLocal,

project/build.properties

@@ -1 +1 @@
-sbt.version=1.2.8
+sbt.version=1.3.7

project/plugins.sbt

@@ -2,5 +2,5 @@ resolvers ++= Seq(
   Resolver.sonatypeRepo("snapshots")
 )
 
-addSbtPlugin("org.scala-exercises" % "sbt-exercise"     % "0.5.0-SNAPSHOT")
+addSbtPlugin("org.scala-exercises" % "sbt-exercise"     % "0.6.0-SNAPSHOT")
 addSbtPlugin("com.47deg"           % "sbt-org-policies" % "0.12.0-M3")

src/main/scala/scalatutorial/sections/FunctionalLoops.scala

@@ -137,10 +137,10 @@ object FunctionalLoops extends ScalaTutorialSection {
    *
    * Complete the following method definition that computes the factorial of a number:
    */
-  def factorialExercise(res0: Int, res1: Int, res2: Int): Unit = {
+  def factorialExercise(res0: Int, res1: Int): Unit = {
     def factorial(n: Int): Int =
       if (n == res0) res1
-      else factorial(n - res2) * n
+      else factorial(n - 1) * n
 
     factorial(3) shouldBe 6
     factorial(4) shouldBe 24

src/main/scala/scalatutorial/sections/HigherOrderFunctions.scala

@@ -161,9 +161,9 @@ object HigherOrderFunctions extends ScalaTutorialSection {
     def sum(f: Int => Int, a: Int, b: Int): Int = {
       def loop(x: Int, acc: Int): Int =
         if (x > b) acc
-        else loop(x + res0, acc + f(x))
-      loop(a, res1)
+        else loop(x + 1, acc + f(x))
+      loop(a, res0)
     }
-    sum(x => x, 1, 10) shouldBe 55
+    sum(x => x, 1, res1) shouldBe 55
   }
 }

src/main/scala/scalatutorial/sections/LazyEvaluation.scala

@@ -43,29 +43,29 @@ object LazyEvaluation extends ScalaTutorialSection {
    *  - Avoid computing the tail of a sequence until it is needed for the evaluation
    *    result (which might be never)
    *
-   * This idea is implemented in a new class, the `Stream`.
+   * This idea is implemented in a new class, the `LazyList`.
    *
-   * Streams are similar to lists, but their tail is evaluated only ''on demand''.
+   * LazyLists are similar to lists, but their elements are evaluated only ''on demand''.
    *
-   * = Defining Streams =
+   * = Defining LazyLists =
    *
-   * Streams are defined from a constant `Stream.empty` and a constructor `Stream.cons`.
+   * LazyLists are defined from a constructor `LazyList.cons`.
    *
    * For instance,
    *
    * {{{
-   *   val xs = Stream.cons(1, Stream.cons(2, Stream.empty))
+   *   val xs = LazyList.cons(1, LazyList.cons(2, LazyList.empty))
    * }}}
    *
-   * = Stream Ranges =
+   * = LazyList Ranges =
    *
-   * Let's try to write a function that returns a `Stream` representing a range of numbers
+   * Let's try to write a function that returns a `LazyList` representing a range of numbers
    * between `lo` and `hi`:
    *
    * {{{
-   *   def streamRange(lo: Int, hi: Int): Stream[Int] =
-   *     if (lo >= hi) Stream.empty
-   *     else Stream.cons(lo, streamRange(lo + 1, hi))
+   *   def llRange(lo: Int, hi: Int): LazyList[Int] =
+   *     if (lo >= hi) LazyList.empty
+   *     else LazyList.cons(lo, llRange(lo + 1, hi))
    * }}}
    *
    * Compare to the same function that produces a list:
@@ -79,113 +79,122 @@ object LazyEvaluation extends ScalaTutorialSection {
    * The functions have almost identical structure yet they evaluate quite differently.
    *
    *  - `listRange(start, end)` will produce a list with `end - start` elements and return it.
-   *  - `streamRange(start, end)` returns a single object of type `Stream` with `start` as head element.
+   *  - `llRange(start, end)` returns a single object of type `LazyList` with `start` as head element.
    *    - The other elements are only computed when they are needed, where
    *      “needed” means that someone calls `tail` on the stream.
    *
-   * = Methods on Streams =
+   * = Methods on LazyLists =
    *
-   * `Stream` supports almost all methods of `List`.
+   * `LazyList` supports almost all methods of `List`.
    *
    * For instance, to find the second prime number between 1000 and 10000:
    *
    * {{{
-   *   (streamRange(1000, 10000) filter isPrime)(1)
+   *   (llRange(1000, 10000) filter isPrime)(1)
    * }}}
    *
    * The one major exception is `::`.
    *
-   * `x :: xs` always produces a list, never a stream.
+   * `x :: xs` always produces a list, never a lazy list.
    *
-   * There is however an alternative operator `#::` which produces a stream.
+   * There is however an alternative operator `#::` which produces a lazy list.
    *
    * {{{
-   *   x #:: xs  ==   Stream.cons(x, xs)
+   *   x #:: xs  ==   LazyList.cons(x, xs)
    * }}}
    *
    * `#::` can be used in expressions as well as patterns.
    *
-   * = Implementation of Streams =
+   * = Implementation of LazyLists =
    *
-   * The implementation of streams is quite close to the one of lists.
+   * The implementation of lazy lists is quite close to the one of lists.
    *
-   * Here's the trait `Stream`:
+   * Here's the class `LazyList`:
    *
    * {{{
-   *   trait Stream[+T] extends Seq[T] {
-   *     def isEmpty: Boolean
-   *     def head: T
-   *     def tail: Stream[T]
-   *     …
+   *   final class LazyList[+A] ... extends ... {
+   *     override def isEmpty: Boolean = ...
+   *     override def head: A = ...
+   *     override def tail: LazyList[A] = ...
+   *     ...
    *   }
    * }}}
    *
    * As for lists, all other methods can be defined in terms of these three.
    *
-   * Concrete implementations of streams are defined in the `Stream` companion object.
+   * Concrete implementations of streams are defined in the `LazyList.State` companion object.
    * Here's a first draft:
    *
    * {{{
-   *   object Stream {
-   *     def cons[T](hd: T, tl: => Stream[T]) = new Stream[T] {
-   *       def isEmpty = false
-   *       def head = hd
-   *       def tail = tl
-   *       override def toString = "Stream(" + hd + ", ?)"
-   *     }
-   *     val empty = new Stream[Nothing] {
-   *       def isEmpty = true
-   *       def head = throw new NoSuchElementException("empty.head")
-   *       def tail = throw new NoSuchElementException("empty.tail")
-   *       override def toString = "Stream()"
+   *   private object State {
+   *     object Empty extends State[Nothing] {
+   *       def head: Nothing = throw new NoSuchElementException("head of empty lazy list")
+   *       def tail: LazyList[Nothing] = throw new UnsupportedOperationException("tail of empty lazy list")
    *     }
+   *
+   *     final class Cons[A](val head: A, val tail: LazyList[A]) extends State[A]
    *   }
    * }}}
    *
-   * The only important difference between the implementations of `List` and `Stream`
-   * concern `tl`, the second parameter of `Stream.cons`.
+   * The only important difference between the implementations of `List` and `LazyList`
+   * concern `tail`, the second parameter of `LazyList.cons`.
    *
-   * For streams, this is a by-name parameter: the type of `tl` starts with `=>`. In such
+   * For lazy lists, this is a by-name parameter: the type of `tail` starts with `=>`. In such
    * a case, this parameter is evaluated by following the rules of the call-by-name model.
    *
-   * That's why the second argument to `Stream.cons` is not evaluated at the point of call.
+   * That's why the second argument to `LazyList.cons` is not evaluated at the point of call.
    *
-   * Instead, it will be evaluated each time someone calls `tail` on a `Stream` object.
+   * Instead, it will be evaluated each time someone calls `tail` on a `LazyList` object.
    *
-   * The other stream methods are implemented analogously to their list counterparts.
+   * In Scala 2.13, LazyList (previously Stream) became fully lazy from head to tail. To make it possible,
+   * methods (`filter`, `flatMap`...) are implemented in a way where the head is not being evaluated if is
+   * not explicitly indicated.
    *
    * For instance, here's `filter`:
    *
    * {{{
-   *   class Stream[+T] {
-   *     …
-   *     def filter(p: T => Boolean): Stream[T] =
-   *       if (isEmpty) this
-   *       else if (p(head)) cons(head, tail.filter(p))
-   *       else tail.filter(p)
+   *   object LazyList extends SeqFactory[LazyList] {
+   *     ...
+   *     private def filterImpl[A](ll: LazyList[A], p: A => Boolean, isFlipped: Boolean): LazyList[A] = {
+   *     // DO NOT REFERENCE `ll` ANYWHERE ELSE, OR IT WILL LEAK THE HEAD
+   *     var restRef = ll                         // val restRef = new ObjectRef(ll)
+   *     newLL {
+   *       var elem: A = null.asInstanceOf[A]
+   *       var found   = false
+   *       var rest    = restRef                  // var rest = restRef.elem
+   *       while (!found && !rest.isEmpty) {
+   *         elem    = rest.head
+   *         found   = p(elem) != isFlipped
+   *         rest    = rest.tail
+   *         restRef = rest                       // restRef.elem = rest
+   *       }
+   *       if (found) sCons(elem, filterImpl(rest, p, isFlipped)) else State.Empty
+   *     }
    *   }
    * }}}
    *
    * = Exercise =
    *
-   * Consider the following modification of `streamRange`. When you write
-   * `streamRange(1, 10).take(3).toList` what is the value of `rec`?
+   * Consider the following modification of `llRange`. When you write
+   * `llRange(1, 10).take(3).toList` what is the value of `rec`?
+   *
+   * Be careful, head is evaluating too!
    */
-  def streamRangeExercise(res0: Int): Unit = {
+  def llRangeExercise(res0: Int): Unit = {
     var rec = 0
-    def streamRange(lo: Int, hi: Int): Stream[Int] = {
+    def llRange(lo: Int, hi: Int): LazyList[Int] = {
       rec = rec + 1
-      if (lo >= hi) Stream.empty
-      else Stream.cons(lo, streamRange(lo + 1, hi))
+      if (lo >= hi) LazyList.empty
+      else LazyList.cons(lo, llRange(lo + 1, hi))
     }
-    streamRange(1, 10).take(3).toList
+    llRange(1, 10).take(3).toList
     rec shouldBe res0
   }
 
   /**
    * = Lazy Evaluation =
    *
-   * The proposed `Stream` implementation suffers from a serious potential performance
+   * The proposed `LazyList` implementation suffers from a serious potential performance
    * problem: If `tail` is called several times, the corresponding stream
    * will be recomputed each time.
    *
@@ -210,18 +219,6 @@ object LazyEvaluation extends ScalaTutorialSection {
    *   lazy val x = expr
    * }}}
    *
-   * = Lazy Vals and Streams =
-   *
-   * Using a lazy value for `tail`, `Stream.cons` can be implemented more efficiently:
-   *
-   * {{{
-   *   def cons[T](hd: T, tl: => Stream[T]) = new Stream[T] {
-   *     def head = hd
-   *     lazy val tail = tl
-   *     …
-   *   }
-   * }}}
-   *
    * == Exercise ==
    */
   def lazyVal(res0: String): Unit = {

src/main/scala/scalatutorial/sections/StandardLibrary.scala

@@ -295,7 +295,7 @@ object StandardLibrary extends ScalaTutorialSection {
     def triple(x: Int): Int = 3 * x
 
     def tripleEither(x: Either[String, Int]): Either[String, Int] =
-      x.right.map(triple)
+      x.map(triple)
 
     tripleEither(Right(1)) shouldBe res0
     tripleEither(Left("not a number")) shouldBe res1

src/main/scala/scalatutorial/sections/SyntacticConveniences.scala

@@ -342,7 +342,7 @@ object SyntacticConveniences extends ScalaTutorialSection {
    */
   def repeatedParameters(res0: Double): Unit = {
     def average(x: Int, xs: Int*): Double =
-      (x :: xs.to[List]).sum.toDouble / (xs.size + 1)
+      (x :: xs.to(List)).sum.toDouble / (xs.size + 1)
 
     average(1) shouldBe 1.0
     average(1, 2) shouldBe 1.5