Added exercise for polymorphic functions / HOF

src/main/scala/fpinscalalib/FPinScalaLibrary.scala

@@ -4,7 +4,7 @@ import org.scalaexercises.definitions._
 
 /** Set of exercises based on Manning's "Functional Programming in Scala" (aka "The Red Book")
   *
-  * @param name fpinscala
+  * @param name fp_in_scala
   */
 object FPinScalaLibrary extends Library {
   override def owner = "scala-exercises"

src/main/scala/fpinscalalib/GettingStartedWithFPSection.scala

@@ -2,23 +2,23 @@ package fpinscalalib
 
 import org.scalatest.{FlatSpec, Matchers}
 
-/** @param name gettingstartedwithfp
+/** @param name getting_started_with_functional_programming
   */
 object GettingStartedWithFPSection extends FlatSpec with Matchers with org.scalaexercises.definitions.Section {
 
   /**
     * = Tail-recursive functions =
     *
     * We're going to introduce some of the basic techniques for how to write functional programs. Let's start by writing
-    * loops using `tail-recursive functions`. For instance, let's take a look on how to functionally write a function that
+    * loops using tail-recursive functions. For instance, let's take a look on how to functionally write a function that
     * calculates the factorial of a given number.
     *
     * {{{
     * def factorial(n: Int): Int = {
     * @annotation.tailrec
     * def go(n: Int, acc: Int): Int =
     *   if (n <= 0) acc
-    *   else go(n-1, n*acc)
+    *   else go(n - 1, n * acc)
     *   go(n, 1)
     * }
     * }}}
@@ -33,19 +33,19 @@ object GettingStartedWithFPSection extends FlatSpec with Matchers with org.scala
     * return the value of `acc` if `n <= 0`).
     *
     * Scala is able to detect this sort of self-recursion and compiles it to the same sort of bytecode as would be emitted
-    * by a `while` loop, as long as the recursive call is in `tail position`. The basic idea is that this optimization
+    * by a `while` loop, as long as the recursive call is in tail position. The basic idea is that this optimization
     * (tail call elimination) is applied when there's no additional work left to do after the recursive call returns.
     *
     * Let's do the same with a function to call the nth number from the Fibonacci sequence. The first two numbers
-    * are 0 and 1. Then, the nth number is always the sum of the previous two, i.e.: 0, 1, 1, 2, 3, 5... The `fib`
+    * are 0 and 1. Then, the nth number is always the sum of the previous two, i.e.: 0, 1, 1, 2, 3, 5... The fib`
     * function starts by calling its `loop` helper function with the initial values of `n` (the position in the sequence
     * we need to calculate), and the previous and current values in the sequence.
     *
     * {{{
     *   def fib(n: Int): Int = {
     *   @annotation.tailrec
     *   def loop(n: Int, prev: Int, cur: Int): Int =
-    *     if (n == ???) prev
+    *     if (n <= ???) prev
     *     else loop(n - ???, cur, prev + cur)
     *   loop(n, 0, 1)
     * }
@@ -55,12 +55,11 @@ object GettingStartedWithFPSection extends FlatSpec with Matchers with org.scala
     * way. What should the missing expressions for the trivial case and the recursive call be?
     */
 
-
   def fibAssert(res0: Int, res1: Int) {
     def fib(n: Int): Int = {
       @annotation.tailrec
       def loop(n: Int, prev: Int, cur: Int): Int =
-        if (n == res0) prev
+        if (n <= res0) prev
         else loop(n - res1, cur, prev + cur)
       loop(n, 0, 1)
     }
@@ -71,7 +70,7 @@ object GettingStartedWithFPSection extends FlatSpec with Matchers with org.scala
   /**
     * = Polymorphic and higher-order functions =
     *
-    * Polymorphic functions allow us to write code that works for **any** type it's given. For instance, take a look at
+    * Polymorphic functions allow us to write code that works for any type it's given. For instance, take a look at
     * `findFirst`, a function that finds the first index in an array where the key occurs (or `-1` if it doesn't exist),
     * implemented more generally by accepting a function to use for testing a particular `A` value.
     *
@@ -91,7 +90,63 @@ object GettingStartedWithFPSection extends FlatSpec with Matchers with org.scala
     *
     * To write a polymorphic function as a method, we introduce a comma-separated list of type parameters, surrounded by
     * square brackets (here, just a single `[A]`), following the name of the function, in this case `findFirst`.
+    *
+    * = Higher-order functions =
+    *
+    * Let's see an example of a higher-order function (HOF):
+    * {{{
+    *   def formatResult(name: String, n: Int, f: Int => Int) = {
+    *     val msg = "The %s of %d is %d."
+    *     msg.format(name, n, f(n))
+    *   }
+    * }}}
+    *
+    * Our `formatResult` HOF takes another function called `f`. We give a type to `f`, as we would for any other parameter.
+    * Its type is `Int => Int`, which indicates that `f` expects an integer argument and will also return an integer.
+    *
+    * Let's create a polymorphic, tail-recursive higher-order function that checks if an array is sorted, according to
+    * a given comparison function that will be passed as a parameter:
+    *
+    * {{{
+    *   def isSorted[A](as: Array[A], ordering: (A, A) => Boolean): Boolean = {
+    *     @annotation.tailrec
+    *     def go(n: Int): Boolean =
+    *       if (n >= as.length - 1) true
+    *       else if (ordering(as(n), as(n + 1))) false
+    *       else go(n + 1)
+    *
+    *     go(0)
+    *    }
+    * }}}
+    *
+    * When using HOFs, it's often convenient to be able to call these functions with anonymous functions, rather than
+    * having to supply some existing named function. For instance, using the previously implemented `findFirst`:
+    *
+    * {{{
+    *   findFirst(Array(7, 9, 13), (x: Int) => x == 9)
+    * }}}
+    *
+    * The syntax `(x: Int) => x == 9` is a `function literal` or `anonymous function`, defining a function that takes one
+    * argument `x` of type `Int` and returns a `Boolean` indicating whether `x` is equal to 9.
+    *
+    * Let's do the same with `isSorted`. After taking a detailed look at its implementation, what would be the results of
+    * applying the following anonymous functions to it?
     */
 
+  def isSortedAssert(res0: Boolean, res1: Boolean, res2: Boolean): Unit = {
+    def isSorted[A](as: Array[A], ordering: (A, A) => Boolean): Boolean = {
+      @annotation.tailrec
+      def go(n: Int): Boolean =
+        if (n >= as.length - 1) true
+        else if (ordering(as(n), as(n + 1))) false
+        else go(n + 1)
+
+      go(0)
+    }
+
+    isSorted(Array(1, 3, 5, 7), (x: Int, y: Int) => x > y) shouldBe res0
+    isSorted(Array(7, 5, 3, 1), (x: Int, y: Int) => x < y) shouldBe res1
+    isSorted(Array("Scala", "Exercises"), (x: String, y: String) => x.length > y.length) shouldBe res2
+  }
 }
 

src/test/scala/exercises/Test.scala

@@ -0,0 +1,8 @@
+package exercises
+
+/**
+  * Created by Javi on 11/7/16.
+  */
+class Test {
+
+}

src/test/scala/exercises/fpinscala/GettingStartedWithFPSpec.scala

@@ -0,0 +1,8 @@
+package exercises.fpinscala
+
+/**
+  * Created by Javi on 11/7/16.
+  */
+class GettingStartedWithFPSpec {
+
+}