composition

src/main/scala/stdlib/Composition.scala

@@ -0,0 +1,42 @@
+/*
+ * scala-exercises - exercises-stdlib
+ * Copyright (C) 2015-2016 47 Degrees, LLC. <http://www.47deg.com>
+ */
+
+package stdlib
+
+import org.scalatest._
+
+/** @param name composition
+ *
+ */
+object Composition
+    extends FlatSpec
+    with Matchers
+    with org.scalaexercises.definitions.Section {
+
+  /** Function `composition` is a functional programming and mathematical transformation that through the combination of two existing functions generates a new function. Mathematically speaking we refer at composition as: `g(x) O f(x) = f(g(x))`
+    * Scala provides a method called compose that can be used this way:
+   */
+  def composeFunctions(res0: String) {
+    def f(s: String) = "f(" + s + ")"
+    def g(s: String) = "g(" + s + ")"
+
+    val composedFunction = f _ compose g _  //Here we compose the functions
+
+    composedFunction("Scala Exercises") should be(res0)
+  }
+
+  /**
+    * There is also a method andThen, closely related to compose.
+    * The only difference between andThen and compose is that the order of evaluation for compose is right to left.
+    */
+  def andThenFunctions(res0: String) {
+    def f(s: String) = "f(" + s + ")"
+    def g(s: String) = "g(" + s + ")"
+
+    val composedFunction = g _ andThen f _
+
+    composedFunction("Scala Exercises") should be(res0)
+  }
+}

src/main/scala/stdlib/StdLib.scala

@@ -29,6 +29,7 @@ object StdLib extends org.scalaexercises.definitions.Library {
     PatternMatching,
     CaseClasses,
     Ranges,
+    Composition,
     PartiallyAppliedFunctions,
     PartialFunctions,
     Implicits,

src/test/scala/stdlib/CompositionSpec.scala

@@ -0,0 +1,33 @@
+/*
+ * scala-exercises - exercises-stdlib
+ * Copyright (C) 2015-2016 47 Degrees, LLC. <http://www.47deg.com>
+ */
+
+package stdlib
+
+import org.scalacheck.Shapeless._
+import org.scalaexercises.Test
+import org.scalatest.Spec
+import org.scalatest.prop.Checkers
+import shapeless.HNil
+
+class CompositionSpec extends Spec with Checkers {
+  def `compose functions` = {
+    check(
+      Test.testSuccess(
+        Composition.composeFunctions _,
+        "f(g(Scala Exercises))" :: HNil
+      )
+    )
+  }
+
+  def `andthen functions` = {
+    check(
+      Test.testSuccess(
+        Composition.andThenFunctions _,
+        "f(g(Scala Exercises))" :: HNil
+      )
+    )
+  }
+
+}