From 39f49cd0d334fa37c5cbd89261e55510911d64b4 Mon Sep 17 00:00:00 2001
From: Fabio Scotto di Santolo <fabio.scottodisantolo@nttdata.com>
Date: Thu, 23 Jan 2020 15:44:29 +0100
Subject: [PATCH] Finito Capitolo 2 del libro

---
 functional-programming-java/pom.xml           | 25 +++++-----
 .../src/main/java/org/gym/fp/fpjava/Main.java |  7 +++
 .../org/gym/fp/fpjava/demo/FunctionDemo.java  | 49 +++++++++++++++++++
 .../gym/fp/fpjava/type/BinaryOperator.java    |  4 ++
 .../java/org/gym/fp/fpjava/type/Function.java | 49 +++++++++++++++++++
 .../java/org/gym/fp/fpjava/type/Tuple.java    | 11 +++++
 6 files changed, 133 insertions(+), 12 deletions(-)
 create mode 100644 functional-programming-java/src/main/java/org/gym/fp/fpjava/demo/FunctionDemo.java
 create mode 100644 functional-programming-java/src/main/java/org/gym/fp/fpjava/type/BinaryOperator.java
 create mode 100644 functional-programming-java/src/main/java/org/gym/fp/fpjava/type/Function.java
 create mode 100644 functional-programming-java/src/main/java/org/gym/fp/fpjava/type/Tuple.java

diff --git a/functional-programming-java/pom.xml b/functional-programming-java/pom.xml
index 46b2ac2..8e30f35 100644
--- a/functional-programming-java/pom.xml
+++ b/functional-programming-java/pom.xml
@@ -1,19 +1,20 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <project xmlns="http://maven.apache.org/POM/4.0.0"
-				 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-				 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
-	<modelVersion>4.0.0</modelVersion>
-	<artifactId>functional-programming-java</artifactId>
-	<packaging>jar</packaging>
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>functional-programming-java</artifactId>
+    <description>Questo modulo contiene gli esercizi e le prove dal libro "Functional Programmin in Java"</description>
+    <packaging>jar</packaging>
 
-	<parent>
-		<artifactId>fpgym</artifactId>
-		<groupId>org.gym.fp</groupId>
-		<version>1.0</version>
-	</parent>
+    <parent>
+        <artifactId>fpgym</artifactId>
+        <groupId>org.gym.fp</groupId>
+        <version>1.0</version>
+    </parent>
 
-	<properties>
-		<maven.compiler.source>8</maven.compiler.source>
+    <properties>
+        <maven.compiler.source>8</maven.compiler.source>
 		<maven.compiler.target>8</maven.compiler.target>
 	</properties>
 
diff --git a/functional-programming-java/src/main/java/org/gym/fp/fpjava/Main.java b/functional-programming-java/src/main/java/org/gym/fp/fpjava/Main.java
index c650a98..3e605fd 100644
--- a/functional-programming-java/src/main/java/org/gym/fp/fpjava/Main.java
+++ b/functional-programming-java/src/main/java/org/gym/fp/fpjava/Main.java
@@ -1,4 +1,11 @@
 package org.gym.fp.fpjava;
 
+import org.gym.fp.fpjava.demo.FunctionDemo;
+
 public class Main {
+
+    public static void main(String[] args) {
+        FunctionDemo.run();
+    }
+
 }
diff --git a/functional-programming-java/src/main/java/org/gym/fp/fpjava/demo/FunctionDemo.java b/functional-programming-java/src/main/java/org/gym/fp/fpjava/demo/FunctionDemo.java
new file mode 100644
index 0000000..15324f7
--- /dev/null
+++ b/functional-programming-java/src/main/java/org/gym/fp/fpjava/demo/FunctionDemo.java
@@ -0,0 +1,49 @@
+package org.gym.fp.fpjava.demo;
+
+import org.gym.fp.fpjava.type.BinaryOperator;
+import org.gym.fp.fpjava.type.Function;
+import org.gym.fp.fpjava.type.Tuple;
+
+public class FunctionDemo {
+
+    // Es. 2.12
+    public static final Function<Integer, Integer> factorial =
+            n -> n <= 1 ? n : n * FunctionDemo.factorial.apply(n - 1);
+
+    public static void run() {
+        // function composition
+        Function<Integer, Integer> triple = n -> n * 3;
+        Function<Integer, Integer> square = n -> n * n;
+        Function<Integer, Integer> compose = square.compose(triple);
+        System.out.println(compose.apply(2));
+        System.out.println(square.andThen(triple).apply(2));
+
+        // function currying
+        BinaryOperator add = x -> y -> x + y;
+        System.out.println(add.apply(3).apply(5));
+        System.out.println(func().apply("ciao").apply("mondo").apply("questo").apply("currying"));
+
+        // partial function
+        Function<Double, Function<Double, Double>> addTax = x -> y -> y + y / 100 * x;
+        Function<Double, Double> add9PercentTax = addTax.apply(9.0);
+        System.out.println(add9PercentTax.apply(90.0));
+
+        // recursive function
+        System.out.println(factorial.apply(5));
+    }
+
+    // ES. 2.9
+    private static <A, B, C, D> Function<A, Function<B, Function<C, Function<D, String>>>> func() {
+        return a -> b -> c -> d -> String.format("%s %s %s %s", a, b, c, d);
+    }
+
+    // ES. 2.10
+    private static <A, B, C> Function<A, Function<B, C>> curry(Function<Tuple<A, B>, C> fn) {
+        return a -> b -> fn.apply(new Tuple<>(a, b));
+    }
+
+    // ES. 2.11
+    private static <T, U, V> Function<U, Function<T, V>> swap(Function<T, Function<U, V>> fn) {
+        return u -> t -> fn.apply(t).apply(u);
+    }
+}
diff --git a/functional-programming-java/src/main/java/org/gym/fp/fpjava/type/BinaryOperator.java b/functional-programming-java/src/main/java/org/gym/fp/fpjava/type/BinaryOperator.java
new file mode 100644
index 0000000..fc582d0
--- /dev/null
+++ b/functional-programming-java/src/main/java/org/gym/fp/fpjava/type/BinaryOperator.java
@@ -0,0 +1,4 @@
+package org.gym.fp.fpjava.type;
+
+public interface BinaryOperator extends Function<Integer, Function<Integer, Integer>> {
+}
diff --git a/functional-programming-java/src/main/java/org/gym/fp/fpjava/type/Function.java b/functional-programming-java/src/main/java/org/gym/fp/fpjava/type/Function.java
new file mode 100644
index 0000000..be373fb
--- /dev/null
+++ b/functional-programming-java/src/main/java/org/gym/fp/fpjava/type/Function.java
@@ -0,0 +1,49 @@
+package org.gym.fp.fpjava.type;
+
+public interface Function<T, U> {
+    U apply(T arg);
+
+    default <V> Function<V, U> compose(Function<V, T> f) {
+        return x -> apply(f.apply(x));
+    }
+
+    default <V> Function<T, V> andThen(Function<U, V> f) {
+        return x -> f.apply(apply(x));
+    }
+
+    default <R> Function<U, R> partialA(T a, Function<T, Function<U, R>> f) {
+        return f.apply(a);
+    }
+
+    default <R> Function<T, R> partialB(U b, Function<T, Function<U, R>> f) {
+        return a -> f.apply(a).apply(b);
+    }
+
+    static <T> Function<T, T> identity() {
+        return x -> x;
+    }
+
+    static <T, U, V> Function<V, U> compose(Function<T, U> f, Function<V, T> g) {
+        return x -> f.apply(g.apply(x));
+    }
+
+    static <T, U, V> Function<T, V> andThen(Function<T, U> f, Function<U, V> g) {
+        return x -> g.apply(f.apply(x));
+    }
+
+    static <T, U, V> Function<Function<T, U>, Function<Function<U, V>, Function<T, V>>> compose() {
+        return x -> y -> y.compose(x);
+    }
+
+    static <T, U, V> Function<Function<T, U>, Function<Function<V, T>, Function<V, U>>> andThen() {
+        return x -> y -> y.andThen(x);
+    }
+
+    static <T, U, V> Function<Function<T, U>, Function<Function<U, V>, Function<T, V>>> higherAndThen() {
+        return x -> y -> z -> y.apply(x.apply(z));
+    }
+
+    static <T, U, V> Function<Function<U, V>, Function<Function<T, U>, Function<T, V>>> higherCompose() {
+        return (Function<U, V> x) -> (Function<T, U> y) -> (T z) -> x.apply(y.apply(z));
+    }
+}
diff --git a/functional-programming-java/src/main/java/org/gym/fp/fpjava/type/Tuple.java b/functional-programming-java/src/main/java/org/gym/fp/fpjava/type/Tuple.java
new file mode 100644
index 0000000..4aa36d1
--- /dev/null
+++ b/functional-programming-java/src/main/java/org/gym/fp/fpjava/type/Tuple.java
@@ -0,0 +1,11 @@
+package org.gym.fp.fpjava.type;
+
+public class Tuple<A, B> {
+    private final A _1;
+    private final B _2;
+
+    public Tuple(A first, B second) {
+        this._1 = first;
+        this._2 = second;
+    }
+}