From 41f9227afd14f86d65a7a97950e712f09068422b Mon Sep 17 00:00:00 2001
From: Paul Campbell <pcampbell@kemitix.net>
Date: Sun, 10 Dec 2017 10:40:31 +0000
Subject: [PATCH] Allow Functor.map() to return type-safe Functor subclasses

---
 src/main/java/net/kemitix/mon/Functor.java  | 7 ++++---
 src/main/java/net/kemitix/mon/Identity.java | 2 +-
 src/main/java/net/kemitix/mon/Mon.java      | 2 +-
 3 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/src/main/java/net/kemitix/mon/Functor.java b/src/main/java/net/kemitix/mon/Functor.java
index 0111ca1..7dc4fa1 100644
--- a/src/main/java/net/kemitix/mon/Functor.java
+++ b/src/main/java/net/kemitix/mon/Functor.java
@@ -34,18 +34,19 @@ import java.util.function.Function;
  * </ul>
  *
  * @param <T> the type of the Functor
+ * @param <F> the type of the mapped Functor
  *
  * @author Tomasz Nurkiewicz (?@?.?)
  */
-public interface Functor<T> {
+public interface Functor<T, F extends Functor<?, ?>> {
 
     /**
      * Applies the function to the value within the Functor, returning the result within a Functor.
      *
      * @param f   the function to apply
-     * @param <R> the type of the result of the function
+     * @param <R> the type of the content of the mapped functor
      *
      * @return a Functor containing the result of the function {@code f} applied to the value
      */
-    <R> Functor<R> map(Function<T, R> f);
+    <R> F map(Function<T, R> f);
 }
diff --git a/src/main/java/net/kemitix/mon/Identity.java b/src/main/java/net/kemitix/mon/Identity.java
index 1300d7d..4b4df37 100644
--- a/src/main/java/net/kemitix/mon/Identity.java
+++ b/src/main/java/net/kemitix/mon/Identity.java
@@ -33,7 +33,7 @@ import java.util.function.Function;
  * @author Paul Campbell (pcampbell@kemitix.net)
  */
 @RequiredArgsConstructor
-class Identity<T> implements Functor<T> {
+class Identity<T> implements Functor<T, Identity<?>> {
 
     private final T value;
 
diff --git a/src/main/java/net/kemitix/mon/Mon.java b/src/main/java/net/kemitix/mon/Mon.java
index 8abe82d..b591044 100644
--- a/src/main/java/net/kemitix/mon/Mon.java
+++ b/src/main/java/net/kemitix/mon/Mon.java
@@ -37,7 +37,7 @@ import java.util.function.Supplier;
  * @author Paul Campbell (pcampbell@kemitix.net)
  */
 @RequiredArgsConstructor(access = AccessLevel.PROTECTED)
-public class Mon<T> implements Functor<T> {
+public class Mon<T> implements Functor<T, Mon<?>> {
 
     /**
      * The value.