Add Around combinator

src/main/java/net/kemitix/mon/combinator/Around.java

@@ -0,0 +1,101 @@
+/**
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2017 Paul Campbell
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of this software
+ * and associated documentation files (the "Software"), to deal in the Software without restriction,
+ * including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all copies
+ * or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+ * INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE
+ * AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+ * DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+package net.kemitix.mon.combinator;
+
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.function.BiConsumer;
+import java.util.function.Function;
+
+/**
+ * Around pattern combinator.
+ *
+ * <p>Original from http://boundsofjava.com/newsletter/003-introducing-combinators-part1</p>
+ *
+ * @param <T> the argument type
+ * @param <R> the result type
+ *
+ * @author Federico Peralta Schaffner (fps@boundsofjava.com)
+ */
+@FunctionalInterface
+public interface Around<T, R> extends
+        Function<
+                Function<T, R>,
+                Function<
+                        BiConsumer<Around.Executable<R>, T>,
+                        Function<T, R>>> {
+
+    /**
+     * Decorates a function with an BiConsumer that will be supplier with an executable to perform the function, and the
+     * argument that will be applied when executed.
+     *
+     * @param function the function to apply the argument to and return the result value of
+     * @param around   the bi-consumer that will supplied with the executable and the argument
+     * @param <T>      the argument type
+     * @param <R>      the result type
+     *
+     * @return a partially applied Function that will take an argument, and the result of applying it to function
+     */
+    static <T, R> Function<T, R> decorate(
+            final Function<T, R> function,
+            final BiConsumer<Executable<R>, T> around
+                                         ) {
+        return Around.<T, R>create().apply(function)
+                .apply(around);
+    }
+
+    /**
+     * Create an Around curried function.
+     *
+     * @param <T> the argument type
+     * @param <R> the result type
+     *
+     * @return a curried function that will execute the around function, passing an executable and the invocations
+     * argument. The around function must {@code execute()} the executable and may capture the result.
+     */
+    static <T, R> Around<T, R> create() {
+        return function -> around -> argument -> {
+            final AtomicReference<R> result = new AtomicReference<>();
+            final Executable<R> callback = () -> {
+                result.set(function.apply(argument));
+                return result.get();
+            };
+            around.accept(callback, argument);
+            return result.get();
+        };
+    }
+
+    /**
+     * The executable that will be supplied to the around function to trigger the surrounded function.
+     *
+     * @param <R> the return type of the function
+     */
+    @FunctionalInterface
+    interface Executable<R> {
+
+        /**
+         * Executes the function.
+         *
+         * @return the result of applying the function
+         */
+        R execute();
+    }
+}

src/test/java/net/kemitix/mon/combinator/AfterTest.java

@@ -16,7 +16,6 @@ public class AfterTest {
         final List<String> events = new ArrayList<>();
         final Function<Integer, Integer> squareDecorated =
                 After.decorate(i -> function(i, events), (v, r) -> after(v, r, events));
-
         //when
         final Integer result = squareDecorated.apply(2);
         //then

src/test/java/net/kemitix/mon/combinator/AroundTest.java

@@ -0,0 +1,46 @@
+package net.kemitix.mon.combinator;
+
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.function.Function;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class AroundTest {
+
+    @Test
+    public void canCreateAnAroundCombinator() {
+        //given
+        final List<String> events = new ArrayList<>();
+        final Function<Integer, Integer> squareDecorated =
+                Around.decorate(
+                        i -> function(i, events),
+                        (executable, argument) -> around(executable, argument, events)
+                               );
+        //when
+        final Integer result = squareDecorated.apply(2);
+        //then
+        assertThat(result).isEqualTo(4);
+        assertThat(events).containsExactly("around before 2", "function", "around after 4");
+    }
+
+    private void around(
+            final Around.Executable<Integer> executable,
+            final Integer argument,
+            final List<String> events
+                       ) {
+        events.add("around before " + argument);
+        final Integer result = executable.execute();
+        events.add("around after " + result);
+    }
+
+    private static Integer function(
+            final Integer argument,
+            final List<String> events
+                                   ) {
+        events.add("function");
+        return argument * argument;
+    }
+}