From 62a61213cc664a6704dd38bd6dfa2bd1ea97f774 Mon Sep 17 00:00:00 2001
From: Paul Campbell <pcampbell@kemitix.net>
Date: Sat, 23 Jun 2018 18:22:05 +0100
Subject: [PATCH] Added Result.maybe()

---
 src/main/java/net/kemitix/mon/result/Err.java |  7 +++
 .../java/net/kemitix/mon/result/Result.java   | 10 +++
 .../java/net/kemitix/mon/result/Success.java  | 10 +++
 src/test/java/net/kemitix/mon/ResultTest.java | 61 +++++++++++++++++++
 4 files changed, 88 insertions(+)

diff --git a/src/main/java/net/kemitix/mon/result/Err.java b/src/main/java/net/kemitix/mon/result/Err.java
index a94318d..76431ac 100644
--- a/src/main/java/net/kemitix/mon/result/Err.java
+++ b/src/main/java/net/kemitix/mon/result/Err.java
@@ -22,9 +22,11 @@
 package net.kemitix.mon.result;
 
 import lombok.RequiredArgsConstructor;
+import net.kemitix.mon.maybe.Maybe;
 
 import java.util.function.Consumer;
 import java.util.function.Function;
+import java.util.function.Predicate;
 
 /**
  * An Error Result.
@@ -60,4 +62,9 @@ class Err<T> implements Result<T> {
     public void match(final Consumer<T> onSuccess, final Consumer<Throwable> onError) {
         onError.accept(error);
     }
+
+    @Override
+    public Result<Maybe<T>> maybe(final Predicate<T> predicate) {
+        return Result.error(error);
+    }
 }
diff --git a/src/main/java/net/kemitix/mon/result/Result.java b/src/main/java/net/kemitix/mon/result/Result.java
index 3bf2a81..37775a9 100644
--- a/src/main/java/net/kemitix/mon/result/Result.java
+++ b/src/main/java/net/kemitix/mon/result/Result.java
@@ -21,8 +21,11 @@
 
 package net.kemitix.mon.result;
 
+import net.kemitix.mon.maybe.Maybe;
+
 import java.util.function.Consumer;
 import java.util.function.Function;
+import java.util.function.Predicate;
 
 /**
  * An Either type for holding a result or an error (exception).
@@ -97,4 +100,11 @@ public interface Result<T> {
      */
     void match(Consumer<T> onSuccess, Consumer<Throwable> onError);
 
+    /**
+     * Wraps the value within the Result in a Maybe, either a Just if the predicate is true, or Nothing.
+     *
+     * @param predicate the test to decide
+     * @return a Result containing a Maybe that may or may not contain a value
+     */
+    Result<Maybe<T>> maybe(Predicate<T> predicate);
 }
diff --git a/src/main/java/net/kemitix/mon/result/Success.java b/src/main/java/net/kemitix/mon/result/Success.java
index 414c487..80dbea5 100644
--- a/src/main/java/net/kemitix/mon/result/Success.java
+++ b/src/main/java/net/kemitix/mon/result/Success.java
@@ -22,9 +22,11 @@
 package net.kemitix.mon.result;
 
 import lombok.RequiredArgsConstructor;
+import net.kemitix.mon.maybe.Maybe;
 
 import java.util.function.Consumer;
 import java.util.function.Function;
+import java.util.function.Predicate;
 
 /**
  * A Successful Result.
@@ -60,4 +62,12 @@ class Success<T> implements Result<T> {
     public void match(final Consumer<T> onSuccess, final Consumer<Throwable> onError) {
         onSuccess.accept(value);
     }
+
+    @Override
+    public Result<Maybe<T>> maybe(final Predicate<T> predicate) {
+        if (predicate.test(value)) {
+            return Result.ok(Maybe.just(value));
+        }
+        return Result.ok(Maybe.nothing());
+    }
 }
diff --git a/src/test/java/net/kemitix/mon/ResultTest.java b/src/test/java/net/kemitix/mon/ResultTest.java
index 4de77e3..1bbb9e8 100644
--- a/src/test/java/net/kemitix/mon/ResultTest.java
+++ b/src/test/java/net/kemitix/mon/ResultTest.java
@@ -1,10 +1,13 @@
 package net.kemitix.mon;
 
 import lombok.RequiredArgsConstructor;
+import net.kemitix.mon.maybe.Maybe;
 import net.kemitix.mon.result.Result;
 import org.assertj.core.api.WithAssertions;
 import org.junit.Test;
 
+import java.util.function.Predicate;
+
 public class ResultTest implements WithAssertions {
 
     @Test
@@ -134,6 +137,64 @@ public class ResultTest implements WithAssertions {
         );
     }
 
+    @Test
+    public void successMaybe_whenPasses_isSuccessJust() {
+        //given
+        final Result<Integer> okResult = Result.ok(1);
+        //when
+        final Result<Maybe<Integer>> maybeResult = okResult.maybe(value -> value >= 0);
+        //then
+        assertThat(maybeResult.isOkay()).isTrue();
+        maybeResult.match(
+                success -> assertThat(success.toOptional()).contains(1),
+                error -> fail("not an error")
+        );
+    }
+
+    @Test
+    public void successMaybe_whenFails_isSuccessNothing() {
+        //given
+        final Result<Integer> okResult = Result.ok(1);
+        //when
+        final Result<Maybe<Integer>> maybeResult = okResult.maybe(value -> value >= 4);
+        //then
+        assertThat(maybeResult.isOkay()).isTrue();
+        maybeResult.match(
+                success -> assertThat(success.toOptional()).isEmpty(),
+                error -> fail("not an error")
+        );
+    }
+
+    @Test
+    public void errorMaybe_whenPasses_isError() {
+        //given
+        final RuntimeException exception = new RuntimeException();
+        final Result<Integer> errorResult = Result.error(exception);
+        //when
+        final Result<Maybe<Integer>> maybeResult = errorResult.maybe(value -> value >= 0);
+        //then
+        assertThat(maybeResult.isError()).isTrue();
+        maybeResult.match(
+                success -> fail("not a success"),
+                error -> assertThat(error).isSameAs(exception)
+        );
+    }
+
+    @Test
+    public void errorMaybe_whenFails_isError() {
+        //given
+        final RuntimeException exception = new RuntimeException();
+        final Result<Integer> errorResult = Result.error(exception);
+        //when
+        final Result<Maybe<Integer>> maybeResult = errorResult.maybe(value -> value >= 4);
+        //then
+        assertThat(maybeResult.isError()).isTrue();
+        maybeResult.match(
+                success -> fail("not a success"),
+                error -> assertThat(error).isSameAs(exception)
+        );
+    }
+
     @Test
     public void useCase_whenOkay_thenReturnSuccess() {
         //given