From e853894382a1992deae5e4b90c80535ad9354a20 Mon Sep 17 00:00:00 2001
From: Paul Campbell <pcampbell@kemitix.net>
Date: Sat, 28 Jul 2018 16:35:40 +0100
Subject: [PATCH] Condition.flatMap() added with Monad tests

---
 .../net/kemitix/conditional/Condition.java    | 10 +++++
 .../conditional/ConditionMonadTest.java       | 45 +++++++++++++++++++
 2 files changed, 55 insertions(+)
 create mode 100644 src/test/java/net/kemitix/conditional/ConditionMonadTest.java

diff --git a/src/main/java/net/kemitix/conditional/Condition.java b/src/main/java/net/kemitix/conditional/Condition.java
index 8bfee11..4fe2d15 100644
--- a/src/main/java/net/kemitix/conditional/Condition.java
+++ b/src/main/java/net/kemitix/conditional/Condition.java
@@ -21,6 +21,7 @@
 
 package net.kemitix.conditional;
 
+import java.util.function.Function;
 import java.util.function.Supplier;
 
 /**
@@ -183,4 +184,13 @@ public interface Condition {
     @SuppressWarnings(SuppressHelper.CS_ILLEGALTHROWS)
     void otherwiseThrow(Exception exception) throws Exception;
 
+    /**
+     * Apply the function to the Condtion, resulting an another Condition.
+     *
+     * @param f the function to apply
+     * @return a new Condition
+     */
+    default Condition flatMap(final Function<Boolean, Condition> f) {
+        return f.apply(isTrue());
+    }
 }
diff --git a/src/test/java/net/kemitix/conditional/ConditionMonadTest.java b/src/test/java/net/kemitix/conditional/ConditionMonadTest.java
new file mode 100644
index 0000000..74307b2
--- /dev/null
+++ b/src/test/java/net/kemitix/conditional/ConditionMonadTest.java
@@ -0,0 +1,45 @@
+package net.kemitix.conditional;
+
+import org.assertj.core.api.WithAssertions;
+import org.junit.Test;
+
+import java.util.function.Function;
+
+public class ConditionMonadTest implements WithAssertions {
+
+    private final boolean v = true;
+    private final Function<Boolean, Condition> f = i -> r(true);
+    private final Function<Boolean, Condition> g = i -> r(i);
+
+    private static Condition r(boolean v) {
+        return Condition.where(v);
+    }
+
+    @Test
+    public void leftIdentity() {
+        assertThat(
+                r(v).flatMap(f)
+        ).isEqualTo(
+                f.apply(v)
+        );
+    }
+
+    @Test
+    public void rightIdentity() {
+        assertThat(
+                r(v).flatMap(x -> r(x))
+        ).isEqualTo(
+                r(v)
+        );
+    }
+
+    @Test
+    public void associativity() {
+        assertThat(
+                r(v).flatMap(f).flatMap(g)
+        ).isEqualTo(
+                r(v).flatMap(x -> f.apply(x).flatMap(g))
+        );
+    }
+
+}