Fixes #417. Add fj.data.State.bind(F<A, State<S, B>>).

drewctaylorgtri · drewctaylorgtri · commit 849dc39c6d5e · 2020-08-06T10:37:44.000-04:00
diff --git a/core/src/main/java/fj/data/State.java b/core/src/main/java/fj/data/State.java
@@ -111,6 +111,24 @@ public State<S, A> withs(F<S, S> f) {
     return suspended(s -> runF.f(f.f(s)));
   }
 
+  /**
+   * Bind the given function across this state.
+   *
+   * @param f   the given function
+   * @param <B> the type of the output value
+   * @return the state
+   */
+  public <B> State<S, B> bind(F<A, State<S, B>> f) {
+    return flatMap(f);
+  }
+
+  /**
+   * Bind the given function across this state.
+   *
+   * @param f   the given function
+   * @param <B> the type of the output value
+   * @return the state
+   */
   public <B> State<S, B> flatMap(F<A, State<S, B>> f) {
     return suspended(s -> runF.f(s).bind(result -> Trampoline.pure(f.f(result._2()).run(result._1()))));
   }
diff --git a/core/src/test/java/fj/data/StateTest.java b/core/src/test/java/fj/data/StateTest.java
@@ -2,14 +2,47 @@
 
 import org.junit.Test;
 
+import static fj.P.p;
+import static org.junit.Assert.assertEquals;
+
 /**
  * Created by MarkPerry on 18/12/2014.
  */
 public class StateTest {
 
-    @Test
-    public void map() {
+  @Test
+  public void testBind() {
+    assertEquals(p(2, "one"), state().run(1));
+    assertEquals(p(3, "two"), state().run(2));
+    assertEquals(p(4, "three"), state().run(3));
+    assertEquals(p(2, "?"), state().bind(state -> State.constant("?")).run(1));
+    assertEquals(p(3, "?"), state().bind(state -> State.constant("?")).run(2));
+    assertEquals(p(4, "?"), state().bind(state -> State.constant("?")).run(3));
+  }
+
+  @Test
+  public void testFlatMap() {
+    assertEquals(p(2, "one"), state().run(1));
+    assertEquals(p(3, "two"), state().run(2));
+    assertEquals(p(4, "three"), state().run(3));
+    assertEquals(p(2, "?"), state().flatMap(state -> State.constant("?")).run(1));
+    assertEquals(p(3, "?"), state().flatMap(state -> State.constant("?")).run(2));
+    assertEquals(p(4, "?"), state().flatMap(state -> State.constant("?")).run(3));
+  }
 
-    }
+  private static final State<Integer, String> state() {
+    return State.<Integer, String>unit(i -> p(i + 1, toLapine(i)));
+  }
 
+  private static String toLapine(
+      final int i) {
+    return i == 1 ?
+        "one" :
+        i == 2 ?
+            "two" :
+            i == 3 ?
+                "three" :
+                i == 4 ?
+                    "four" : "hrair";
+  }
 }
