Merge pull request #246 from puffnfresh/feature/semigroups

jbgi · jbgi · commit 9c58bfe8204c · 2016-05-13T13:24:22.000+02:00
Some more Semigroup and Monoid instances
diff --git a/core/src/main/java/fj/Monoid.java b/core/src/main/java/fj/Monoid.java
@@ -4,6 +4,8 @@
 import static fj.Function.flip;
 import fj.data.Array;
 import fj.data.List;
+import fj.data.IO;
+import fj.data.IOFunctions;
 import fj.data.Natural;
 import fj.data.Option;
 import fj.data.Set;
@@ -384,6 +386,18 @@ public static <A> Monoid<Array<A>> arrayMonoid() {
     return monoid(Semigroup.<A>arraySemigroup(), Array.<A>empty());
   }
 
+  /**
+   * A monoid for IO values.
+   */
+  public static <A> Monoid<IO<A>> ioMonoid(final Monoid <A> ma) {
+    return monoid(Semigroup.ioSemigroup(ma.semigroup()), IOFunctions.unit(ma.zero()));
+  }
+
+  /**
+   * A monoid for the Unit value.
+   */
+  public static final Monoid<Unit> unitMonoid = monoid(Semigroup.unitSemigroup, Unit.unit());
+
   /**
    * A monoid for sets.
    *
diff --git a/core/src/main/java/fj/Semigroup.java b/core/src/main/java/fj/Semigroup.java
@@ -2,6 +2,8 @@
 
 import fj.data.Array;
 import fj.data.List;
+import fj.data.IO;
+import fj.data.IOFunctions;
 import fj.data.Natural;
 import fj.data.NonEmptyList;
 import fj.data.Option;
@@ -262,6 +264,20 @@ public static <A> Semigroup<A> semigroup(final F2<A, A, A> sum) {
   public static final Semigroup<StringBuilder> stringBuilderSemigroup =
       semigroup((s1, s2) -> new StringBuilder(s1).append(s2));
 
+  /**
+   * A semigroup which always uses the "first" (left-hand side) value.
+   */
+  public static <A> Semigroup<A> firstSemigroup() {
+      return semigroup((a1, a2) -> a1);
+  }
+
+  /**
+   * A semigroup which always uses the "last" (right-hand side) value.
+   */
+  public static <A> Semigroup<A> lastSemigroup() {
+    return semigroup((a1, a2) -> a2);
+  }
+
   /**
    * A semigroup for functions.
    *
@@ -355,6 +371,13 @@ public static <A, B> Semigroup<P2<A, B>> p2Semigroup(final Semigroup<A> sa, fina
     return semigroup((a1, a2) -> P.lazy(() -> sa.sum(a1._1(), a2._1()), () -> sb.sum(a1._2(), a2._2())));
   }
 
+  /**
+   * A semigroup for IO values.
+   */
+  public static <A> Semigroup<IO<A>> ioSemigroup(final Semigroup <A> sa) {
+      return semigroup((a1, a2) -> IOFunctions.liftM2(a1, a2, sa::sum));
+  }
+
   /**
    * A semigroup for the Unit value.
    */
