Add "dual" methods on Semigroup and Monoid

puffnfresh · puffnfresh · commit 30da36c20aab · 2016-05-26T15:59:31.000+10:00
diff --git a/core/src/main/java/fj/Monoid.java b/core/src/main/java/fj/Monoid.java
@@ -11,6 +11,7 @@
 import fj.data.Set;
 import fj.data.Stream;
 
+import static fj.Function.flip;
 import static fj.Semigroup.multiply1p;
 import static fj.data.Stream.iterableStream;
 
@@ -191,6 +192,13 @@ public A join(final Iterable<A> as, final A a) {
            s.foldLeft1(Function.compose(sum, flip(sum).f(a)));
   }
 
+  /**
+   * Swaps the arguments when summing.
+   */
+  public Monoid<A> dual() {
+    return monoid(flip(sum), zero);
+  }
+
   /**
    * Constructs a monoid from the given sum function and zero value, which must follow the monoidal
    * laws.
diff --git a/core/src/main/java/fj/Semigroup.java b/core/src/main/java/fj/Semigroup.java
@@ -14,6 +14,7 @@
 import java.math.BigInteger;
 
 import static fj.Function.curry;
+import static fj.Function.flip;
 
 /**
  * Implementations must satisfy the law of associativity:
@@ -97,6 +98,13 @@ static <A> A multiply1p(F<A, F<A, A>> sum, int n, A a) {
   }
 
   /**
+  /**
+   * Swaps the arguments when summing.
+   */
+  public Semigroup<A> dual() {
+    return semigroup(flip(sum));
+  }
+
    * Constructs a semigroup from the given function.
    *
    * @param sum The function to construct this semigroup with.
