Added monoid composition

Runar Bjarnason · Runar Bjarnason · commit 35b55ff6ab65 · 2015-04-17T18:41:17.000-04:00
diff --git a/.gitignore b/.gitignore
@@ -13,5 +13,4 @@ build
 .DS_Store
 MANIFEST.MF
 */bin/**
-
-
+*~
diff --git a/core/src/main/java/fj/Monoid.java b/core/src/main/java/fj/Monoid.java
@@ -1,7 +1,6 @@
 package fj;
 
 import static fj.Function.curry;
-import static fj.Function.compose;
 import static fj.Function.flip;
 import fj.data.Array;
 import fj.data.List;
@@ -20,7 +19,7 @@
  * <ul>
  * <li><em>Left Identity</em>; forall x. sum(zero(), x) == x</li>
  * <li><em>Right Identity</em>; forall x. sum(x, zero()) == x</li>
- * <li><em>Associativity</em>; forall x. forall y. forall z. sum(sum(x, y), z) == sum(x, sum(y, z))</li>
+ * <li><em>Associativity</em>; forall x y z. sum(sum(x, y), z) == sum(x, sum(y, z))</li>
  * </ul>
  *
  * @version %build.number%
@@ -34,6 +33,14 @@ private Monoid(final F<A, F<A, A>> sum, final A zero) {
     this.zero = zero;
   }
 
+  /**
+   * Composes this monoid with another.
+   */
+  public <B> Monoid<P2<A,B>>compose(Monoid<B> m) {
+    return monoid((P2<A,B> x) -> (P2<A,B> y) ->
+      P.p(sum(x._1(), y._1()), m.sum(x._2(), y._2())), P.p(zero, m.zero));
+  }
+
   /**
    * Returns a semigroup projection of this monoid.
    *
@@ -176,7 +183,7 @@ public A join(final Iterable<A> as, final A a) {
     final Stream<A> s = iterableStream(as);
     return s.isEmpty() ?
            zero :
-           s.foldLeft1(compose(sum, flip(sum).f(a)));
+           s.foldLeft1(Function.compose(sum, flip(sum).f(a)));
   }
 
   /**
