collectionConcat monoid

Author: jyrimatti

pom.xml

@@ -2,7 +2,7 @@
     <modelVersion>4.0.0</modelVersion>
     <groupId>fi.solita.utils</groupId>
     <artifactId>functional-utils</artifactId>
-    <version>0.12.64</version>
+    <version>0.12.66</version>
     <build>
         <resources>
                 <resource>

src/main/java/fi/solita/utils/functional/Monoids.java

@@ -5,6 +5,7 @@
 import static fi.solita.utils.functional.Collections.newList;
 import static fi.solita.utils.functional.Functional.concat;
 
+import java.util.Collection;
 import java.util.Comparator;
 import java.util.List;
 import java.util.Map;
@@ -81,6 +82,23 @@ public Iterable<T> zero() {
         };
     }
 
+    public static final <T> Monoid<Collection<T>> collectionConcat() {
+        return new Monoid<Collection<T>>() {
+            public Collection<T> apply(Map.Entry<? extends Collection<T>, ? extends Collection<T>> t) {
+                if (t.getKey() == null || t.getKey().isEmpty()) {
+                    return t.getValue();
+                } else if (t.getValue() == null || t.getValue().isEmpty()) {
+                    return t.getKey();
+                }
+                return newList(concat(t.getKey(), t.getValue()));
+            }
+
+            public Collection<T> zero() {
+                return emptyList();
+            }
+        };
+    }
+    
     public static final <T> Monoid<List<T>> listConcat() {
         return new Monoid<List<T>>() {
             public List<T> apply(Map.Entry<? extends List<T>, ? extends List<T>> t) {

src/main/java/fi/solita/utils/functional/SemiGroups.java

@@ -111,6 +111,10 @@ public static final <T> SemiGroup<List<T>> listConcat() {
         return Monoids.listConcat();
     }
 
+    public static final <T> SemiGroup<Collection<T>> collectionConcat() {
+        return Monoids.collectionConcat();
+    }
+    
     public static final <T> SemiGroup<Iterable<T>> iterableConcat() {
         return Monoids.iterableConcat();
     }