#163: Implemented Validation.sequence using a semigroup

mperry · mperry · commit 35f81787f2d3 · 2015-07-04T00:46:54.000+10:00
diff --git a/core/src/main/java/fj/data/Validation.java b/core/src/main/java/fj/data/Validation.java
@@ -200,6 +200,18 @@ public <A> Validation<E, A> sequence(final Validation<E, A> v) {
     return bind(Function.<T, Validation<E, A>>constant(v));
   }
 
+  /**
+   * If list contains a failure, returns a failure of the reduction of
+   * all the failures using the semigroup, otherwise returns the successful list.
+   */
+  public static <E, A> Validation<E, List<A>> sequence(final Semigroup<E> s, final List<Validation<E, A>> list) {
+    if (list.exists(v -> v.isFail())) {
+      return Validation.<E, List<A>>fail(list.filter(v -> v.isFail()).map(v -> v.fail()).foldLeft1((e1, e2) -> s.sum(e1, e2)));
+    } else {
+      return Validation.success(list.foldLeft((List<A> acc, Validation<E, A> v) -> acc.cons(v.success()), List.nil()).reverse());
+    }
+  }
+
   /**
    * Returns <code>None</code> if this is a failure or if the given predicate <code>p</code> does not hold for the
    * success value, otherwise, returns a success in <code>Some</code>.
diff --git a/props-core/src/test/java/fj/data/properties/ValidationProperties.java b/props-core/src/test/java/fj/data/properties/ValidationProperties.java
@@ -1,6 +1,7 @@
 package fj.data.properties;
 
 import fj.P2;
+import fj.Semigroup;
 import fj.data.List;
 import fj.data.Validation;
 import fj.test.Arbitrary;
@@ -45,4 +46,18 @@ public Property sequenceReduce() {
         });
     }
 
+    public Property sequence() {
+        return Property.property(arbList(arbValidation(arbString, arbInteger)), list -> {
+            Validation<String, List<Integer>> v = Validation.sequence(Semigroup.stringSemigroup, list);
+            Property p1 = implies(list.exists((Validation<String, Integer> v2) -> v2.isFail()), () -> prop(v.isFail()));
+            boolean b = list.forall((Validation<String, Integer> v2) -> v2.isSuccess());
+            Property p2 = implies(b, () -> {
+                List<Integer> l2 = list.map((Validation<String, Integer> v2) -> v2.success());
+                boolean b2 = v.success().equals(l2);
+                return prop(b2);
+            });
+            return p1.and(p2);
+        });
+    }
+
 }
