1 files changed, 29 insertions, 3 deletions
diff --git a/src/main/java/au/id/zancanaro/javacheck/Generator.java b/src/main/java/au/id/zancanaro/javacheck/Generator.java
index 9d88020..103bc69 100644
--- a/src/main/java/au/id/zancanaro/javacheck/Generator.java
+++ b/src/main/java/au/id/zancanaro/javacheck/Generator.java
@@ -4,6 +4,7 @@ import java.util.Iterator;
 import java.util.List;
 import java.util.Random;
 import java.util.function.Function;
+import java.util.function.Predicate;
 
 /**
  * Generators are a way of producing random objects and their associated shrink
@@ -63,13 +64,13 @@ public interface Generator<T> {
      * @return A {@link Generator} returning a {@link List}
      */
     @SafeVarargs
-    static <T> Generator<List<T>> tuple(Generator<T>... generators) {
+    static <T> Generator<List<T>> tuple(Generator<? extends T>... generators) {
         return (random, size) -> {
             @SuppressWarnings("unchecked")
             ShrinkTree<T>[] result = (ShrinkTree<T>[]) new ShrinkTree[generators.length];
             int index = 0;
-            for (Generator<T> generator : generators) {
-                result[index++] = generator.generate(random, size);
+            for (Generator<? extends T> generator : generators) {
+                result[index++] = generator.generate(random, size).map(x -> (T) x);
             }
             return ShrinkTree.zip(Function.identity(), result);
         };
@@ -130,6 +131,31 @@ public interface Generator<T> {
         return (random, size) -> ShrinkTree.join(this.generate(random, size).map(action).map(g -> g.generate(random, size)));
     }
 
+    /**
+     * Filter the results of this generator to only those matching a given
+     * predicate.
+     *
+     * suchThat will keep trying the generator until either it provides a valid
+     * value, or a stack overflow error occurs.
+     *
+     * <b>Only use this method with predicates which are very likely to
+     * match.</b>
+     *
+     * @param predicate The predicate to match
+     * @return A new generator resulting from filtering this generator to only
+     * terms which match the given predicate
+     */
+    default Generator<T> suchThat(Predicate<T> predicate) {
+        return (random, size) -> {
+            ShrinkTree<T> result = this.generate(random, size);
+            if (predicate.test(result.getValue())) {
+                return result.filter(predicate);
+            } else {
+                return this.suchThat(predicate).generate(random, size);
+            }
+        };
+    }
+
     default Iterator<T> sample(Random random, int maxSize) {
         return new GeneratorSampleIterator<>(this, random, maxSize);
     }