Add fj.test.Gen.streamOf(Gen<A>), similar to fj.test.Gen.listOf(Gen<A>, int); returns a generator of streams whose values come from the given generator.

drewctaylor · drewctaylor · commit 560db2e6d9de · 2020-05-19T20:34:01.000-04:00
diff --git a/quickcheck/src/main/java/fj/test/Gen.java b/quickcheck/src/main/java/fj/test/Gen.java
@@ -9,6 +9,7 @@
 import fj.data.Array;
 import fj.data.List;
 import fj.data.Option;
+import fj.data.Stream;
 import fj.function.Effect1;
 
 import static fj.Bottom.error;
@@ -497,6 +498,18 @@ public static <A> Gen<List<A>> listOf1(final Gen<A> g) {
     return listOf(g, 1);
   }
 
+  /**
+   * Returns a generator of streams whose values come from the given generator.
+   *
+   * @param g   the generator to produce values from for the returned generator
+   * @param <A> the type of the generator
+   *
+   * @return A generator of streams whose values come from the given generator.
+   */
+  public static <A> Gen<Stream<A>> streamOf(final Gen<A> g) {
+    return gen(i -> r -> Stream.cons(g.gen(i, r), () -> streamOf(g).gen(i, r)));
+  }
+
   /**
    * Returns a generator that picks one element from the given list. If the given list is empty, then the
    * returned generator will never produce a value.
diff --git a/quickcheck/src/test/java/fj/test/GenTest.java b/quickcheck/src/test/java/fj/test/GenTest.java
@@ -1,16 +1,19 @@
 package fj.test;
 
 import fj.data.List;
+import fj.data.Stream;
 import fj.function.Effect1;
 import org.junit.Test;
 
 import static fj.Ord.charOrd;
 import static fj.data.List.list;
 import static fj.data.List.range;
-import static fj.test.Gen.selectionOf;
 import static fj.test.Gen.combinationOf;
-import static fj.test.Gen.wordOf;
 import static fj.test.Gen.permutationOf;
+import static fj.test.Gen.pickOne;
+import static fj.test.Gen.selectionOf;
+import static fj.test.Gen.streamOf;
+import static fj.test.Gen.wordOf;
 import static fj.test.Rand.standard;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertTrue;
@@ -175,6 +178,15 @@ public void testWordOf_four() {
     });
   }
 
+  @Test
+  public void testStreamOf() {
+    final Gen<Stream<Character>> instance = streamOf(pickOne(AS));
+    testPick(100, instance.map(stream -> stream.take(4).toList()), actual -> {
+      assertEquals(4, actual.length());
+      assertTrue(actual.forall(actualA -> AS.exists(a -> a.equals(actualA))));
+    });
+  }
+
   private static <A> void testPick(int n, Gen<List<A>> instance, Effect1<List<A>> test) {
     range(0, n).map(ignore -> instance.gen(0, standard)).foreachDoEffect(test);
   }
