HList#cons static factory method auto-promotes

jnape · jnape · commit 974fa525019b · 2019-11-20T09:57:15.000-06:00
diff --git a/src/main/java/com/jnape/palatable/lambda/adt/hlist/HList.java b/src/main/java/com/jnape/palatable/lambda/adt/hlist/HList.java
@@ -1,5 +1,7 @@
 package com.jnape.palatable.lambda.adt.hlist;
 
+import com.jnape.palatable.lambda.functions.builtin.fn1.Downcast;
+
 import java.util.Objects;
 
 /**
@@ -63,7 +65,7 @@ public static HNil nil() {
      * @return the newly created HList
      */
     public static <Head, Tail extends HList> HCons<Head, Tail> cons(Head head, Tail tail) {
-        return new HCons<>(head, tail);
+        return Downcast.<HCons<Head, Tail>, HCons<Head, ? extends HList>>downcast(tail.cons(head));
     }
 
     /**
@@ -271,7 +273,7 @@ public final boolean equals(Object other) {
             if (other instanceof HCons) {
                 HCons<?, ?> that = (HCons<?, ?>) other;
                 return this.head.equals(that.head)
-                        && this.tail.equals(that.tail);
+                    && this.tail.equals(that.tail);
             }
             return false;
         }
diff --git a/src/test/java/com/jnape/palatable/lambda/adt/hlist/HListTest.java b/src/test/java/com/jnape/palatable/lambda/adt/hlist/HListTest.java
@@ -2,15 +2,9 @@
 
 import org.junit.Test;
 
-import static com.jnape.palatable.lambda.adt.hlist.HList.cons;
-import static com.jnape.palatable.lambda.adt.hlist.HList.nil;
-import static com.jnape.palatable.lambda.adt.hlist.HList.singletonHList;
-import static com.jnape.palatable.lambda.adt.hlist.HList.tuple;
-import static org.junit.Assert.assertEquals;
-import static org.junit.Assert.assertFalse;
-import static org.junit.Assert.assertNotEquals;
-import static org.junit.Assert.assertSame;
-import static org.junit.Assert.assertTrue;
+import static com.jnape.palatable.lambda.adt.hlist.HList.*;
+import static org.hamcrest.CoreMatchers.instanceOf;
+import static org.junit.Assert.*;
 
 public class HListTest {
 
@@ -36,6 +30,18 @@ public void convenienceStaticFactoryMethods() {
         assertEquals(nil().cons(false).cons(4.0).cons("3").cons('2').cons(1), tuple(1, '2', "3", 4.0, false));
     }
 
+    @Test
+    public void autoPromotion() {
+        assertThat(cons(1, nil()), instanceOf(SingletonHList.class));
+        assertThat(cons(1, singletonHList(1)), instanceOf(Tuple2.class));
+        assertThat(cons(1, tuple(1, 1)), instanceOf(Tuple3.class));
+        assertThat(cons(1, tuple(1, 1, 1)), instanceOf(Tuple4.class));
+        assertThat(cons(1, tuple(1, 1, 1, 1)), instanceOf(Tuple5.class));
+        assertThat(cons(1, tuple(1, 1, 1, 1, 1)), instanceOf(Tuple6.class));
+        assertThat(cons(1, tuple(1, 1, 1, 1, 1, 1)), instanceOf(Tuple7.class));
+        assertThat(cons(1, tuple(1, 1, 1, 1, 1, 1, 1)), instanceOf(Tuple8.class));
+    }
+
     @Test
     public void nilReusesInstance() {
         assertSame(nil(), nil());
