Convert tail calls to kotlin

src/main/kotlin/recursion/TailCall.java

@@ -1,25 +0,0 @@
-package recursion;
-
-import java.util.stream.Stream;
-
-@FunctionalInterface
-public interface TailCall<T> {
-
-    TailCall<T> apply();
-
-    default boolean isComplete() {
-        return false;
-    }
-
-    default T result() {
-        throw new UnsupportedOperationException();
-    }
-
-    default T invoke() {
-        return Stream.iterate(this, TailCall::apply)
-                     .filter(TailCall::isComplete)
-                     .findFirst()
-                     .get()
-                     .result();
-    }
-}

src/main/kotlin/recursion/TailCall.kt

@@ -0,0 +1,13 @@
+package recursion
+
+@FunctionalInterface
+interface TailCall<T> {
+
+    fun isComplete() = false
+    fun apply(): TailCall<T>
+    fun result(): T = throw UnsupportedOperationException()
+
+    operator fun invoke() = generateSequence(this) { it.apply() }
+        .first { it.isComplete() }
+        .result()
+}

src/main/kotlin/recursion/TailCalls.java

@@ -1,28 +0,0 @@
-package recursion;
-
-public class TailCalls {
-
-    public static <T> TailCall<T> tailCall(TailCall<T> nextCall) {
-        return nextCall;
-    }
-
-    public static <T> TailCall<T> done(T value) {
-        return new TailCall<T>() {
-
-            @Override
-            public boolean isComplete() {
-                return true;
-            }
-
-            @Override
-            public T result() {
-                return value;
-            }
-
-            @Override
-            public TailCall<T> apply() {
-                throw new UnsupportedOperationException();
-            }
-        };
-    }
-}

src/main/kotlin/recursion/TailCalls.kt

@@ -0,0 +1,16 @@
+package recursion
+
+object TailCalls {
+
+    @JvmStatic
+    fun <T> tailCall(nextCall: () -> TailCall<T>) = object : TailCall<T> {
+        override fun apply() = nextCall()
+    }
+
+    @JvmStatic
+    fun <T> done(value: T) = object : TailCall<T> {
+        override fun isComplete() = true
+        override fun result() = value
+        override fun apply() = throw UnsupportedOperationException()
+    }
+}

src/test/kotlin/recursion/TailCallTest.kt

@@ -8,13 +8,13 @@ class TailCallTest {
 
     @Test
     fun `tailCall does not support result`() {
-        val tailCall = TailCall<Any> { null }
+        val tailCall = object : TailCall<Nothing?> { override fun apply() = done(null) }
 
         assertThrows(UnsupportedOperationException::class.java) { tailCall.result() }
     }
 
     @Test
     fun `done does not support apply`() {
-        assertThrows(UnsupportedOperationException::class.java) { done<Any>(null).apply() }
+        assertThrows(UnsupportedOperationException::class.java) { done(null).apply() }
     }
 }