Tail recursion abstraction

functional-programming-java/src/main/java/org/gym/fp/fpjava/Main.java

@@ -2,12 +2,14 @@ package org.gym.fp.fpjava;
 
 import org.gym.fp.fpjava.demo.AbstractControlStructureDemo;
 import org.gym.fp.fpjava.demo.FunctionDemo;
+import org.gym.fp.fpjava.demo.RecursionDemo;
 
 public class Main {
 
     public static void main(String[] args) {
         FunctionDemo.run();
         AbstractControlStructureDemo.run();
+        RecursionDemo.run();
     }
 
 }

functional-programming-java/src/main/java/org/gym/fp/fpjava/demo/RecursionDemo.java

@@ -0,0 +1,23 @@
+package org.gym.fp.fpjava.demo;
+
+import org.gym.fp.fpjava.type.TailCall;
+
+import static org.gym.fp.fpjava.type.TailCall.ret;
+import static org.gym.fp.fpjava.type.TailCall.suspend;
+
+public class RecursionDemo {
+
+    public static void run() {
+        System.out.println(add(10, 1000));
+    }
+
+    public static int add(int x, int y) {
+        return addRec(x, y).eval();
+    }
+
+    private static TailCall<Integer> addRec(int x, int y) {
+        return y == 0
+                ? ret(x)
+                : suspend(() -> addRec(x + 1, y - 1));
+    }
+}

functional-programming-java/src/main/java/org/gym/fp/fpjava/type/TailCall.java

@@ -0,0 +1,71 @@
+package org.gym.fp.fpjava.type;
+
+public abstract class TailCall<T> {
+
+    private TailCall() {
+    }
+
+    public static <T> TailCall<T> ret(T t) {
+        return new Return<>(t);
+    }
+
+    public static <T> TailCall<T> suspend(Supplier<TailCall<T>> s) {
+        return new Suspend<>(s);
+    }
+
+    public abstract TailCall<T> resume();
+
+    public abstract T eval();
+
+    public abstract boolean isSuspend();
+
+    private static class Return<T> extends TailCall<T> {
+        private final T t;
+
+        public Return(T t) {
+            this.t = t;
+        }
+
+        @Override
+        public TailCall<T> resume() {
+            throw new IllegalStateException("Return has no resume");
+        }
+
+        @Override
+        public T eval() {
+            return t;
+        }
+
+        @Override
+        public boolean isSuspend() {
+            return false;
+        }
+    }
+
+    private static class Suspend<T> extends TailCall<T> {
+        private final Supplier<TailCall<T>> resume;
+
+        public Suspend(Supplier<TailCall<T>> resume) {
+            this.resume = resume;
+        }
+
+        @Override
+        public TailCall<T> resume() {
+            return resume.get();
+        }
+
+        @Override
+        public T eval() {
+            TailCall<T> tailRec = this;
+            while (tailRec.isSuspend()) {
+                tailRec = tailRec.resume();
+            }
+            return tailRec.eval();
+        }
+
+        @Override
+        public boolean isSuspend() {
+            return true;
+        }
+    }
+}