tyler569 · January 12, 2020 18:50 · Jan 12, 2020
diff --git a/monad.java b/monad.java
@@ -0,0 +1,69 @@
+import java.util.function.*;
+
+interface Monad<T> {
+    Monad<T> bind(Function<T, Monad<T>> fn);
+}
+
+class maybe<T> implements Monad<T> {
+    boolean valid;
+    T value;
+
+    maybe(boolean v, T val) {
+        valid = v;
+        value = val;
+    }
+
+    maybe() {
+        valid = false;
+        value = null;
+    }
+
+    public Monad<T> bind(Function<T, Monad<T>> fn) {
+        if (valid) {
+            return fn.apply(value);
+        } else {
+            return None();
+        }
+    }
+
+    public static <U> maybe<U> Just(U value) {
+        return new maybe<U>(true, value);
+    }
+
+    public static <U> maybe<U> None() {
+        return new maybe<U>();
+    }
+
+    public String toString() {
+        if (valid) {
+            return "Some(" + value.toString() + ")";
+        } else {
+            return "None()";
+        }
+    }
+}
+
+public class monad {
+    static maybe<Integer> incEven(Integer j) {
+        if (j % 2 == 0) {
+            return maybe.None();
+        } else {
+            return maybe.Just(j + 1);
+        }
+    }
+
+    static maybe<Integer> half(Integer j) {
+        return maybe.Just(j / 2);
+    }
+
+    public static void main(String[] args) {
+        for (int i=0; i<20; i++) {
+            Monad<Integer> some = maybe.Just(i);
+            Monad<Integer> g = some.bind((j) -> incEven(j));
+            Monad<Integer> h = g.bind((j) -> half(j));
+            Monad<Integer> k = h.bind((j) -> incEven(j));
+
+            System.out.println(k);
+        }
+    }
+}