Adding EndoK monoid

jnape · jnape · commit 435f3a5a4b8f · 2019-10-13T15:31:01.000-05:00
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -6,6 +6,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/).
 ## [Unreleased]
 ### Added
 - `Writer`, the writer monad
+- `EndoK`, a monoid formed under endomorphism for any monad
 
 ## [5.0.0] - 2019-09-23
 ### Changed
diff --git a/src/main/java/com/jnape/palatable/lambda/monoid/builtin/EndoK.java b/src/main/java/com/jnape/palatable/lambda/monoid/builtin/EndoK.java
@@ -0,0 +1,59 @@
+package com.jnape.palatable.lambda.monoid.builtin;
+
+import com.jnape.palatable.lambda.functions.Fn1;
+import com.jnape.palatable.lambda.functions.builtin.fn3.FoldLeft;
+import com.jnape.palatable.lambda.functions.specialized.MonoidFactory;
+import com.jnape.palatable.lambda.functions.specialized.Pure;
+import com.jnape.palatable.lambda.monad.MonadRec;
+import com.jnape.palatable.lambda.monad.SafeT;
+import com.jnape.palatable.lambda.monoid.Monoid;
+
+import static com.jnape.palatable.lambda.functions.specialized.Kleisli.kleisli;
+import static com.jnape.palatable.lambda.monad.SafeT.safeT;
+
+/**
+ * The monoid formed under monadic endomorphism.
+ *
+ * @param <M>  the {@link MonadRec} witness
+ * @param <A>  the carrier type
+ * @param <MA> the fully witnessed {@link MonadRec} type
+ */
+public final class EndoK<M extends MonadRec<?, M>, A, MA extends MonadRec<A, M>> implements
+        MonoidFactory<Pure<M>, Fn1<A, MA>> {
+
+    private static final EndoK<?, ?, ?> INSTANCE = new EndoK<>();
+
+    @Override
+    public Monoid<Fn1<A, MA>> checkedApply(Pure<M> pureM) {
+        return new Monoid<Fn1<A, MA>>() {
+            @Override
+            public Fn1<A, MA> identity() {
+                return pureM::apply;
+            }
+
+            @Override
+            public Fn1<A, MA> checkedApply(Fn1<A, MA> f, Fn1<A, MA> g) {
+                return a -> kleisli(f).andThen(g::apply).apply(a);
+            }
+
+            @Override
+            public <B> Fn1<A, MA> foldMap(Fn1<? super B, ? extends Fn1<A, MA>> fn, Iterable<B> bs) {
+                return a -> FoldLeft.foldLeft((f, b) -> f.fmap(ma -> ma.flatMap(a_ -> safeT(fn.apply(b).apply(a_)))),
+                                              safeT(identity()).fmap(SafeT::safeT),
+                                              bs)
+                        .<Fn1<A, SafeT<M, A>>>runSafeT()
+                        .apply(a)
+                        .runSafeT();
+            }
+        };
+    }
+
+    @SuppressWarnings("unchecked")
+    public static <M extends MonadRec<?, M>, A, MA extends MonadRec<A, M>> EndoK<M, A, MA> endoK() {
+        return (EndoK<M, A, MA>) INSTANCE;
+    }
+
+    public static <M extends MonadRec<?, M>, A, MA extends MonadRec<A, M>> Monoid<Fn1<A, MA>> endoK(Pure<M> pureM) {
+        return EndoK.<M, A, MA>endoK().apply(pureM);
+    }
+}
diff --git a/src/test/java/com/jnape/palatable/lambda/monoid/builtin/EndoKTest.java b/src/test/java/com/jnape/palatable/lambda/monoid/builtin/EndoKTest.java
@@ -0,0 +1,34 @@
+package com.jnape.palatable.lambda.monoid.builtin;
+
+import com.jnape.palatable.lambda.functions.Fn1;
+import com.jnape.palatable.lambda.functor.builtin.Identity;
+import com.jnape.palatable.lambda.monoid.Monoid;
+import org.junit.Test;
+
+import static com.jnape.palatable.lambda.functions.builtin.fn2.Replicate.replicate;
+import static com.jnape.palatable.lambda.functor.builtin.Identity.pureIdentity;
+import static com.jnape.palatable.lambda.monoid.builtin.EndoK.endoK;
+import static org.junit.Assert.assertEquals;
+import static testsupport.Constants.STACK_EXPLODING_NUMBER;
+
+public class EndoKTest {
+
+    @Test
+    public void identity() {
+        Monoid<Fn1<Integer, Identity<Integer>>> endoK = endoK(pureIdentity());
+        assertEquals(new Identity<>(1), endoK.identity().apply(1));
+    }
+
+    @Test
+    public void monoid() {
+        Monoid<Fn1<Integer, Identity<Integer>>> endoK = endoK(pureIdentity());
+        assertEquals(new Identity<>(3),
+                     endoK.apply(x -> new Identity<>(x + 1), x -> new Identity<>(x + 2)).apply(0));
+    }
+
+    @Test
+    public void stackSafe() {
+        Monoid<Fn1<Integer, Identity<Integer>>> endoK = endoK(pureIdentity());
+        assertEquals(new Identity<>(0), endoK.reduceLeft(replicate(STACK_EXPLODING_NUMBER, Identity::new)).apply(0));
+    }
+}
