Futumorphism

jnape · jnape · commit 5d4235725893 · 2017-03-05T14:29:33.000-06:00
diff --git a/src/main/java/com/jnape/palatable/lambda/recursionschemes/builtin/Futumorphism.java b/src/main/java/com/jnape/palatable/lambda/recursionschemes/builtin/Futumorphism.java
@@ -0,0 +1,43 @@
+package com.jnape.palatable.lambda.recursionschemes.builtin;
+
+import com.jnape.palatable.lambda.functions.Fn1;
+import com.jnape.palatable.lambda.functions.Fn2;
+import com.jnape.palatable.lambda.functor.Functor;
+import com.jnape.palatable.lambda.recursionschemes.Coalgebra;
+import com.jnape.palatable.lambda.recursionschemes.Fix;
+import com.jnape.palatable.lambda.recursionschemes.Free;
+
+import java.util.function.Function;
+
+import static com.jnape.palatable.lambda.functions.builtin.fn1.Id.id;
+import static com.jnape.palatable.lambda.recursionschemes.Free.pure;
+import static com.jnape.palatable.lambda.recursionschemes.builtin.Anamorphism.ana;
+
+public final class Futumorphism<A, F extends Functor, FR extends Functor<Free<A, F, ?>, F>> implements Fn2<Function<A, FR>, A, Fix<F, ? extends Functor<? extends Fix<F, ?>, F>>> {
+
+    private static final Futumorphism INSTANCE = new Futumorphism();
+
+    private Futumorphism() {
+    }
+
+    @Override
+    @SuppressWarnings({"unchecked", "RedundantCast"})
+    public Fix<F, ? extends Functor<? extends Fix<F, ?>, F>> apply(Function<A, FR> fn, A a) {
+        return ana((Coalgebra<Free<A, F, ?>, FR>) free -> (FR) free.match(fn, id()), pure(a));
+    }
+
+    @SuppressWarnings("unchecked")
+    public static <A, F extends Functor, FR extends Functor<Free<A, F, ?>, F>> Futumorphism<A, F, FR> futu() {
+        return INSTANCE;
+    }
+
+    public static <A, F extends Functor, FR extends Functor<Free<A, F, ?>, F>> Fn1<A, Fix<F, ? extends Functor<? extends Fix<F, ?>, F>>> futu(
+            Function<A, FR> fn) {
+        return Futumorphism.<A, F, FR>futu().apply(fn);
+    }
+
+    public static <A, F extends Functor, FR extends Functor<Free<A, F, ?>, F>> Fix<F, ? extends Functor<? extends Fix<F, ?>, F>> futu(
+            Function<A, FR> fn, A a) {
+        return futu(fn).apply(a);
+    }
+}
diff --git a/src/test/java/com/jnape/palatable/lambda/recursionschemes/builtin/FutumorphismTest.java b/src/test/java/com/jnape/palatable/lambda/recursionschemes/builtin/FutumorphismTest.java
@@ -0,0 +1,21 @@
+package com.jnape.palatable.lambda.recursionschemes.builtin;
+
+import org.junit.Test;
+
+import static com.jnape.palatable.lambda.recursionschemes.Fix.fix;
+import static com.jnape.palatable.lambda.recursionschemes.Free.pure;
+import static com.jnape.palatable.lambda.recursionschemes.builtin.Futumorphism.futu;
+import static org.junit.Assert.assertEquals;
+import static testsupport.recursion.NatF.s;
+import static testsupport.recursion.NatF.z;
+
+public class FutumorphismTest {
+
+    @Test
+    public void unfoldingToGreatestFixedPointWithOptimizations() {
+        assertEquals(fix(s(fix(s(fix(s(fix(s(fix(s(fix(z()))))))))))),
+                     futu(x -> x == 1 ? z() : x % 2 == 0
+                             ? s(pure(x / 2))
+                             : s(pure(x * 3 + 1)), 5));
+    }
+}
