Apomorphism

jnape · jnape · commit 4c5a6d765b55 · 2017-02-26T14:31:19.000-06:00
diff --git a/src/main/java/com/jnape/palatable/lambda/recursionschemes/builtin/Apomorphism.java b/src/main/java/com/jnape/palatable/lambda/recursionschemes/builtin/Apomorphism.java
@@ -0,0 +1,41 @@
+package com.jnape.palatable.lambda.recursionschemes.builtin;
+
+import com.jnape.palatable.lambda.adt.coproduct.CoProduct2;
+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.Fix;
+
+import java.util.function.Function;
+
+import static com.jnape.palatable.lambda.functions.builtin.fn1.Id.id;
+import static com.jnape.palatable.lambda.recursionschemes.Fix.fix;
+
+public final class Apomorphism<A, F extends Functor, FT extends Functor<? extends CoProduct2<A, ? extends Fix<F, ? extends Functor<Fix<F, ?>, F>>>, F>>
+        implements Fn2<Function<A, FT>, A, Fix<F, Functor<Fix<F, ?>, F>>> {
+
+    private static final Apomorphism INSTANCE = new Apomorphism();
+
+    private Apomorphism() {
+    }
+
+    @Override
+    public Fix<F, Functor<Fix<F, ?>, F>> apply(Function<A, FT> fn, A a) {
+        return fix(fn.apply(a).fmap(cp2 -> cp2.match(apo(fn), id())));
+    }
+
+    @SuppressWarnings("unchecked")
+    public static <A, F extends Functor, FT extends Functor<? extends CoProduct2<A, ? extends Fix<F, ? extends Functor<Fix<F, ?>, F>>>, F>> Apomorphism<A, F, FT> apo() {
+        return INSTANCE;
+    }
+
+    public static <A, F extends Functor, FT extends Functor<? extends CoProduct2<A, ? extends Fix<F, ? extends Functor<Fix<F, ?>, F>>>, F>> Fn1<A, Fix<F, Functor<Fix<F, ?>, F>>> apo(
+            Function<A, FT> fn) {
+        return Apomorphism.<A, F, FT>apo().apply(fn);
+    }
+
+    public static <A, F extends Functor, FT extends Functor<? extends CoProduct2<A, ? extends Fix<F, ? extends Functor<Fix<F, ?>, F>>>, F>> Fix<F, Functor<Fix<F, ?>, F>> apo(
+            Function<A, FT> fn, A a) {
+        return apo(fn).apply(a);
+    }
+}
diff --git a/src/test/java/com/jnape/palatable/lambda/recursionschemes/builtin/ApomorphismTest.java b/src/test/java/com/jnape/palatable/lambda/recursionschemes/builtin/ApomorphismTest.java
@@ -0,0 +1,25 @@
+package com.jnape.palatable.lambda.recursionschemes.builtin;
+
+import com.jnape.palatable.lambda.adt.coproduct.CoProduct2;
+import com.jnape.palatable.lambda.recursionschemes.Fix;
+import org.junit.Test;
+import testsupport.recursion.NatF;
+
+import java.util.function.Function;
+
+import static com.jnape.palatable.lambda.adt.choice.Choice2.a;
+import static com.jnape.palatable.lambda.adt.choice.Choice2.b;
+import static com.jnape.palatable.lambda.recursionschemes.Fix.fix;
+import static com.jnape.palatable.lambda.recursionschemes.builtin.Apomorphism.apo;
+import static org.junit.Assert.assertEquals;
+
+public class ApomorphismTest {
+
+    @Test
+    public void unfoldingToGreatestFixedPointWithShortCircuit() {
+        Function<Integer, NatF<CoProduct2<Integer, Fix<NatF, NatF<Fix<NatF, ?>>>>>> unfold =
+                x -> NatF.s(x < 3 ? a(x + 1) : b(fix(NatF.z())));
+
+        assertEquals(Fix.<NatF, NatF<Fix<NatF, ?>>>fix(NatF.s(fix(NatF.s(fix(NatF.s(fix(NatF.z()))))))), apo(unfold, 1));
+    }
+}
