Adding static Pure and Lift instances for IterateT

jnape · jnape · commit c3f5652ba3fd · 2020-07-30T14:32:21.000-05:00
diff --git a/src/main/java/com/jnape/palatable/lambda/monad/transformer/builtin/IterateT.java b/src/main/java/com/jnape/palatable/lambda/monad/transformer/builtin/IterateT.java
@@ -8,6 +8,7 @@
 import com.jnape.palatable.lambda.functions.Fn1;
 import com.jnape.palatable.lambda.functions.Fn2;
 import com.jnape.palatable.lambda.functions.recursion.RecursiveResult;
+import com.jnape.palatable.lambda.functions.specialized.Lift;
 import com.jnape.palatable.lambda.functions.specialized.Pure;
 import com.jnape.palatable.lambda.functor.Applicative;
 import com.jnape.palatable.lambda.functor.builtin.Lazy;
@@ -451,4 +452,29 @@ public static <A> IterateT<IO<?>, A> fromIterator(Iterator<A> as) {
             return nothing();
         }), io(() -> as));
     }
+
+    /**
+     * The canonical {@link Pure} instance for {@link IterateT}.
+     *
+     * @param pureM the argument {@link Monad} {@link Pure}
+     * @param <M>   the argument {@link Monad} witness
+     * @return the {@link Pure} instance
+     */
+    public static <M extends MonadRec<?, M>> Pure<IterateT<M, ?>> pureIterateT(Pure<M> pureM) {
+        return new Pure<IterateT<M, ?>>() {
+            @Override
+            public <A> IterateT<M, A> checkedApply(A a) {
+                return liftIterateT().apply(pureM.<A, MonadRec<A, M>>apply(a));
+            }
+        };
+    }
+
+    /**
+     * {@link Lift} for {@link IterateT}.
+     *
+     * @return the {@link Monad} lifted into {@link IterateT}
+     */
+    public static Lift<IterateT<?, ?>> liftIterateT() {
+        return IterateT::singleton;
+    }
 }
diff --git a/src/test/java/com/jnape/palatable/lambda/monad/transformer/builtin/IterateTTest.java b/src/test/java/com/jnape/palatable/lambda/monad/transformer/builtin/IterateTTest.java
@@ -13,11 +13,7 @@
 import com.jnape.palatable.traitor.runners.Traits;
 import org.junit.Test;
 import org.junit.runner.RunWith;
-import testsupport.traits.ApplicativeLaws;
-import testsupport.traits.Equivalence;
-import testsupport.traits.FunctorLaws;
-import testsupport.traits.MonadLaws;
-import testsupport.traits.MonadRecLaws;
+import testsupport.traits.*;
 
 import java.util.ArrayList;
 import java.util.Collection;
@@ -34,14 +30,9 @@
 import static com.jnape.palatable.lambda.functions.recursion.RecursiveResult.terminate;
 import static com.jnape.palatable.lambda.functor.builtin.Identity.pureIdentity;
 import static com.jnape.palatable.lambda.functor.builtin.Lazy.lazy;
-import static com.jnape.palatable.lambda.functor.builtin.Writer.listen;
-import static com.jnape.palatable.lambda.functor.builtin.Writer.pureWriter;
-import static com.jnape.palatable.lambda.functor.builtin.Writer.tell;
-import static com.jnape.palatable.lambda.functor.builtin.Writer.writer;
+import static com.jnape.palatable.lambda.functor.builtin.Writer.*;
 import static com.jnape.palatable.lambda.io.IO.io;
-import static com.jnape.palatable.lambda.monad.transformer.builtin.IterateT.empty;
-import static com.jnape.palatable.lambda.monad.transformer.builtin.IterateT.singleton;
-import static com.jnape.palatable.lambda.monad.transformer.builtin.IterateT.unfold;
+import static com.jnape.palatable.lambda.monad.transformer.builtin.IterateT.*;
 import static com.jnape.palatable.lambda.monoid.builtin.AddAll.addAll;
 import static com.jnape.palatable.lambda.monoid.builtin.Join.join;
 import static com.jnape.palatable.traitor.framework.Subjects.subjects;
@@ -53,9 +44,7 @@
 import static org.junit.Assert.assertThat;
 import static testsupport.Constants.STACK_EXPLODING_NUMBER;
 import static testsupport.matchers.IOMatcher.yieldsValue;
-import static testsupport.matchers.IterateTMatcher.isEmpty;
-import static testsupport.matchers.IterateTMatcher.iterates;
-import static testsupport.matchers.IterateTMatcher.iteratesAll;
+import static testsupport.matchers.IterateTMatcher.*;
 import static testsupport.traits.Equivalence.equivalence;
 
 @RunWith(Traits.class)
@@ -242,4 +231,20 @@ public void concatIsStackSafe() {
         assertEquals(new Identity<>(10_000),
                      bigIterateT.fold((x, y) -> new Identity<>(x + y), new Identity<>(0)));
     }
+
+    @Test
+    public void staticPure() {
+        assertEquals(new Identity<>(singletonList(1)),
+                     pureIterateT(pureIdentity())
+                         .<Integer, IterateT<Identity<?>, Integer>>apply(1)
+                         .<List<Integer>, Identity<List<Integer>>>toCollection(ArrayList::new));
+    }
+
+    @Test
+    public void staticLift() {
+        assertEquals(new Identity<>(singletonList(1)),
+                     liftIterateT()
+                         .<Integer, Identity<?>, IterateT<Identity<?>, Integer>>apply(new Identity<>(1))
+                         .<List<Integer>, Identity<List<Integer>>>toCollection(ArrayList::new));
+    }
 }
