Adding AutoBracket, an AutoClosable constrained Bracket

orenmnero · jnape · commit 9d115e50b5c5 · 2019-10-12T15:10:20.000-05:00
diff --git a/src/main/java/com/jnape/palatable/lambda/functions/builtin/fn2/AutoBracket.java b/src/main/java/com/jnape/palatable/lambda/functions/builtin/fn2/AutoBracket.java
@@ -0,0 +1,48 @@
+package com.jnape.palatable.lambda.functions.builtin.fn2;
+
+import com.jnape.palatable.lambda.functions.Fn1;
+import com.jnape.palatable.lambda.functions.Fn2;
+import com.jnape.palatable.lambda.functions.builtin.fn3.Bracket;
+import com.jnape.palatable.lambda.io.IO;
+
+import static com.jnape.palatable.lambda.functions.builtin.fn3.Bracket.bracket;
+import static com.jnape.palatable.lambda.io.IO.io;
+
+/**
+ * Given an {@link IO} yielding some {@link AutoCloseable} type <code>A</code> and a kleisli arrow from that type to a
+ * new {@link IO} of type <code>B</code>, attempt to provision the <code>A</code>, applying the body operation if
+ * provisioning was successful and ensuring that {@link AutoCloseable#close} is called regardless of whether the body
+ * succeeds or fails.
+ * <p>
+ * This is the canonical {@link Bracket bracketing} operation for {@link AutoCloseable AutoCloseables}.
+ *
+ * @param <A> the initial {@link AutoCloseable} value type to map and clean up
+ * @param <B> the resulting type
+ * @see Bracket
+ */
+public final class AutoBracket<A extends AutoCloseable, B> implements
+        Fn2<IO<A>, Fn1<? super A, ? extends IO<B>>, IO<B>> {
+
+    private static final AutoBracket<?, ?> INSTANCE = new AutoBracket<>();
+
+    private AutoBracket() {
+    }
+
+    @Override
+    public IO<B> checkedApply(IO<A> io, Fn1<? super A, ? extends IO<B>> bodyIO) {
+        return bracket(io, a -> io(a::close), bodyIO);
+    }
+
+    @SuppressWarnings("unchecked")
+    public static <A extends AutoCloseable, B> AutoBracket<A, B> autoBracket() {
+        return (AutoBracket<A, B>) INSTANCE;
+    }
+
+    public static <A extends AutoCloseable, B> Fn1<Fn1<? super A, ? extends IO<B>>, IO<B>> autoBracket(IO<A> io) {
+        return AutoBracket.<A, B>autoBracket().apply(io);
+    }
+
+    public static <A extends AutoCloseable, B> IO<B> autoBracket(IO<A> io, Fn1<? super A, ? extends IO<B>> bodyIO) {
+        return AutoBracket.<A, B>autoBracket(io).apply(bodyIO);
+    }
+}
diff --git a/src/test/java/com/jnape/palatable/lambda/functions/builtin/fn2/AutoBracketTest.java b/src/test/java/com/jnape/palatable/lambda/functions/builtin/fn2/AutoBracketTest.java
@@ -0,0 +1,48 @@
+package com.jnape.palatable.lambda.functions.builtin.fn2;
+
+import com.jnape.palatable.lambda.adt.Unit;
+import com.jnape.palatable.lambda.io.IO;
+import org.junit.Before;
+import org.junit.Test;
+
+import java.util.concurrent.atomic.AtomicInteger;
+
+import static com.jnape.palatable.lambda.functions.builtin.fn2.AutoBracket.autoBracket;
+import static com.jnape.palatable.lambda.io.IO.io;
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertThat;
+import static testsupport.matchers.IOMatcher.throwsException;
+import static testsupport.matchers.IOMatcher.yieldsValue;
+
+public class AutoBracketTest {
+
+    private AtomicInteger closedCounter;
+    private AutoCloseable autoCloseable;
+
+    @Before
+    public void setUp() {
+        closedCounter = new AtomicInteger(0);
+        autoCloseable = closedCounter::incrementAndGet;
+    }
+
+    @Test
+    public void closeWhenDone() {
+        IO<Integer> bracketed = autoBracket(io(autoCloseable), closeable -> io(1));
+
+        assertEquals(0, closedCounter.get());
+        assertThat(bracketed, yieldsValue(equalTo(1)));
+        assertEquals(1, closedCounter.get());
+    }
+
+    @Test
+    public void closeOnException() {
+        RuntimeException cause = new RuntimeException();
+
+        IO<Unit> bracketed = autoBracket(io(autoCloseable), closeable -> IO.throwing(cause));
+
+        assertEquals(0, closedCounter.get());
+        assertThat(bracketed, throwsException(equalTo(cause)));
+        assertEquals(1, closedCounter.get());
+    }
+}
