Adding HMapLens, lenses for HMaps

jnape · jnape · commit f0009b832c1b · 2018-03-30T17:13:01.000-05:00
diff --git a/src/main/java/com/jnape/palatable/lambda/lens/lenses/HMapLens.java b/src/main/java/com/jnape/palatable/lambda/lens/lenses/HMapLens.java
@@ -0,0 +1,27 @@
+package com.jnape.palatable.lambda.lens.lenses;
+
+import com.jnape.palatable.lambda.adt.Maybe;
+import com.jnape.palatable.lambda.adt.hmap.HMap;
+import com.jnape.palatable.lambda.adt.hmap.TypeSafeKey;
+import com.jnape.palatable.lambda.lens.Lens;
+
+import static com.jnape.palatable.lambda.lens.Lens.simpleLens;
+
+/**
+ * Lenses that operate on {@link HMap}s.
+ */
+public final class HMapLens {
+    private HMapLens() {
+    }
+
+    /**
+     * A lens that focuses on a value at a {@link TypeSafeKey}&lt;A&gt; in an {@link HMap}, as a {@link Maybe}.
+     *
+     * @param key the key
+     * @param <A> the value type at the key
+     * @return the lens
+     */
+    public static <A> Lens.Simple<HMap, Maybe<A>> valueAt(TypeSafeKey<A> key) {
+        return simpleLens(m -> m.get(key), (m, maybeA) -> maybeA.fmap(a -> m.put(key, a)).orElseGet(() -> m.remove(key)));
+    }
+}
diff --git a/src/test/java/com/jnape/palatable/lambda/lens/lenses/HMapLensTest.java b/src/test/java/com/jnape/palatable/lambda/lens/lenses/HMapLensTest.java
@@ -0,0 +1,30 @@
+package com.jnape.palatable.lambda.lens.lenses;
+
+import com.jnape.palatable.lambda.adt.hmap.TypeSafeKey;
+import org.junit.Test;
+
+import static com.jnape.palatable.lambda.adt.Maybe.just;
+import static com.jnape.palatable.lambda.adt.Maybe.nothing;
+import static com.jnape.palatable.lambda.adt.hmap.HMap.emptyHMap;
+import static com.jnape.palatable.lambda.adt.hmap.HMap.hMap;
+import static com.jnape.palatable.lambda.adt.hmap.HMap.singletonHMap;
+import static com.jnape.palatable.lambda.adt.hmap.TypeSafeKey.typeSafeKey;
+import static java.util.Arrays.asList;
+import static testsupport.assertion.LensAssert.assertLensLawfulness;
+
+public class HMapLensTest {
+
+    @Test
+    public void valueAt() {
+        TypeSafeKey<String> key = typeSafeKey();
+        assertLensLawfulness(HMapLens.valueAt(key),
+                             asList(emptyHMap(),
+                                    singletonHMap(key, "foo"),
+                                    hMap(key, "foo",
+                                         typeSafeKey(), "bar"),
+                                    singletonHMap(typeSafeKey(), "bar")),
+                             asList(nothing(),
+                                    just("foo"),
+                                    just("bar")));
+    }
+}
