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Adding MergeHMaps, a Monoid merging HMaps by using key-specific semigroups
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‎CHANGELOG.md

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### Changed
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- `HList#cons` static factory method auto-promotes to specialized `HList` if there is one
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### Added
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- `MergeHMaps`, a `Monoid` that merges `HMap`s by merging the values via key-specified `Semigroup`s
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## [5.1.0] - 2019-10-13
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### Changed
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- All monad transformers that can support composable parallelism do support it

‎src/main/java/com/jnape/palatable/lambda/monoid/builtin/MergeHMaps.java

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package com.jnape.palatable.lambda.monoid.builtin;
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import com.jnape.palatable.lambda.adt.Maybe;
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import com.jnape.palatable.lambda.adt.hmap.HMap;
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import com.jnape.palatable.lambda.adt.hmap.TypeSafeKey;
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import com.jnape.palatable.lambda.functions.Fn1;
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import com.jnape.palatable.lambda.functions.Fn2;
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import com.jnape.palatable.lambda.functions.builtin.fn3.FoldLeft;
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import com.jnape.palatable.lambda.monoid.Monoid;
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import com.jnape.palatable.lambda.semigroup.Semigroup;
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import java.util.Map;
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import static com.jnape.palatable.lambda.adt.Maybe.just;
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import static com.jnape.palatable.lambda.adt.Maybe.maybe;
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import static com.jnape.palatable.lambda.functions.builtin.fn2.Map.map;
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import static com.jnape.palatable.lambda.monoid.builtin.Last.last;
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import static com.jnape.palatable.lambda.monoid.builtin.Present.present;
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import static com.jnape.palatable.lambda.optics.functions.Set.set;
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import static com.jnape.palatable.lambda.optics.lenses.MapLens.valueAt;
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import static java.util.Arrays.asList;
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import static java.util.Collections.emptyMap;
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/**
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* A {@link Monoid} instance formed by merging {@link HMap HMaps} using the chosen
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* <code>{@link TypeSafeKey} -&gt; {@link Semigroup}</code>
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* {@link MergeHMaps#key(TypeSafeKey, Semigroup) mappings}, defaulting to {@link Last} in case no
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* {@link Semigroup} has been chosen for a given {@link TypeSafeKey}.
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*/
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public final class MergeHMaps implements Monoid<HMap> {
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private final Map<TypeSafeKey<?, ?>, Fn2<HMap, HMap, HMap>> bindings;
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private final Φ<Fn2<HMap, HMap, HMap>> defaultBinding;
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private MergeHMaps(Map<TypeSafeKey<?, ?>, Fn2<HMap, HMap, HMap>> bindings,
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Φ<Fn2<HMap, HMap, HMap>> defaultBinding) {
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this.bindings = bindings;
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this.defaultBinding = defaultBinding;
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}
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public <A> MergeHMaps key(TypeSafeKey<?, A> key, Semigroup<A> semigroup) {
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return new MergeHMaps(set(valueAt(key), just(merge(key, present(semigroup))), bindings), defaultBinding);
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}
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@Override
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public HMap identity() {
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return HMap.emptyHMap();
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}
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@Override
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public HMap checkedApply(HMap x, HMap y) throws Throwable {
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return reduceLeft(asList(x, y));
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}
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@Override
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public <B> HMap foldMap(Fn1<? super B, ? extends HMap> fn, Iterable<B> bs) {
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return FoldLeft.foldLeft((acc, m) -> FoldLeft.foldLeft((result, k) -> maybe(bindings.get(k))
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.orElseGet(() -> defaultBinding.eliminate(k))
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.apply(result, m), acc, m.keys()), identity(), map(fn, bs));
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}
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public static MergeHMaps mergeHMaps() {
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return new MergeHMaps(emptyMap(), new Φ<Fn2<HMap, HMap, HMap>>() {
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@Override
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public <A> Fn2<HMap, HMap, HMap> eliminate(TypeSafeKey<?, A> key) {
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return merge(key, last());
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}
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});
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}
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private static <A> Fn2<HMap, HMap, HMap> merge(TypeSafeKey<?, A> key, Semigroup<Maybe<A>> semigroup) {
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return (x, y) -> semigroup.apply(x.get(key), y.get(key))
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.fmap(a -> x.put(key, a))
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.orElse(x);
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}
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@SuppressWarnings({"NonAsciiCharacters"})
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private interface Φ<R> {
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<A> R eliminate(TypeSafeKey<?, A> key);
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}
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}

‎src/test/java/com/jnape/palatable/lambda/monoid/builtin/MergeHMapsTest.java

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package com.jnape.palatable.lambda.monoid.builtin;
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import com.jnape.palatable.lambda.adt.hmap.TypeSafeKey;
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import org.junit.Test;
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import static com.jnape.palatable.lambda.adt.hmap.HMap.emptyHMap;
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import static com.jnape.palatable.lambda.adt.hmap.HMap.hMap;
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import static com.jnape.palatable.lambda.adt.hmap.HMap.singletonHMap;
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import static com.jnape.palatable.lambda.adt.hmap.TypeSafeKey.typeSafeKey;
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import static com.jnape.palatable.lambda.monoid.builtin.Join.join;
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import static com.jnape.palatable.lambda.monoid.builtin.MergeHMaps.mergeHMaps;
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import static java.util.Arrays.asList;
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import static org.junit.Assert.assertEquals;
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public class MergeHMapsTest {
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@Test
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public void allKeysAccountedFor() {
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TypeSafeKey.Simple<String> stringKey = typeSafeKey();
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MergeHMaps mergeHMaps = mergeHMaps().key(stringKey, join());
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assertEquals(emptyHMap(), mergeHMaps.apply(emptyHMap(), emptyHMap()));
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assertEquals(singletonHMap(stringKey, "foo"),
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mergeHMaps.apply(singletonHMap(stringKey, "foo"), emptyHMap()));
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assertEquals(singletonHMap(stringKey, "foobar"),
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mergeHMaps.apply(singletonHMap(stringKey, "foo"),
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singletonHMap(stringKey, "bar")));
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}
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@Test
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public void unaccountedForKeyUsesLastByDefault() {
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TypeSafeKey.Simple<String> stringKey = typeSafeKey();
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assertEquals(singletonHMap(stringKey, "foo"),
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mergeHMaps().apply(singletonHMap(stringKey, "foo"), emptyHMap()));
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assertEquals(singletonHMap(stringKey, "bar"),
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mergeHMaps().apply(emptyHMap(), singletonHMap(stringKey, "bar")));
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assertEquals(singletonHMap(stringKey, "bar"),
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mergeHMaps().apply(singletonHMap(stringKey, "foo"), singletonHMap(stringKey, "bar")));
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}
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@Test
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public void sparseKeysAcrossMaps() {
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TypeSafeKey.Simple<String> stringKey = typeSafeKey();
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TypeSafeKey.Simple<Integer> intKey = typeSafeKey();
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TypeSafeKey.Simple<Boolean> boolKey = typeSafeKey();
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MergeHMaps mergeHMaps = mergeHMaps()
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.key(stringKey, join())
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.key(intKey, Integer::sum);
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assertEquals(hMap(stringKey, "foobar",
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intKey, 3,
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boolKey, false),
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mergeHMaps.reduceLeft(asList(singletonHMap(stringKey, "foo"),
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singletonHMap(intKey, 1),
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singletonHMap(boolKey, true),
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hMap(stringKey, "bar",
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intKey, 2,
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boolKey, false))));
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}
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}

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