From 85a76c4d1df3ff1eafd8bd9257d4f4ea2506e69a Mon Sep 17 00:00:00 2001
From: Glavo <zjx001202@gmail.com>
Date: Mon, 23 Oct 2023 02:44:23 +0800
Subject: [PATCH] Close #71: Create Either::bifold

---
 .../src/main/java/kala/control/Either.java    | 984 +++++++++---------
 1 file changed, 510 insertions(+), 474 deletions(-)

diff --git a/kala-base/src/main/java/kala/control/Either.java b/kala-base/src/main/java/kala/control/Either.java
index ce600e2c..ddfe6a7c 100644
--- a/kala-base/src/main/java/kala/control/Either.java
+++ b/kala-base/src/main/java/kala/control/Either.java
@@ -1,474 +1,510 @@
-package kala.control;
-
-import kala.annotations.Covariant;
-import kala.annotations.Sealed;
-import kala.annotations.UnstableName;
-import org.jetbrains.annotations.Contract;
-import org.jetbrains.annotations.NotNull;
-
-import java.io.Serializable;
-import java.util.NoSuchElementException;
-import java.util.Objects;
-import java.util.function.Consumer;
-import java.util.function.Function;
-
-@Sealed(subclasses = {Either.Left.class, Either.Right.class})
-@SuppressWarnings("unchecked")
-public abstract class Either<@Covariant A, @Covariant B> implements Serializable {
-    Either() {
-    }
-
-    @Contract(value = "_ -> param1", pure = true)
-    public static <A, B> Either<A, B> narrow(Either<? extends A, ? extends B> either) {
-        return (Either<A, B>) either;
-    }
-
-    @Contract(value = "_ -> param1", pure = true)
-    public static <A, B> Either.Left<A, B> narrow(Either.Left<? extends A, ? extends B> left) {
-        return (Either.Left<A, B>) left;
-    }
-
-    @Contract(value = "_ -> param1", pure = true)
-    public static <A, B> Either.Right<A, B> narrow(Either.Right<? extends A, ? extends B> right) {
-        return (Either.Right<A, B>) right;
-    }
-
-    @Contract("_ -> new")
-    public static <A, B> Either.@NotNull Left<A, B> left(A value) {
-        return new Left<>(value);
-    }
-
-    @Contract("_ -> new")
-    public static <A, B> Either.@NotNull Right<A, B> right(B value) {
-        return new Right<>(value);
-    }
-
-    @UnstableName
-    public static <T> T join(@NotNull Either<? extends T, ? extends T> either) {
-        if (either.isLeft()) {
-            return either.getLeftValue();
-        } else {
-            return either.getRightValue();
-        }
-    }
-
-    public abstract boolean isLeft();
-
-    public abstract boolean isRight();
-
-    public abstract A getLeftValue();
-
-    public abstract @NotNull Option<A> getLeftOption();
-
-    public abstract B getRightValue();
-
-    public abstract @NotNull Option<B> getRightOption();
-
-    public abstract <U, V> @NotNull Either<U, V> map(
-            @NotNull Function<? super A, ? extends U> leftMapper,
-            @NotNull Function<? super B, ? extends V> rightMapper
-    );
-
-    public abstract <U> @NotNull Either<U, B> mapLeft(@NotNull Function<? super A, ? extends U> mapper);
-
-    public abstract <U> @NotNull Either<A, U> mapRight(@NotNull Function<? super B, ? extends U> mapper);
-
-    public abstract <U> U fold(
-            @NotNull Function<? super A, ? extends U> leftMapper,
-            @NotNull Function<? super B, ? extends U> rightMapper
-    );
-
-    @Contract("-> new")
-    public abstract @NotNull Either<B, A> swap();
-
-    @Contract("-> new")
-    public abstract @NotNull Result<B, A> toResult();
-
-    @Contract(" -> new")
-    public final @NotNull LeftProjection leftProjection() {
-        return this.new LeftProjection();
-    }
-
-    @Contract(" -> new")
-    public final @NotNull RightProjection rightProjection() {
-        return this.new RightProjection();
-    }
-
-    public abstract void forEach(
-            @NotNull Consumer<? super A> leftConsumer,
-            @NotNull Consumer<? super B> rightConsumer
-    );
-
-    public final static class Left<@Covariant A, @Covariant B> extends Either<A, B> {
-        private static final long serialVersionUID = -1160729620210301179L;
-
-        private static final int HASH_MAGIC = -1951578063;
-
-        private final A value;
-
-        Left(A value) {
-            this.value = value;
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @Override
-        public boolean isLeft() {
-            return true;
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @Override
-        public boolean isRight() {
-            return false;
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @Override
-        public A getLeftValue() {
-            return value;
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @Override
-        public @NotNull Option<A> getLeftOption() {
-            return Option.some(value);
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @Override
-        @Contract("-> fail")
-        public B getRightValue() {
-            throw new NoSuchElementException("Either.Left.getRightValue");
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @Override
-        public @NotNull Option<B> getRightOption() {
-            return Option.none();
-        }
-
-        @Override
-        public <U, V> @NotNull Either<U, V> map(
-                @NotNull Function<? super A, ? extends U> leftMapper,
-                @NotNull Function<? super B, ? extends V> rightMapper) {
-            return Either.left(leftMapper.apply(value));
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @Override
-        public <U> @NotNull Either<U, B> mapLeft(@NotNull Function<? super A, ? extends U> mapper) {
-            Objects.requireNonNull(mapper);
-            return Either.left(mapper.apply(value));
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @Override
-        public <U> @NotNull Either<A, U> mapRight(@NotNull Function<? super B, ? extends U> mapper) {
-            return (Either<A, U>) this;
-        }
-
-        @Override
-        public <U> U fold(
-                @NotNull Function<? super A, ? extends U> leftMapper,
-                @NotNull Function<? super B, ? extends U> rightMapper) {
-            return leftMapper.apply(value);
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @Override
-        public @NotNull Either<B, A> swap() {
-            return Either.right(value);
-        }
-
-        @Override
-        public @NotNull Result<B, A> toResult() {
-            return Result.err(value);
-        }
-
-        @Override
-        public void forEach(@NotNull Consumer<? super A> leftConsumer, @NotNull Consumer<? super B> rightConsumer) {
-            leftConsumer.accept(value);
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @Override
-        public boolean equals(Object obj) {
-            if (this == obj) {
-                return true;
-            }
-            if (!(obj instanceof Left<?, ?>)) {
-                return false;
-            }
-
-            return Objects.equals(value, ((Left<?, ?>) obj).value);
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @Override
-        public int hashCode() {
-            return Objects.hashCode(value) + HASH_MAGIC;
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @Override
-        public String toString() {
-            return "Either.Left[" + value + "]";
-        }
-    }
-
-    public final static class Right<@Covariant A, @Covariant B> extends Either<A, B> {
-        private static final long serialVersionUID = -3372589401685464421L;
-
-        private static final int HASH_MAGIC = 1973604283;
-
-        private final B value;
-
-        Right(B value) {
-            this.value = value;
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @Override
-        public boolean isLeft() {
-            return false;
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @Override
-        public boolean isRight() {
-            return true;
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @Override
-        @Contract("-> fail")
-        public A getLeftValue() {
-            throw new NoSuchElementException("Either.Right.getLeftValue");
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @Override
-        public @NotNull Option<A> getLeftOption() {
-            return Option.none();
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @Override
-        public B getRightValue() {
-            return value;
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @Override
-        public @NotNull Option<B> getRightOption() {
-            return Option.some(value);
-        }
-
-        @Override
-        public <U, V> @NotNull Either<U, V> map(
-                @NotNull Function<? super A, ? extends U> leftMapper,
-                @NotNull Function<? super B, ? extends V> rightMapper
-        ) {
-            return Either.right(rightMapper.apply(value));
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @Override
-        public <U> @NotNull Either<U, B> mapLeft(@NotNull Function<? super A, ? extends U> mapper) {
-            return (Either<U, B>) this;
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @Override
-        public <U> @NotNull Either<A, U> mapRight(@NotNull Function<? super B, ? extends U> mapper) {
-            Objects.requireNonNull(mapper);
-            return Either.right(mapper.apply(value));
-        }
-
-        @Override
-        public <U> U fold(
-                @NotNull Function<? super A, ? extends U> leftMapper,
-                @NotNull Function<? super B, ? extends U> rightMapper) {
-            return rightMapper.apply(value);
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @Override
-        public @NotNull Either<B, A> swap() {
-            return Either.left(value);
-        }
-
-        @Override
-        public @NotNull Result<B, A> toResult() {
-            return Result.ok(value);
-        }
-
-        @Override
-        public void forEach(@NotNull Consumer<? super A> leftConsumer, @NotNull Consumer<? super B> rightConsumer) {
-            rightConsumer.accept(value);
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @Override
-        public boolean equals(Object obj) {
-            if (this == obj) {
-                return true;
-            }
-            if (!(obj instanceof Right<?, ?>)) {
-                return false;
-            }
-
-            return Objects.equals(value, ((Right<?, ?>) obj).value);
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @Override
-        public int hashCode() {
-            return Objects.hashCode(value) + HASH_MAGIC;
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @Override
-        public String toString() {
-            return "Either.Right[" + value + "]";
-        }
-    }
-
-    public abstract class Projection<@Covariant T> implements OptionContainer<T> {
-        Projection() {
-        }
-
-        private static final int HASH_MAGIC = 905770825;
-
-        public final @NotNull Either<A, B> getEither() {
-            return Either.this;
-        }
-
-        @Override
-        public boolean equals(Object o) {
-            if (this == o) {
-                return true;
-            }
-            if (o == null || getClass() != o.getClass()) {
-                return false;
-            }
-
-            return Either.this.equals(((Projection<?>) o).getEither());
-        }
-
-        @Override
-        public final int hashCode() {
-            return Either.this.hashCode() + HASH_MAGIC;
-        }
-    }
-
-    public final class LeftProjection extends Projection<A> {
-        @Override
-        public boolean isDefined() {
-            return Either.this.isLeft();
-        }
-
-        @Override
-        public A get() {
-            return Either.this.getLeftValue();
-        }
-
-        @Override
-        @NotNull
-        public <U> Either<U, B>.LeftProjection map(@NotNull Function<? super A, ? extends U> mapper) {
-            Objects.requireNonNull(mapper);
-            if (isDefined()) {
-                return Either.<U, B>left(mapper.apply(getLeftValue())).new LeftProjection();
-            }
-            return (Either<U, B>.LeftProjection) this;
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @NotNull
-        @Override
-        public String toString() {
-            return Either.this + ".LeftProjection";
-        }
-    }
-
-    public final class RightProjection extends Projection<B> {
-        @Override
-        public boolean isDefined() {
-            return Either.this.isRight();
-        }
-
-        @Override
-        public B get() {
-            return Either.this.getRightValue();
-        }
-
-        @Override
-        @NotNull
-        public <U> Either<A, U>.RightProjection map(@NotNull Function<? super B, ? extends U> mapper) {
-            Objects.requireNonNull(mapper);
-            if (isDefined()) {
-                return Either.<A, U>right(mapper.apply(getRightValue())).new RightProjection();
-            }
-            return (Either<A, U>.RightProjection) this;
-        }
-
-        /**
-         * {@inheritDoc}
-         */
-        @NotNull
-        @Override
-        public String toString() {
-            return Either.this + ".RightProjection";
-        }
-    }
-}
+package kala.control;
+
+import kala.annotations.Covariant;
+import kala.annotations.Sealed;
+import kala.annotations.UnstableName;
+import org.jetbrains.annotations.Contract;
+import org.jetbrains.annotations.NotNull;
+
+import java.io.Serializable;
+import java.util.NoSuchElementException;
+import java.util.Objects;
+import java.util.function.BiFunction;
+import java.util.function.Consumer;
+import java.util.function.Function;
+
+@Sealed(subclasses = {Either.Left.class, Either.Right.class})
+@SuppressWarnings("unchecked")
+public abstract class Either<@Covariant A, @Covariant B> implements Serializable {
+    Either() {
+    }
+
+    @Contract(value = "_ -> param1", pure = true)
+    public static <A, B> Either<A, B> narrow(Either<? extends A, ? extends B> either) {
+        return (Either<A, B>) either;
+    }
+
+    @Contract(value = "_ -> param1", pure = true)
+    public static <A, B> Either.Left<A, B> narrow(Either.Left<? extends A, ? extends B> left) {
+        return (Either.Left<A, B>) left;
+    }
+
+    @Contract(value = "_ -> param1", pure = true)
+    public static <A, B> Either.Right<A, B> narrow(Either.Right<? extends A, ? extends B> right) {
+        return (Either.Right<A, B>) right;
+    }
+
+    @Contract("_ -> new")
+    public static <A, B> Either.@NotNull Left<A, B> left(A value) {
+        return new Left<>(value);
+    }
+
+    @Contract("_ -> new")
+    public static <A, B> Either.@NotNull Right<A, B> right(B value) {
+        return new Right<>(value);
+    }
+
+    @UnstableName
+    public static <T> T join(@NotNull Either<? extends T, ? extends T> either) {
+        if (either.isLeft()) {
+            return either.getLeftValue();
+        } else {
+            return either.getRightValue();
+        }
+    }
+
+    public abstract boolean isLeft();
+
+    public abstract boolean isRight();
+
+    public abstract A getLeftValue();
+
+    public abstract @NotNull Option<A> getLeftOption();
+
+    public abstract B getRightValue();
+
+    public abstract @NotNull Option<B> getRightOption();
+
+    public abstract <U, V> @NotNull Either<U, V> map(
+            @NotNull Function<? super A, ? extends U> leftMapper,
+            @NotNull Function<? super B, ? extends V> rightMapper
+    );
+
+    public abstract <U> @NotNull Either<U, B> mapLeft(@NotNull Function<? super A, ? extends U> mapper);
+
+    public abstract <U> @NotNull Either<A, U> mapRight(@NotNull Function<? super B, ? extends U> mapper);
+
+    public abstract <U> U fold(
+            @NotNull Function<? super A, ? extends U> leftMapper,
+            @NotNull Function<? super B, ? extends U> rightMapper
+    );
+
+    public abstract <C, D, U> U bifold(
+            @NotNull Either<? extends C, ? extends D> other,
+            U defaultValue,
+            @NotNull BiFunction<? super A, ? super C, ? extends U> leftMapper,
+            @NotNull BiFunction<? super B, ? super D, ? extends U> rightMapper
+    );
+
+    @Contract("-> new")
+    public abstract @NotNull Either<B, A> swap();
+
+    @Contract("-> new")
+    public abstract @NotNull Result<B, A> toResult();
+
+    @Contract(" -> new")
+    public final @NotNull LeftProjection leftProjection() {
+        return this.new LeftProjection();
+    }
+
+    @Contract(" -> new")
+    public final @NotNull RightProjection rightProjection() {
+        return this.new RightProjection();
+    }
+
+    public abstract void forEach(
+            @NotNull Consumer<? super A> leftConsumer,
+            @NotNull Consumer<? super B> rightConsumer
+    );
+
+    public final static class Left<@Covariant A, @Covariant B> extends Either<A, B> {
+        private static final long serialVersionUID = -1160729620210301179L;
+
+        private static final int HASH_MAGIC = -1951578063;
+
+        private final A value;
+
+        Left(A value) {
+            this.value = value;
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public boolean isLeft() {
+            return true;
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public boolean isRight() {
+            return false;
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public A getLeftValue() {
+            return value;
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public @NotNull Option<A> getLeftOption() {
+            return Option.some(value);
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        @Contract("-> fail")
+        public B getRightValue() {
+            throw new NoSuchElementException("Either.Left.getRightValue");
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public @NotNull Option<B> getRightOption() {
+            return Option.none();
+        }
+
+        @Override
+        public <U, V> @NotNull Either<U, V> map(
+                @NotNull Function<? super A, ? extends U> leftMapper,
+                @NotNull Function<? super B, ? extends V> rightMapper) {
+            return Either.left(leftMapper.apply(value));
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public <U> @NotNull Either<U, B> mapLeft(@NotNull Function<? super A, ? extends U> mapper) {
+            Objects.requireNonNull(mapper);
+            return Either.left(mapper.apply(value));
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public <U> @NotNull Either<A, U> mapRight(@NotNull Function<? super B, ? extends U> mapper) {
+            return (Either<A, U>) this;
+        }
+
+        @Override
+        public <U> U fold(
+                @NotNull Function<? super A, ? extends U> leftMapper,
+                @NotNull Function<? super B, ? extends U> rightMapper) {
+            return leftMapper.apply(value);
+        }
+
+        @Override
+        public <C, D, U> U bifold(
+                @NotNull Either<? extends C, ? extends D> other,
+                U defaultValue,
+                @NotNull BiFunction<? super A, ? super C, ? extends U> leftMapper,
+                @NotNull BiFunction<? super B, ? super D, ? extends U> rightMapper) {
+
+            if (other instanceof Left) {
+                return leftMapper.apply(this.value, ((Left<? extends C, ? extends D>) other).value);
+            } else {
+                return defaultValue;
+            }
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public @NotNull Either<B, A> swap() {
+            return Either.right(value);
+        }
+
+        @Override
+        public @NotNull Result<B, A> toResult() {
+            return Result.err(value);
+        }
+
+        @Override
+        public void forEach(@NotNull Consumer<? super A> leftConsumer, @NotNull Consumer<? super B> rightConsumer) {
+            leftConsumer.accept(value);
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public boolean equals(Object obj) {
+            if (this == obj) {
+                return true;
+            }
+            if (!(obj instanceof Left<?, ?>)) {
+                return false;
+            }
+
+            return Objects.equals(value, ((Left<?, ?>) obj).value);
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public int hashCode() {
+            return Objects.hashCode(value) + HASH_MAGIC;
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public String toString() {
+            return "Either.Left[" + value + "]";
+        }
+    }
+
+    public final static class Right<@Covariant A, @Covariant B> extends Either<A, B> {
+        private static final long serialVersionUID = -3372589401685464421L;
+
+        private static final int HASH_MAGIC = 1973604283;
+
+        private final B value;
+
+        Right(B value) {
+            this.value = value;
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public boolean isLeft() {
+            return false;
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public boolean isRight() {
+            return true;
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        @Contract("-> fail")
+        public A getLeftValue() {
+            throw new NoSuchElementException("Either.Right.getLeftValue");
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public @NotNull Option<A> getLeftOption() {
+            return Option.none();
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public B getRightValue() {
+            return value;
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public @NotNull Option<B> getRightOption() {
+            return Option.some(value);
+        }
+
+        @Override
+        public <U, V> @NotNull Either<U, V> map(
+                @NotNull Function<? super A, ? extends U> leftMapper,
+                @NotNull Function<? super B, ? extends V> rightMapper
+        ) {
+            return Either.right(rightMapper.apply(value));
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public <U> @NotNull Either<U, B> mapLeft(@NotNull Function<? super A, ? extends U> mapper) {
+            return (Either<U, B>) this;
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public <U> @NotNull Either<A, U> mapRight(@NotNull Function<? super B, ? extends U> mapper) {
+            Objects.requireNonNull(mapper);
+            return Either.right(mapper.apply(value));
+        }
+
+        @Override
+        public <U> U fold(
+                @NotNull Function<? super A, ? extends U> leftMapper,
+                @NotNull Function<? super B, ? extends U> rightMapper) {
+            return rightMapper.apply(value);
+        }
+
+        @Override
+        public <C, D, U> U bifold(
+                @NotNull Either<? extends C, ? extends D> other,
+                U defaultValue,
+                @NotNull BiFunction<? super A, ? super C, ? extends U> leftMapper,
+                @NotNull BiFunction<? super B, ? super D, ? extends U> rightMapper) {
+
+            if (other instanceof Right) {
+                return rightMapper.apply(this.value, ((Right<? extends C, ? extends D>) other).value);
+            } else {
+                return defaultValue;
+            }
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public @NotNull Either<B, A> swap() {
+            return Either.left(value);
+        }
+
+        @Override
+        public @NotNull Result<B, A> toResult() {
+            return Result.ok(value);
+        }
+
+        @Override
+        public void forEach(@NotNull Consumer<? super A> leftConsumer, @NotNull Consumer<? super B> rightConsumer) {
+            rightConsumer.accept(value);
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public boolean equals(Object obj) {
+            if (this == obj) {
+                return true;
+            }
+            if (!(obj instanceof Right<?, ?>)) {
+                return false;
+            }
+
+            return Objects.equals(value, ((Right<?, ?>) obj).value);
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public int hashCode() {
+            return Objects.hashCode(value) + HASH_MAGIC;
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public String toString() {
+            return "Either.Right[" + value + "]";
+        }
+    }
+
+    public abstract class Projection<@Covariant T> implements OptionContainer<T> {
+        Projection() {
+        }
+
+        private static final int HASH_MAGIC = 905770825;
+
+        public final @NotNull Either<A, B> getEither() {
+            return Either.this;
+        }
+
+        @Override
+        public boolean equals(Object o) {
+            if (this == o) {
+                return true;
+            }
+            if (o == null || getClass() != o.getClass()) {
+                return false;
+            }
+
+            return Either.this.equals(((Projection<?>) o).getEither());
+        }
+
+        @Override
+        public final int hashCode() {
+            return Either.this.hashCode() + HASH_MAGIC;
+        }
+    }
+
+    public final class LeftProjection extends Projection<A> {
+        @Override
+        public boolean isDefined() {
+            return Either.this.isLeft();
+        }
+
+        @Override
+        public A get() {
+            return Either.this.getLeftValue();
+        }
+
+        @Override
+        @NotNull
+        public <U> Either<U, B>.LeftProjection map(@NotNull Function<? super A, ? extends U> mapper) {
+            Objects.requireNonNull(mapper);
+            if (isDefined()) {
+                return Either.<U, B>left(mapper.apply(getLeftValue())).new LeftProjection();
+            }
+            return (Either<U, B>.LeftProjection) this;
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @NotNull
+        @Override
+        public String toString() {
+            return Either.this + ".LeftProjection";
+        }
+    }
+
+    public final class RightProjection extends Projection<B> {
+        @Override
+        public boolean isDefined() {
+            return Either.this.isRight();
+        }
+
+        @Override
+        public B get() {
+            return Either.this.getRightValue();
+        }
+
+        @Override
+        @NotNull
+        public <U> Either<A, U>.RightProjection map(@NotNull Function<? super B, ? extends U> mapper) {
+            Objects.requireNonNull(mapper);
+            if (isDefined()) {
+                return Either.<A, U>right(mapper.apply(getRightValue())).new RightProjection();
+            }
+            return (Either<A, U>.RightProjection) this;
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @NotNull
+        @Override
+        public String toString() {
+            return Either.this + ".RightProjection";
+        }
+    }
+}