Scott encoded Either

Either.java

package se.foo.bar;

import java.util.Optional;
import java.util.function.Function;
import java.util.function.Supplier;
import java.util.stream.Stream;

interface Either<L, R> {
    <T> T either(Function<L, T> left, Function<R, T> right);

    <T> Either<L, T> map(Function<R, T> mapper);

    <T> Either<L, T> flatMap(Function<R, Either<L, T>> fn);

    boolean isLeft();

    boolean isRight();

    Stream<R> toStream();

    Optional<R> toOptional();

    R orElse(R other);

    R orElseGet(Supplier<R> other);

    L fromLeft(L l);

    R fromRight(R r);
    
    static <T> Either<Throwable, T> tryCatch(Supplier<T> supplier) {
        try {
            return Either.Right(supplier.get());
        } catch (Throwable t) {
            return Either.Left(t);
        }
    }

    static <L, R> Either<L, R> Left(L a) {
        return new Either<L, R>() {
            @Override
            public <C> C either(Function<L, C> l, Function<R, C> r) {
                return l.apply(a);
            }

            @Override
            public <T> Either<L, T> map(Function<R, T> mapper) {
                return Left(a);
            }

            @Override
            public <T> Either<L, T> flatMap(Function<R, Either<L, T>> fn) {
                return Left(a);
            }

            @Override
            public boolean isLeft() {
                return true;
            }

            @Override
            public boolean isRight() {
                return false;
            }

            @Override
            public Stream<R> toStream() {
                return Stream.empty();
            }

            @Override
            public Optional<R> toOptional() {
                return Optional.empty();
            }

            @Override
            public String toString() {
                return String.format("Left(%s)", a);
            }

            public R orElse(R other) {
                return other;
            }

            @Override
            public R orElseGet(Supplier<R> other) {
                return other.get();
            }

            @Override
            public L fromLeft(L __) {
                return a;
            }

            @Override
            public R fromRight(R r) {
                return r;
            }
        };
    }

    static <L, R> Either<L, R> Right(R b) {
        return new Either<L, R>() {
            @Override
            public <T> T either(Function<L, T> l, Function<R, T> r) {
                return r.apply(b);
            }

            @Override
            public <T> Either<L, T> map(Function<R, T> fn) {
                return Right(fn.apply(b));
            }

            @Override
            public <T> Either<L, T> flatMap(Function<R, Either<L, T>> fn) {
                return fn.apply(b);
            }

            @Override
            public boolean isLeft() {
                return false;
            }

            @Override
            public boolean isRight() {
                return true;
            }

            @Override
            public Stream<R> toStream() {
                return Stream.of(b);
            }

            @Override
            public Optional<R> toOptional() {
                return Optional.of(b);
            }

            @Override
            public R orElse(R __) {
                return b;
            }

            @Override
            public R orElseGet(Supplier<R> __) {
                return b;
            }

            @Override
            public L fromLeft(L l) {
                return l;
            }

            @Override
            public R fromRight(R __) {
                return b;
            }

            @Override
            public String toString() {
                return String.format("Right(%s)", b);
            }
        };
    }
}

    @Test
    public void foo() {
        Either<Throwable, Integer> result = getNumber(100).flatMap(n -> getNumber(n - 51));

        int someInt = result.either(this::handleErrors, x -> x);

        System.out.println(someInt); // 777
    }

    private int handleErrors(Throwable fromLeft) {
        if (fromLeft instanceof BelowHundred) {
            return 666;
        }

        if (fromLeft instanceof BelowFifty) {
            return 777;
        }

        return 0;
    }

    private Either<Throwable, Integer> getNumber(int n) {

        return n < 100
                ? n < 50 ? Either.Left(new BelowFifty()) : Either.Left(new BelowHundred())
                : Either.Right(n);
    }

    private static class BelowHundred extends Throwable {}

    private static class BelowFifty extends Throwable {}

e instanceof Right won't work though :/