Using Optional with custom monads or functional programming libraries can enhance code readability and handle null-like scenarios effectively. Here’s how you can integrate Optional with custom monads or functional programming libraries:

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1. Understanding Optional in Functional Context

Optional is essentially a simplified monad used to represent the presence or absence of a value. Custom monads often introduce additional context, like logging (Writer), computation (IO), or error propagation (Either). You need to interoperate by converting between Optional and your custom monads.

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2. Use Case: Wrapping Optional in Custom Monads

You can seamlessly integrate Optional with your monads using the following steps:

a) Lifting Optional into a Monad

If you have an Optional value and want to lift it into another monad (e.g., Either, Try, etc.):

Optional<String> optionalValue = Optional.of("Hello");

Either<String, String> eitherValue = optionalValue
   .map(Either::<String, String>right) // Wrap the value in a Right
   .orElse(Either.left("Default value")); // Provide a Left value for absent option

b) From Custom Monad to Optional

Converting a value from a monadic type back to Optional:

Suppose you are using a library with custom monads like Either<L, R>. To extract the right value into an Optional:

Either<String, String> eitherValue = Either.right("Hello");

Optional<String> optionalValue = eitherValue
   .toOptional(); // Assuming your library has this method

If your library doesn’t support this natively, you can write utility methods:

public static <L, R> Optional<R> eitherToOptional(Either<L, R> either) {
   return either.isRight() ? Optional.of(either.getRight()) : Optional.empty();
}

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3. Higher-Order Functions: Combine Optional with Streams or Collections

Libraries like Vavr or Arrow provide monadic types as part of their standard functional programming suite. Interoperating with them requires mapping and flat-mapping similar to Optional.

Example: Using Vavr’s Option with Java’s Optional

Converting between Java’s Optional and Vavr’s Option:

Optional<String> javaOptional = Optional.of("Functional!");
io.vavr.control.Option<String> vavrOption = io.vavr.control.Option.ofOptional(javaOptional);

// Vice versa: Convert Vavr's Option to Java's Optional
Optional<String> convertedOptional = vavrOption.toJavaOptional();

Example: Handle Streams with Optional

If your monad uses Java functions:

Optional<String> optionalValue = Optional.of("Monad");
List<Optional<String>> optionalList = Arrays.asList(optionalValue);

List<String> unwrappedList = optionalList.stream()
   .flatMap(Optional::stream) // Java 9+ Optional::stream
   .collect(Collectors.toList());

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4. Custom Monad Utility Using Optional

Suppose you want to use Optional in a custom monadic type:

package org.kodejava.util;

import java.util.Optional;
import java.util.function.Function;

public class CustomMonad<T> {
    private final Optional<T> optional;

    public CustomMonad(T value) {
        this.optional = Optional.ofNullable(value);
    }

    public <R> CustomMonad<R> flatMap(Function<T, CustomMonad<R>> mapper) {
        if (optional.isEmpty()) return new CustomMonad<>(null);
        return mapper.apply(optional.get());
    }

    public Optional<T> toOptional() {
        return optional;
    }

    public T getOrElse(T defaultValue) {
        return optional.orElse(defaultValue);
    }
}

Use:

CustomMonad<String> monad = new CustomMonad<>("Hello");

CustomMonad<String> upperCaseMonad = monad.flatMap(
   value -> new CustomMonad<>(value.toUpperCase()));

System.out.println(upperCaseMonad.toOptional().orElse("Fallback"));

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5. Chaining Optional with Monads

If your monad (Optional, Either, or others) supports chaining via flatMap, you can chain operations together efficiently:

Optional<String> optional = Optional.of("Monad");

Optional<Integer> length = optional.flatMap(val -> Optional.of(val.length()));

If chaining involves multiple monads, interconversion techniques (discussed above) become useful.

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6. Error Handling with Optional

When pairing Optional with an error-propagating monad like Either or Try, handle absence cases explicitly:

Optional<String> optional = Optional.empty();

Try<String> result = Try.of(() -> optional.orElseThrow(() -> new RuntimeException("Empty!")));

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Integrating Optional with custom monads or functional programming libraries usually requires interconversion or adapting map/flatMap semantics to maintain behavior. Using third-party libraries like Vavr can further expand the functional possibilities with their enriched monad ecosystem.