MonadError: Handling Errors Gracefully

While a Monad is excellent for sequencing operations that might fail (like with Optional or Either), it doesn't provide a standardised way to inspect or recover from those failures. The MonadError type class fills this gap.

It's a specialised Monad that has a defined error type E, giving you a powerful and abstract API for raising and handling errors within any monadic workflow.

What is it?

A MonadError is a Monad that provides two additional, fundamental operations for working with failures:

1. raiseError(E error): This allows you to construct a failed computation by lifting an error value E directly into the monadic context.
2. handleErrorWith(Kind<F, A> fa, ...): This is the recovery mechanism. It allows you to inspect a potential failure and provide a fallback computation to rescue the workflow.

By abstracting over a specific error type E, MonadError allows you to write generic, resilient code that can work with any data structure capable of representing failure, such as Either<E, A>, Try<A> (where E is Throwable), or even custom error-handling monads.

The interface for MonadError in hkj-api extends Monad:

@NullMarked
public interface MonadError<F, E> extends Monad<F> {

  <A> @NonNull Kind<F, A> raiseError(@Nullable final E error);

  <A> @NonNull Kind<F, A> handleErrorWith(
      final Kind<F, A> ma,
      final Function<? super E, ? extends Kind<F, A>> handler);

  // Default recovery methods like handleError, recover, etc. are also provided
  default <A> @NonNull Kind<F, A> handleError(
      final Kind<F, A> ma,
      final Function<? super E, ? extends A> handler) {
    return handleErrorWith(ma, error -> of(handler.apply(error)));
  }
}

Why is it useful?

MonadError formalises the pattern of "try-catch" in a purely functional way. It lets you build complex workflows that need to handle specific types of errors without coupling your logic to a concrete implementation like Either or Try. You can write a function once, and it will work seamlessly with any data type that has a MonadError instance.

This is incredibly useful for building robust applications, separating business logic from error-handling logic, and providing sensible fallbacks when operations fail.

Example: A Resilient Division Workflow

Let's model a division operation that can fail with a specific error message. We'll use Either<String, A> as our data type, which is a perfect fit for MonadError.

import org.higherkindedj.hkt.Kind;
import org.higherkindedj.hkt.either.Either;
import org.higherkindedj.hkt.either.EitherMonad;
import static org.higherkindedj.hkt.either.EitherKindHelper.EITHER;

// --- Get the MonadError instance for Either<String, ?> ---
MonadError<Either.Witness<String>, String> monadError = EitherMonad.instance();

// A function that performs division, raising a specific error on failure
public Kind<Either.Witness<String>, Integer> safeDivide(int a, int b) {
    if (b == 0) {
        return monadError.raiseError("Cannot divide by zero!");
    }
    return monadError.of(a / b);
}

// --- Scenario 1: A successful division ---
Kind<Either.Witness<String>, Integer> success = safeDivide(10, 2);

// Result: Right(5)
System.out.println(EITHER.narrow(success));


// --- Scenario 2: A failed division ---
Kind<Either.Witness<String>, Integer> failure = safeDivide(10, 0);

// Result: Left(Cannot divide by zero!)
System.out.println(EITHER.narrow(failure));


// --- Scenario 3: Recovering from the failure ---
// We can use handleErrorWith to catch the error and return a fallback value.
Kind<Either.Witness<String>, Integer> recovered = monadError.handleErrorWith(
    failure,
    errorMessage -> {
        System.out.println("Caught an error: " + errorMessage);
        return monadError.of(0); // Recover with a default value of 0
    }
);

// Result: Right(0)
System.out.println(EITHER.narrow(recovered));

In this example, raiseError allows us to create the failure case in a clean, declarative way, while handleErrorWith provides a powerful mechanism for recovery, making our code more resilient and predictable.