The MaybeMonad:

Handling Optional Values with Non-Null Guarantee

What You'll Learn

How Maybe provides null-safe optional values with guaranteed non-null contents
The difference between Maybe and Optional (non-null guarantee in Just)
Using Maybe as a MonadError with Unit as the error type
Chaining operations with automatic Nothing propagation
Building robust pipelines that handle absence gracefully

See Example Code:

Purpose

How do you handle optional values in Java without falling into the null pointer trap? The Maybe<T> type in Higher-Kinded-J provides an elegant solution—representing a value that might be present (Just<T>) or absent (Nothing<T>), with one crucial guarantee: a Just<T> will never hold null.

The Maybe<T> type is conceptually similar to java.util.Optional<T> but with a key distinction: a Just<T> is guaranteed to hold a non-null value. This strictness helps prevent NullPointerExceptions when a value is asserted to be present. Maybe.fromNullable(T value) or MaybeMonad.of(T value) should be used if the input value could be null, as these will correctly produce a Nothing in such cases.

The MaybeMonad provides a monadic interface for Maybe, allowing for functional composition and integration with the Higher-Kinded Type (HKT) system. This facilitates chaining operations that may or may not yield a value, propagating the Nothing state automatically.

Key benefits include:

Explicit Optionality with Non-Null Safety: Just<T> guarantees its contained value is not null. Nothing<T> clearly indicates absence.
Functional Composition: Enables elegant chaining of operations using map, flatMap, and ap, where Nothing short-circuits computations.
HKT Integration: MaybeKind<A> (the HKT wrapper for Maybe<A>) and MaybeMonad allow Maybe to be used with generic functions and type classes that expect Kind<F, A>, Functor<F>, Applicative<F>, Monad<M>, or MonadError<M, E>.
Error Handling for Absence: MaybeMonad implements MonadError<MaybeKind.Witness, Unit>. Nothing is treated as the "error" state, with Unit as the phantom error type, signifying absence.

It implements MonadError<MaybeKind.Witness, Unit>, which transitively includes Monad<MaybeKind.Witness>, Applicative<MaybeKind.Witness>, and Functor<MaybeKind.Witness>.

Structure

Related Types

For working with Java's standard Optional type in the HKT system, see Optional Monad. For error handling with typed error values, see Either Monad.

How to Use `MaybeMonad` and `Maybe`

Creating Instances

Maybe<A> instances can be created directly using static factory methods on Maybe, or via MaybeMonad for HKT integration. Since Just<T> and Nothing<T> directly implement MaybeKind<T>, they are first-class participants in the HKT simulation with zero runtime overhead for widen/narrow operations.

Direct Maybe Creation:

Maybe.just(@NonNull T value)

Creates a Just holding a non-null value. Throws NullPointerException if value is null.

Maybe<String> justHello = Maybe.just("Hello"); // Just("Hello")
Maybe<String> illegalJust = Maybe.just(null); // Throws NullPointerException

Maybe.nothing()

Returns a singleton Nothing instance.

Maybe<Integer> noInt = Maybe.nothing(); // Nothing

Maybe.fromNullable(@Nullable T value)

Creates Just(value) if value is non-null, otherwise Nothing.

Maybe<String> fromPresent = Maybe.fromNullable("Present"); // Just("Present")
Maybe<String> fromNull = Maybe.fromNullable(null);     // Nothing

MaybeKindHelper (for HKT wrapping):

MaybeKindHelper.widen(Maybe maybe)

Converts a Maybe<A> to Kind<MaybeKind.Witness, A>. Since Just and Nothing directly implement MaybeKind, this performs a null check and type-safe cast (zero overhead—no wrapper object allocation).

Kind<MaybeKind.Witness, String> kindJust = MAYBE.widen(Maybe.just("Wrapped"));
Kind<MaybeKind.Witness,Integer> kindNothing = MAYBE.widen(Maybe.nothing());

MAYBE.just(@NonNull A value)

Convenience for widen(Maybe.just(value)).

MAYBE.nothing()

Convenience for widen(Maybe.nothing()).

MaybeMonad Instance Methods:

maybeMonad.of(@Nullable A value)

Lifts a value into Kind<MaybeKind.Witness, A>. Uses Maybe.fromNullable() internally.

MaybeMonad maybeMonad = MaybeMonad.INSTANCE;
Kind<MaybeKind.Witness, String> kindFromMonad = maybeMonad.of("Monadic"); // Just("Monadic")
Kind<MaybeKind.Witness, String> kindNullFromMonad = maybeMonad.of(null);   // Nothing

maybeMonad.raiseError(@Nullable Unit error)

Creates a Kind<MaybeKind.Witness, E> representing Nothing. The error (Unit) argument is ignored.

Kind<MaybeKind.Witness, Double> errorKind = maybeMonad.raiseError(Unit.INSTANCE); // Nothing

Unwrapping MaybeKind

To get the underlying Maybe<A> from a MaybeKind<A>, use MAYBE.narrow():

MaybeKind<String> kindJust = MAYBE.just("Example");
Maybe<String> unwrappedMaybe = MAYBE.narrow(kindJust); // Just("Example")
System.out.println("Unwrapped: " + unwrappedMaybe);

MaybeKind<Integer> kindNothing = MAYBE.nothing();
Maybe<Integer> unwrappedNothing = MAYBE.narrow(kindNothing); // Nothing
System.out.println("Unwrapped Nothing: " + unwrappedNothing);

Interacting with `Maybe` values

The Maybe interface itself provides useful methods:

isJust(): Returns true if it's a Just.
isNothing(): Returns true if it's a Nothing.
get(): Returns the value if Just, otherwise throws NoSuchElementException. Use with caution.
orElse(@NonNull T other): Returns the value if Just, otherwise returns other.
orElseGet(@NonNull Supplier<? extends @NonNull T> other): Returns the value if Just, otherwise invokes other.get().
The Maybe interface also has its own map and flatMap methods, which are similar in behaviour to those on MaybeMonad but operate directly on Maybe instances.

Key Operations (via `MaybeMonad`)

Example: map(Function<A, B> f, Kind<MaybeKind.Witness, A> ma)

Applies f to the value inside ma if it's Just. If ma is Nothing, or if f returns null (which Maybe.fromNullable then converts to Nothing), the result is Nothing.

void mapExample() {
  MaybeMonad maybeMonad = MaybeMonad.INSTANCE;
  Kind<MaybeKind.Witness, Integer> justNum = MAYBE.just(10);
  Kind<MaybeKind.Witness, Integer> nothingNum = MAYBE.nothing();

  Function<Integer, String> numToString = n -> "Val: " + n;
  Kind<MaybeKind.Witness, String> justStr = maybeMonad.map(numToString, justNum); // Just("Val: 10")
  Kind<MaybeKind.Witness, String> nothingStr = maybeMonad.map(numToString, nothingNum); // Nothing

  Function<Integer, String> numToNull = n -> null;
  Kind<MaybeKind.Witness, String> mappedToNull = maybeMonad.map(numToNull, justNum); // Nothing

  System.out.println("Map (Just): " + MAYBE.narrow(justStr));
  System.out.println("Map (Nothing): " + MAYBE.narrow(nothingStr));
  System.out.println("Map (To Null): " + MAYBE.narrow(mappedToNull));
}

Example: flatMap(Function<A, Kind<MaybeKind.Witness, B>> f, Kind<MaybeKind.Witness, A> ma)

If ma is Just(a), applies f to a. f must return a Kind<MaybeKind.Witness, B>. If ma is Nothing, or f returns Nothing, the result is Nothing.

void flatMapExample() {
  MaybeMonad maybeMonad = MaybeMonad.INSTANCE;
  Function<String, Kind<MaybeKind.Witness, Integer>> parseString = s -> {
    try {
      return MAYBE.just(Integer.parseInt(s));
    } catch (NumberFormatException e) {
      return MAYBE.nothing();
    }
  };

  Kind<MaybeKind.Witness, String> justFiveStr = MAYBE.just("5");
  Kind<MaybeKind.Witness, Integer> parsedJust = maybeMonad.flatMap(parseString, justFiveStr); // Just(5)

  Kind<MaybeKind.Witness, String> justNonNumStr = MAYBE.just("abc");
  Kind<MaybeKind.Witness, Integer> parsedNonNum = maybeMonad.flatMap(parseString, justNonNumStr); // Nothing

  System.out.println("FlatMap (Just): " + MAYBE.narrow(parsedJust));
  System.out.println("FlatMap (NonNum): " + MAYBE.narrow(parsedNonNum));
}

Example: ap(Kind<MaybeKind.Witness, Function<A, B>> ff, Kind<MaybeKind.Witness, A> fa)

If ff is Just(f) and fa is Just(a), applies f to a. Otherwise, Nothing.

void apExample() {
  MaybeMonad maybeMonad = MaybeMonad.INSTANCE;
  Kind<MaybeKind.Witness, Integer> justNum = MAYBE.just(10);
  Kind<MaybeKind.Witness, Integer> nothingNum = MAYBE.nothing();
  Kind<MaybeKind.Witness, Function<Integer, String>> justFunc = MAYBE.just(i -> "Result: " + i);
  Kind<MaybeKind.Witness, Function<Integer, String>> nothingFunc = MAYBE.nothing();

  Kind<MaybeKind.Witness, String> apApplied = maybeMonad.ap(justFunc, justNum); // Just("Result: 10")
  Kind<MaybeKind.Witness, String> apNothingFunc = maybeMonad.ap(nothingFunc, justNum); // Nothing
  Kind<MaybeKind.Witness, String> apNothingVal = maybeMonad.ap(justFunc, nothingNum); // Nothing

  System.out.println("Ap (Applied): " + MAYBE.narrow(apApplied));
  System.out.println("Ap (Nothing Func): " + MAYBE.narrow(apNothingFunc));
  System.out.println("Ap (Nothing Val): " + MAYBE.narrow(apNothingVal));
}

Example: handleErrorWith(Kind<MaybeKind.Witness, A> ma, Function<Void, Kind<MaybeKind.Witness, A>> handler)

If ma is Just, it's returned. If ma is Nothing (the "error" state), handler is invoked (with Unit.INSTANCE for Unit) to provide a recovery MaybeKind.

void handleErrorWithExample() {
  MaybeMonad maybeMonad = MaybeMonad.INSTANCE;
  Function<Unit, Kind<MaybeKind.Witness, String>> recover = v -> MAYBE.just("Recovered");

  Kind<MaybeKind.Witness, String> handledJust = maybeMonad.handleErrorWith(MAYBE.just("Original"), recover); // Just("Original")
  Kind<MaybeKind.Witness, String> handledNothing = maybeMonad.handleErrorWith(MAYBE.nothing(), recover);    // Just("Recovered")

  System.out.println("HandleError (Just): " + MAYBE.narrow(handledJust));
  System.out.println("HandleError (Nothing): " + MAYBE.narrow(handledNothing));
}

Example: Using MaybeMonad

A complete example demonstrating generic usage:

public void monadExample() {
  MaybeMonad maybeMonad = MaybeMonad.INSTANCE;

  // 1. Create MaybeKind instances
  Kind<MaybeKind.Witness, Integer> presentIntKind = MAYBE.just(100);
  Kind<MaybeKind.Witness, Integer> absentIntKind = MAYBE.nothing();
  Kind<MaybeKind.Witness, String> nullInputStringKind = maybeMonad.of(null); // Becomes Nothing

  // 2. Use map
  Function<Integer, String> intToStatus = n -> "Status: " + n;
  Kind<MaybeKind.Witness, String> mappedPresent = maybeMonad.map(intToStatus, presentIntKind);
  Kind<MaybeKind.Witness, String> mappedAbsent = maybeMonad.map(intToStatus, absentIntKind);

  System.out.println("Mapped (Present): " + MAYBE.narrow(mappedPresent)); // Just(Status: 100)
  System.out.println("Mapped (Absent): " + MAYBE.narrow(mappedAbsent));   // Nothing

  // 3. Use flatMap
  Function<Integer, Kind<MaybeKind.Witness, String>> intToPositiveStatusKind = n ->
      (n > 0) ? maybeMonad.of("Positive: " + n) : MAYBE.nothing();

  Kind<MaybeKind.Witness, String> flatMappedPresent = maybeMonad.flatMap(intToPositiveStatusKind, presentIntKind);
  Kind<MaybeKind.Witness, String> flatMappedZero = maybeMonad.flatMap(intToPositiveStatusKind, maybeMonad.of(0)); // 0 is not > 0

  System.out.println("FlatMapped (Present Positive): " + MAYBE.narrow(flatMappedPresent)); // Just(Positive: 100)
  System.out.println("FlatMapped (Zero): " + MAYBE.narrow(flatMappedZero)); // Nothing

  // 4. Use 'of' and 'raiseError'
  Kind<MaybeKind.Witness, String> fromOf = maybeMonad.of("Direct Value");
  Kind<MaybeKind.Witness, String> fromRaiseError = maybeMonad.raiseError(Unit.INSTANCE); // Creates Nothing
  System.out.println("From 'of': " + MAYBE.narrow(fromOf)); // Just(Direct Value)
  System.out.println("From 'raiseError': " + MAYBE.narrow(fromRaiseError)); // Nothing
  System.out.println("From 'of(null)': " + MAYBE.narrow(nullInputStringKind)); // Nothing


  // 5. Use handleErrorWith
  Function<Void, Kind<MaybeKind.Witness, Integer>> recoverWithDefault =
      v -> maybeMonad.of(-1); // Default value if absent

  Kind<MaybeKind.Witness, Integer> recoveredFromAbsent =
      maybeMonad.handleErrorWith(absentIntKind, recoverWithDefault);
  Kind<MaybeKind.Witness, Integer> notRecoveredFromPresent =
      maybeMonad.handleErrorWith(presentIntKind, recoverWithDefault);

  System.out.println("Recovered (from Absent): " + MAYBE.narrow(recoveredFromAbsent)); // Just(-1)
  System.out.println("Recovered (from Present): " + MAYBE.narrow(notRecoveredFromPresent)); // Just(100)

  // Using the generic processData function
  Kind<MaybeKind.Witness, String> processedPresent = processData(presentIntKind, x -> "Processed: " + x, "N/A", maybeMonad);
  Kind<MaybeKind.Witness, String> processedAbsent = processData(absentIntKind, x -> "Processed: " + x, "N/A", maybeMonad);

  System.out.println("Generic Process (Present): " + MAYBE.narrow(processedPresent)); // Just(Processed: 100)
  System.out.println("Generic Process (Absent): " + MAYBE.narrow(processedAbsent));   // Just(N/A)

  // Unwrap to get back the standard Maybe
  Maybe<String> finalMappedMaybe = MAYBE.narrow(mappedPresent);
  System.out.println("Final unwrapped mapped maybe: " + finalMappedMaybe); // Just(Status: 100)
}

public static <A, B> Kind<MaybeKind.Witness, B> processData(
    Kind<MaybeKind.Witness, A> inputKind,
    Function<A, B> mapper,
    B defaultValueOnAbsence,
    MaybeMonad monad
) {
  // inputKind is now Kind<MaybeKind.Witness, A>, which is compatible with monad.map
  Kind<MaybeKind.Witness, B> mappedKind = monad.map(mapper, inputKind);

  // The result of monad.map is Kind<MaybeKind.Witness, B>.
  // The handler (Unit v) -> monad.of(defaultValueOnAbsence) also produces Kind<MaybeKind.Witness, B>.
  return monad.handleErrorWith(mappedKind, (Unit v) -> monad.of(defaultValueOnAbsence));
}

This example highlights how MaybeMonad facilitates working with optional values in a functional, type-safe manner, especially when dealing with the HKT abstractions and requiring non-null guarantees for present values.

Higher-Kinded Types and Optics for Java