The StateT Transformer:

Monad Transformer

What You'll Learn

How to add stateful computation to any existing monad
Building stack operations that can fail (StateT with Optional)
Understanding the relationship between State and StateT<S, Identity, A>
Creating complex workflows that manage both state and other effects
Using get, set, modify operations within transformer contexts

See Example Code:

The StateT monad transformer is a powerful construct that allows you to add state-management capabilities to an existing monadic context. Think of it as taking the State Monad and making it work on top of another monad, like OptionalKind, EitherKind, or IOKind.

This is incredibly useful when you have computations that are both stateful and involve other effects, such as:

Potentially missing values (Optional)
Operations that can fail (Either, Try)
Side-effecting computations (IO)

What is StateT?

At its core, a StateT<S, F, A> represents a computation that:

Takes an initial state of type S.
Produces a result of type A along with a new state of type S.
And this entire process of producing the (newState, value) pair is itself wrapped in an underlying monadic context F.

So, the fundamental structure of a StateT computation can be thought of as a function: S -> F<StateTuple<S, A>>

Where:

S: The type of the state.
F: The witness type for the underlying monad (e.g., OptionalKind.Witness, IOKind.Witness).
A: The type of the computed value.
StateTuple<S, A>: A simple container holding a pair of (state, value).

Key Classes and Concepts

StateT<S, F, A>: The primary data type representing the stateful computation stacked on monad F. It holds the function S -> Kind<F, StateTuple<S, A>>.
StateTKind<S, F, A>: The Kind representation for StateT, allowing it to be used with higher-kinded-j's typeclasses like Monad. This is what you'll mostly interact with when using StateT in a generic monadic context.
StateTKind.Witness<S, F>: The higher-kinded type witness for StateT<S, F, _>. Note that both the state type S and the underlying monad witness F are part of the StateT witness.
StateTMonad<S, F>: The Monad instance for StateT<S, F, _>. It requires a Monad instance for the underlying monad F to function.
StateTKindHelper: A utility class providing static methods for working with StateTKind, such as narrow (to convert Kind<StateTKind.Witness<S, F>, A> back to StateT<S, F, A>), runStateT, evalStateT, and execStateT.
StateTuple<S, A>: A simple record-like class holding the pair (S state, A value).

Motivation: Why Use StateT?

Imagine you're processing a sequence of items, and for each item:

You need to update some running total (state).
The processing of an item might fail or return no result (e.g., Optional).

Without StateT, you might end up with deeply nested Optional<StateTuple<S, A>> and manually manage both the optionality and the state threading. StateT<S, OptionalKind.Witness, A> elegantly combines these concerns.

Usage

StateTExample.java

Creating StateT Instances

You typically create StateT instances in a few ways:

Directly with StateT.create(): This is the most fundamental way, providing the state function and the underlying monad instance.



// Assume S = Integer (state type), F = OptionalKind.Witness, A = String (value type)
 OptionalMonad optionalMonad = OptionalMonad.INSTANCE;

 Function<Integer, Kind<OptionalKind.Witness, StateTuple<Integer, String>>> runFn =
     currentState -> {
       if (currentState < 0) {
         return OPTIONAL.widen(Optional.empty());
       }
       return OPTIONAL.widen(Optional.of(StateTuple.of(currentState + 1, "Value: " + currentState)));
     };

 StateT<Integer, OptionalKind.Witness, String> stateTExplicit =
     StateT.create(runFn, optionalMonad);

 Kind<StateTKind.Witness<Integer, OptionalKind.Witness>, String> stateTKind =
     stateTExplicit;

Lifting values with StateTMonad.of(): This lifts a pure value A into the StateT context. The state remains unchanged, and the underlying monad F will wrap the result using its own of method.

 StateTMonad<Integer, OptionalKind.Witness> stateTMonad = StateTMonad.instance(optionalMonad);

 Kind<StateTKind.Witness<Integer, OptionalKind.Witness>, String> pureStateT =
     stateTMonad.of("pure value");


 Optional<StateTuple<Integer, String>> pureResult =
     OPTIONAL.narrow(STATE_T.runStateT(pureStateT, 10));
 System.out.println("Pure StateT result: " + pureResult);
// When run with state 10, this will result in Optional.of(StateTuple(10, "pure value"))

Running StateT Computations

To execute a StateT computation and extract the result, you use methods from StateTKindHelper or directly from the StateT object:

runStateT(initialState): Executes the computation with an initialState and returns the result wrapped in the underlying monad: Kind<F, StateTuple<S, A>>.

// Continuing the stateTKind from above:
Kind<OptionalKind.Witness, StateTuple<Integer, String>> resultOptionalTuple =
      StateTKindHelper.runStateT(stateTKind, 10);

  Optional<StateTuple<Integer, String>> actualOptional = OPTIONAL.narrow(resultOptionalTuple);

  if (actualOptional.isPresent()) {
    StateTuple<Integer, String> tuple = actualOptional.get();
    System.out.println("New State (from stateTExplicit): " + tuple.state());
    System.out.println("Value (from stateTExplicit): " + tuple.value());
  } else {
    System.out.println("actualOptional was empty for initial state 10");
  }

  // Example with negative initial state (expecting empty Optional)
  Kind<OptionalKind.Witness, StateTuple<Integer, String>> resultEmptyOptional =
      StateTKindHelper.runStateT(stateTKind, -5);
  Optional<StateTuple<Integer, String>> actualEmpty = OPTIONAL.narrow(resultEmptyOptional);
  // Output: Is empty: true
  System.out.println("Is empty (for initial state -5): " + actualEmpty.isEmpty());

evalStateT(initialState): Executes and gives you Kind<F, A> (the value, discarding the final state).
execStateT(initialState): Executes and gives you Kind<F, S> (the final state, discarding the value).

Composing StateT Actions

Like any monad, StateT computations can be composed using map and flatMap.

map(Function<A, B> fn): Transforms the value A to B within the StateT context, leaving the state transformation logic and the underlying monad F's effect untouched for that step.

    Kind<StateTKind.Witness<Integer, OptionalKind.Witness>, Integer> initialComputation =
      StateT.create(s -> OPTIONAL.widen(Optional.of(StateTuple.of(s + 1, s * 2))), optionalMonad);

  Kind<StateTKind.Witness<Integer, OptionalKind.Witness>, String> mappedComputation =
      stateTMonad.map(
          val -> "Computed: " + val,
          initialComputation);

  // Run mappedComputation with initial state 5:
  // 1. initialComputation runs: state becomes 6, value is 10. Wrapped in Optional.
  // 2. map's function ("Computed: " + 10) is applied to 10.
  // Result: Optional.of(StateTuple(6, "Computed: 10"))
  Optional<StateTuple<Integer, String>> mappedResult =
      OPTIONAL.narrow(STATE_T.runStateT(mappedComputation, 5));
  System.out.print("Mapped result (initial state 5): ");
  mappedResult.ifPresentOrElse(System.out::println, () -> System.out.println("Empty"));
  // Output: StateTuple[state=6, value=Computed: 10]

flatMap(Function<A, Kind<StateTKind.Witness<S, F>, B>> fn): Sequences two StateT computations. The state from the first computation is passed to the second. The effects of the underlying monad F are also sequenced according to F's flatMap.

    // stateTMonad and optionalMonad are defined
  Kind<StateTKind.Witness<Integer, OptionalKind.Witness>, Integer> firstStep =
      StateT.create(s -> OPTIONAL.widen(Optional.of(StateTuple.of(s + 1, s * 10))), optionalMonad);

  Function<Integer, Kind<StateTKind.Witness<Integer, OptionalKind.Witness>, String>> secondStepFn =
      prevValue -> StateT.create(
          s -> {
            if (prevValue > 100) {
              return OPTIONAL.widen(Optional.of(StateTuple.of(s + prevValue, "Large: " + prevValue)));
            } else {
              return OPTIONAL.widen(Optional.empty());
            }
          },
          optionalMonad);

  Kind<StateTKind.Witness<Integer, OptionalKind.Witness>, String> combined =
      stateTMonad.flatMap(secondStepFn, firstStep);

  // Run with initial state 15
  // 1. firstStep(15): state=16, value=150. Wrapped in Optional.of.
  // 2. secondStepFn(150) is called. It returns a new StateT.
  // 3. The new StateT is run with state=16:
  //    Its function: s' (which is 16) -> Optional.of(StateTuple(16 + 150, "Large: 150"))
  //    Result: Optional.of(StateTuple(166, "Large: 150"))
  Optional<StateTuple<Integer, String>> combinedResult =
      OPTIONAL.narrow(STATE_T.runStateT(combined, 15));
  System.out.print("Combined result (initial state 15): ");
  combinedResult.ifPresentOrElse(System.out::println, () -> System.out.println("Empty"));

  // Output: StateTuple[state=166, value=Large: 150]

  // Run with initial state 5
  // 1. firstStep(5): state=6, value=50. Wrapped in Optional.of.
  // 2. secondStepFn(50) is called.
  // 3. The new StateT is run with state=6:
  //    Its function: s' (which is 6) -> Optional.empty()
  //    Result: Optional.empty()
  Optional<StateTuple<Integer, String>> combinedEmptyResult =
      OPTIONAL.narrow(STATE_T.runStateT(combined, 5));
  // Output: true
  System.out.println("Is empty from small initial (state 5 for combined): " + combinedEmptyResult.isEmpty());

ap(ff, fa): Applies a wrapped function to a wrapped value.

Null Handling

Note on Null Handling: The ap method requires the function it extracts from the first StateT computation to be non-null. If the function is null, a NullPointerException will be thrown when the computation is executed. It is the developer's responsibility to ensure that any functions provided within a StateT context are non-null. Similarly, the value from the second computation may be null, and the provided function must be able to handle a null input if that is a valid state.

State-Specific Operations

While higher-kinded-j's StateT provides the core monadic structure, you'll often want common state operations like get, set, modify. These can be constructed using StateT.create or StateTKind.lift.

get(): Retrieves the current state as the value.

public static <S, F> Kind<StateTKind.Witness<S, F>, S> get(Monad<F> monadF) {
  Function<S, Kind<F, StateTuple<S, S>>> runFn = s -> monadF.of(StateTuple.of(s, s));
  return StateT.create(runFn, monadF);
}
// Usage: stateTMonad.flatMap(currentState -> ..., get(optionalMonad))

set(newState, monadF): Replaces the current state with newState. The value is often Void or Unit.

public static <S, F> Kind<StateTKind.Witness<S, F>, Unit> set(S newState, Monad<F> monadF) {
  Function<S, Kind<F, StateTuple<S, Void>>> runFn = s -> monadF.of(StateTuple.of(newState, Unit.INSTANCE));
  return StateT.create(runFn, monadF);
}

modify(f, monadF): Modifies the state using a function.

public static <S, F> Kind<StateTKind.Witness<S, F>, Unit> modify(Function<S, S> f, Monad<F> monadF) {
  Function<S, Kind<F, StateTuple<S, Unit>>> runFn = s -> monadF.of(StateTuple.of(f.apply(s), Unit.INSTANCE));
  return StateT.create(runFn, monadF);
}

gets(f, monadF): Retrieves a value derived from the current state.

public static <S, F, A> Kind<StateTKind.Witness<S, F>, A> gets(Function<S, A> f, Monad<F> monadF) {
Function<S, Kind<F, StateTuple<S, A>>> runFn = s -> monadF.of(StateTuple.of(s, f.apply(s)));
return StateT.create(runFn, monadF);
}

StateTStackExample.java

Let's simulate stack operations where the stack is a List<Integer> and operations might be absent if, for example, popping an empty stack.

public class StateTStackExample {

  private static final OptionalMonad OPT_MONAD = OptionalMonad.INSTANCE;
  private static final StateTMonad<List<Integer>, OptionalKind.Witness> ST_OPT_MONAD =
      StateTMonad.instance(OPT_MONAD);

  // Helper to lift a state function into StateT<List<Integer>, OptionalKind.Witness, A>
  private static <A> Kind<StateTKind.Witness<List<Integer>, OptionalKind.Witness>, A> liftOpt(
      Function<List<Integer>, Kind<OptionalKind.Witness, StateTuple<List<Integer>, A>>> f) {
    return StateTKindHelper.stateT(f, OPT_MONAD);
  }

  // push operation
  public static Kind<StateTKind.Witness<List<Integer>, OptionalKind.Witness>, Unit> push(Integer value) {
    return liftOpt(stack -> {
      List<Integer> newStack = new LinkedList<>(stack);
      newStack.add(0, value); // Add to front
      return OPTIONAL.widen(Optional.of(StateTuple.of(newStack, Unit.INSTANCE)));
    });
  }

  // pop operation
  public static Kind<StateTKind.Witness<List<Integer>, OptionalKind.Witness>, Integer> pop() {
    return liftOpt(stack -> {
      if (stack.isEmpty()) {
        return OPTIONAL.widen(Optional.empty()); // Cannot pop from empty stack
      }
      List<Integer> newStack = new LinkedList<>(stack);
      Integer poppedValue = newStack.remove(0);
      return OPTIONAL.widen(Optional.of(StateTuple.of(newStack, poppedValue)));
    });
  }

  public static void main(String[] args) {
    var computation =
        For.from(ST_OPT_MONAD, push(10))
            .from(_ -> push(20))
            .from(_ -> pop())
            .from(_ -> pop()) // t._3() is the first popped value
            .yield((a, b, p1, p2) -> {
              System.out.println("Popped in order: " + p1 + ", then " + p2);
              return p1 + p2;
            });


    List<Integer> initialStack = Collections.emptyList();
    Kind<OptionalKind.Witness, StateTuple<List<Integer>, Integer>> resultWrapped =
        StateTKindHelper.runStateT(computation, initialStack);

    Optional<StateTuple<List<Integer>, Integer>> resultOpt =
        OPTIONAL.narrow(resultWrapped);

    resultOpt.ifPresentOrElse(
        tuple -> {
          System.out.println("Final value: " + tuple.value());       // Expected: 30
          System.out.println("Final stack: " + tuple.state());       // Expected: [] (empty)
        },
        () -> System.out.println("Computation resulted in empty Optional.")
    );

    // Example of popping an empty stack
    Kind<StateTKind.Witness<List<Integer>, OptionalKind.Witness>, Integer> popEmptyStack = pop();
    Optional<StateTuple<List<Integer>, Integer>> emptyPopResult =
        OPTIONAL.narrow(StateTKindHelper.runStateT(popEmptyStack, Collections.emptyList()));
    System.out.println("Popping empty stack was successful: " + emptyPopResult.isPresent()); // false
  }
}

Key Points:

Relationship to State Monad

The State Monad (State<S, A>) can be seen as a specialised version of StateT. Specifically, State<S, A> is equivalent to StateT<S, Id, A>, where Id is the Identity monad (a monad that doesn't add any effects, simply Id<A> = A). higher-kinded-j provides an Id monad. State<S, A> can be seen as an equivalent to StateT<S, IdKind.Witness, A>.

Higher-Kinded Types and Optics for Java