Higher-Kinded-J: Managing Side Effects with IO

In functional programming, managing side effects (like printing to the console, reading files, making network calls, generating random numbers, or getting the current time) while maintaining purity is a common challenge.

The IO<A> monad in higher-kinded-j provides a way to encapsulate these side-effecting computations, making them first-class values that can be composed and manipulated functionally.

The key idea is that an IO<A> value doesn't perform the side effect immediately upon creation. Instead, it represents a description or recipe for a computation that, when executed, will perform the effect and potentially produce a value of type A. The actual execution is deferred until explicitly requested.

Core Components

The IO Type

The HKT Bridge for IO

Typeclasses for IO

The IO functionality is built upon several related components:

1. IO<A>: The core functional interface. An IO<A> instance essentially wraps a Supplier<A> (or similar function) that performs the side effect and returns a value A. The crucial method is unsafeRunSync(), which executes the encapsulated computation.
2. IOKind<A>: The HKT marker interface (Kind<IOKind.Witness, A>) for IO. This allows IO to be treated as a generic type constructor F in type classes like Functor, Applicative, and Monad. The witness type is IOKind.Witness.
3. IOKindHelper: The essential utility class for working with IO in the HKT simulation. It provides:
   • widen(IO<A>): Wraps a concrete IO<A> instance into its HKT representation IOKind<A>.
   • narrow(Kind<IOKind.Witness, A>): Unwraps an IOKind<A> back to the concrete IO<A>. Throws KindUnwrapException if the input Kind is invalid.
   • delay(Supplier<A>): The primary factory method to create an IOKind<A> by wrapping a side-effecting computation described by a Supplier.
   • unsafeRunSync(Kind<IOKind.Witness, A>): The method to execute the computation described by an IOKind. This is typically called at the "end of the world" in your application (e.g., in the main method) to run the composed IO program.
4. IOFunctor: Implements Functor<IOKind.Witness>. Provides the map operation to transform the result value A of an IO computation without executing the effect.
5. IOApplicative: Extends IOFunctor and implements Applicative<IOKind.Witness>. Provides of (to lift a pure value into IO without side effects) and ap (to apply a function within IO to a value within IO).
6. IOMonad: Extends IOApplicative and implements Monad<IOKind.Witness>. Provides flatMap to sequence IO computations, ensuring effects happen in the intended order.

Purpose and Usage

• Encapsulating Side Effects: Describe effects (like printing, reading files, network calls) as IO values without executing them immediately.
• Maintaining Purity: Functions that create or combine IO values remain pure. They don't perform the effects themselves, they just build up a description of the effects to be performed later.
• Composition: Use map and flatMap (via IOMonad) to build complex sequences of side-effecting operations from smaller, reusable IO actions.
• Deferred Execution: Effects are only performed when unsafeRunSync is called on the final, composed IO value. This separates the description of the program from its execution.

Important Note:IO in this library primarily deals with deferring execution. It does not automatically provide sophisticated error handling like Either or Try, nor does it manage asynchronicity like CompletableFuture. Exceptions thrown during unsafeRunSync will typically propagate unless explicitly handled within the Supplier provided to IOKindHelper.delay.

import org.higherkindedj.hkt.Kind;
import org.higherkindedj.hkt.io.*; 
import org.higherkindedj.hkt.unit.Unit;
import java.util.function.Supplier;
import java.util.Scanner;

// Get the IOMonad instance
IOMonad ioMonad = IOMonad.INSTANCE;

// IO action to print a message
Kind<IOKind.Witness, Unit> printHello = IOKindHelper.delay(() -> {
    System.out.println("Hello from IO!");
    return Unit.INSTANCE;
});

// IO action to read a line from the console
Kind<IOKind.Witness, String> readLine = IOKindHelper.delay(() -> {
    System.out.print("Enter your name: ");
    // Scanner should ideally be managed more robustly in real apps
    try (Scanner scanner = new Scanner(System.in)) {
         return scanner.nextLine();
    }
});

// IO action that returns a pure value (no side effect description here)
Kind<IOKind.Witness, Integer> pureValueIO = ioMonad.of(42);

// IO action that simulates getting the current time (a side effect)
Kind<IOKind.Witness, Long> currentTime = IOKindHelper.delay(System::currentTimeMillis);

// Creating an IO action that might fail internally
Kind<IOKind.Witness, String> potentiallyFailingIO = IOKindHelper.delay(() -> {
   if (Math.random() < 0.5) {
       throw new RuntimeException("Simulated failure!");
   }
   return "Success!";
});

Nothing happens when you create these IOKind values. The Supplier inside delay is not executed.

// (Continuing from above examples)

// Execute printHello
System.out.println("Running printHello:");
IOKindHelper.unsafeRunSync(printHello); // Actually prints "Hello from IO!"

// Execute readLine (will block for user input)
// System.out.println("\nRunning readLine:");
// String name = IOKindHelper.unsafeRunSync(readLine);
// System.out.println("User entered: " + name);

// Execute pureValueIO
System.out.println("\nRunning pureValueIO:");
Integer fetchedValue = IOKindHelper.unsafeRunSync(pureValueIO);
System.out.println("Fetched pure value: " + fetchedValue); // Output: 42

// Execute potentiallyFailingIO
System.out.println("\nRunning potentiallyFailingIO:");
try {
String result = IOKindHelper.unsafeRunSync(potentiallyFailingIO);
   System.out.println("Succeeded: " + result);
} catch (RuntimeException e) {
   System.err.println("Caught expected failure: " + e.getMessage());
   }

// Notice that running the same IO action again executes the effect again
   System.out.println("\nRunning printHello again:");
IOKindHelper.unsafeRunSync(printHello); // Prints "Hello from IO!" again

import org.higherkindedj.hkt.io.IOMonad;
import org.higherkindedj.hkt.unit.Unit;
import java.util.function.Function;

IOMonad ioMonad = IOMonad.INSTANCE;

// --- map example ---
Kind<IOKind.Witness, String> readLineAction = IOKindHelper.delay(() -> "Test Input"); // Simulate input

// Map the result of readLineAction without executing readLine yet
Kind<IOKind.Witness, String> greetAction = ioMonad.map(
    name -> "Hello, " + name + "!", // Function to apply to the result
    readLineAction
);

System.out.println("Greet action created, not executed yet.");
// Now execute the mapped action
String greeting = IOKindHelper.unsafeRunSync(greetAction);
System.out.println("Result of map: " + greeting); // Output: Hello, Test Input!

// --- flatMap example ---
// Action 1: Get name
Kind<IOKind.Witness, String> getName = IOKindHelper.delay(() -> {
    System.out.println("Effect: Getting name...");
    return "Alice";
});

// Action 2 (depends on name): Print greeting
Function<String, Kind<IOKind.Witness, Unit>> printGreeting = name ->
    IOKindHelper.delay(() -> {
        System.out.println("Effect: Printing greeting for " + name);
        System.out.println("Welcome, " + name + "!");
        return Unit.INSTANCE;
    });

// Combine using flatMap
Kind<IOKind.Witness, Void> combinedAction = ioMonad.flatMap(printGreeting, getName);

System.out.println("\nCombined action created, not executed yet.");
// Execute the combined action
IOKindHelper.unsafeRunSync(combinedAction);
// Output:
// Effect: Getting name...
// Effect: Printing greeting for Alice
// Welcome, Alice!

// --- Full Program Example ---
Kind<IOKind.Witness, Unit> program = ioMonad.flatMap(
    ignored -> ioMonad.flatMap( // Chain after printing hello
        name -> ioMonad.map( // Map the result of printing the greeting
            ignored2 -> { System.out.println("Program finished");
              return Unit.INSTANCE; },
              printGreeting.apply(name) // Action 3: Print greeting based on name
        ),
        readLine // Action 2: Read line
    ),
    printHello // Action 1: Print Hello
);

System.out.println("\nComplete IO Program defined. Executing...");
// IOKindHelper.unsafeRunSync(program); // Uncomment to run the full program