Implementing Algebraic Effects Handlers in TypeScript

Algebraic effects and handlers offer a powerful way to manage side effects and control flow in a functional programming style. They allow you to decouple the description of an effect from its execution, enabling flexible and composable side-effect management. This challenge focuses on building a foundational system for algebraic effects in TypeScript.

Problem Description

Your task is to implement a system for algebraic effects and their handlers in TypeScript. This involves defining a mechanism to declare effects and then providing a way to "handle" these effects, allowing different parts of your program to decide how those effects should be resolved.

Key Requirements:

1. Effect Declaration: You need a way to declare an "effect" as a specific type of operation that can be requested. This declaration should include the name of the effect and the type of its arguments and return value.
2. Effect Invocation (Prompting): Create a function or mechanism to "prompt" an effect. When an effect is prompted, it should suspend the current computation and delegate control to a handler. The handler will then decide how to resolve the effect.
3. Handler Definition: Implement a way to define handlers. A handler is a function that can intercept specific effects and provide a resolution for them. Handlers should be able to "resume" the suspended computation with a returned value.
4. Effect Scope: Implement a mechanism to establish a scope within which a handler is active. Any effect prompted within this scope will be routed to the active handler.
5. Composability: The system should support nesting handlers, allowing for more granular control over effect resolution.

Expected Behavior:

When an effect is prompted within a handler's scope:

• The effect is passed to the handler.
• The handler executes its logic.
• The handler can choose to resolve the effect by returning a value. This value is then passed back to the point where the effect was prompted, allowing the suspended computation to continue.
• The handler can also choose to re-throw the effect (if it doesn't know how to handle it) to be caught by an outer handler, or it can transform the effect.

Examples

Example 1: Simple Logging Effect

// Assume you have a system to define and handle effects
// Let's call it 'effectful' for now

// Effect Declaration
const log = effectful.declare<string, void>('log');

// Handler Definition
const consoleLogger = effectful.handler({
  [log.name]: (message: string) => {
    console.log(`[LOG]: ${message}`);
    return effectful.resume(); // Continue computation after logging
  },
  // other effects can be defined here
});

// Usage
const program = effectful.run(
  consoleLogger,
  () => {
    console.log('Starting...');
    log('User logged in');
    console.log('Finishing...');
  }
);

// Expected Output to Console:
// Starting...
// [LOG]: User logged in
// Finishing...

Example 2: Counter Effect with State

// Effect Declaration
const increment = effectful.declare<void, number>('increment');

// Handler Definition
const counterHandler = effectful.handler({
  [increment.name]: (next: () => number) => {
    // Here 'next' represents resuming the interrupted computation.
    // In a simple counter, we might not need to call 'next' directly,
    // but for more complex effects, it's crucial.
    // For this example, let's assume we just want to return a count.

    // A more accurate simulation would involve state management within the handler's closure.
    // Let's illustrate a simplified approach where the handler manages its own state.
    let count = 0;
    return () => {
      count++;
      return count; // Return the current count
    };
  },
});

// Usage
const countProgram = () => {
  const c1 = increment();
  const c2 = increment();
  return [c1, c2];
};

const result = effectful.run(counterHandler, countProgram);

// Expected Output:
// result = [1, 2]

Example 3: Nested Handlers and Interruption

// Effect Declarations
const readLine = effectful.declare<void, string>('readLine');
const writeLine = effectful.declare<string, void>('writeLine');

// Handlers
const inputHandler = effectful.handler({
  [readLine.name]: () => {
    // Simulate reading input
    return "User input simulation";
  },
});

const outputHandler = effectful.handler({
  [writeLine.name]: (message: string) => {
    console.log(`[OUTPUT]: ${message}`);
    return effectful.resume();
  },
});

// Program with nested effects
const interactiveProgram = () => {
  writeLine('Please enter your name:');
  const name = readLine();
  writeLine(`Hello, ${name}!`);
};

// Running with nested handlers
const programWithNesting = effectful.run(
  effectful.compose(inputHandler, outputHandler), // Input handled first, then output
  interactiveProgram
);

// Expected Output to Console:
// [OUTPUT]: Please enter your name:
// [OUTPUT]: Hello, User input simulation!

Constraints

• The implementation should be in TypeScript, leveraging its type system for effect signatures.
• The system should be performant enough for typical application use cases. Avoid excessive overhead.
• The solution should not rely on external libraries for core effect handling logic (e.g., libraries specifically designed for effect systems). You may use general utility libraries if absolutely necessary, but the core effect mechanism must be your own.
• The provided examples should be runnable with your implementation.

Notes

• Think about how to represent suspended computations. Continuations or similar concepts are often involved.
• Consider how to manage state within handlers, especially for effects that require accumulating data or maintaining context.
• The effectful.resume() function (or its equivalent in your design) is crucial for passing control back to the suspended computation.
• TypeScript's control flow analysis might be a challenge. You'll need to ensure type safety and that the compiler understands the flow of control when effects are handled.
• Consider using generics extensively to define effects with arbitrary argument and return types.
• The effectful.declare function is a conceptual placeholder. You'll need to design its actual implementation. Similarly for effectful.handler and effectful.run.
• The concept of "algebraic effects" can be implemented in various ways. Common approaches involve monads, continuations, or trampolines. You are encouraged to explore a clean and understandable implementation.