Effect.ts: Absence as First-Class

Core Concepts • Lesson 5 of 51

5. Generator Syntax - Imperative Style

yield* makes async code look synchronous, but it's still describing absence

Code Example

const program = Effect.gen(function* () {
  // "Get" these values (they don't exist yet)
  const a = yield* Effect.succeed(5);
  const b = yield* Effect.succeed(10);

  // Transform them
  const sum = a + b;

  // Return result
  return `Sum: ${sum}`;
});

// Still just a description!
// Nothing has run yet.

await Effect.runPromise(program);
// NOW it runs: "Sum: 15"

Interactive Example

const program = Effect.gen(function* () {
      const a = yield* Effect.succeed(5);
      const b = yield* Effect.succeed(10);
      return "Sum: " + (a + b);
});
const result = await Effect.runPromise(program);
return 'Generator syntax result: ' + result + '\n\n- a = 5 (described)\n- b = 10 (described)\n- sum computed imperatively\n- Nothing executed until runPromise!';

Explanation

Generator syntax: describing unavailability imperatively.

yield* looks like it "gets" values, but it doesn't! It DESCRIBES getting unavailable values.

No assumptions made:

'a' doesn't exist yet (we describe getting it)
'b' doesn't exist yet (we describe getting it)
'sum' doesn't exist yet (we describe computing it)

The entire function is a DESCRIPTION of working with unavailable data. Nothing executes until runPromise. We're not making assumptions - we're typing unavailability in an imperative style.

Part 5 of 51 in the Effect.ts Absence Modeling series

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