Effect-TS Anti-Patterns

Overview

Effect-TS provides a powerful functional framework, but common imperative habits can bypass its safety guarantees. This skill identifies and fixes patterns that lead to resource leaks, process crashes, and unhandled errors.

When to Use

• During code reviews of Effect-TS implementations.
• When debugging "unhandled promise rejections" or process crashes in Effect code.
• When optimizing performance of parallel operations.

When NOT to use:

• For simple, non-Effect TypeScript code.
• When using other functional libraries (fp-ts, etc.).

Anti-Patterns Reference

Anti-Pattern	Bad Code	Good Code	Why
Run Outside Boundary	await Effect.runPromise(fx) mid-function	yield* fx (compose)	Bypasses error channel & tracing.
Missing yield*	const user = yield fetchUser()	const user = yield* fetchUser()	Yields the Effect object, not the result.
Throwing Errors	if (!ok) throw new Error()	yield* Effect.fail(new Error())	throw creates a "Defect" (crash), not a typed error.
Unbounded Parallelism	Effect.all(tasks) (default)	Effect.all(tasks, { concurrency: 10 })	Prevents OOM and rate limit exhaustion.
Ignoring Errors	Effect.runPromise(fx) (no catch)	fx.pipe(Effect.catchAll(...))	Leads to unhandled rejections.
Manual Cleanup	try { ... } finally { cleanup() }	Effect.acquireRelease(...)	finally doesn't guarantee cleanup on interruption.

Common Mistakes

1. The yield vs yield* Trap

// ❌ BAD: Missing *, yields the Effect object itself
const bad = Effect.gen(function* () {
  const user = yield fetchUser(id); // Returns Effect, not User!
});

// ✅ GOOD: yield* executes the Effect
const good = Effect.gen(function* () {
  const user = yield* fetchUser(id); // Returns User
});

2. Unbounded Parallelism

// ❌ BAD: 10,000 concurrent requests (OOM risk)
const bad = Effect.all(urls.map(fetch));

// ✅ GOOD: Bounded to 10 concurrent
const good = Effect.all(urls.map(fetch), { concurrency: 10 });

3. Running Effects Mid-Code

// ❌ BAD: Effect.runPromise scattered throughout
const bad = async () => {
  const user = await Effect.runPromise(getUser(id));
  const posts = await Effect.runPromise(getPosts(user.id));
};

// ✅ GOOD: Compose effects, run ONCE at boundary
const good = Effect.gen(function* () {
  const user = yield* getUser(id);
  const posts = yield* getPosts(user.id);
  return { user, posts };
});

Rationalization Table

Excuse	Reality
"I'll use concurrency: 'unbounded' to make it fast"	Fast = Crash. Always bound parallelism for external resources.
"I'll just throw for now"	throw bypasses the error channel and makes code untestable.
"It's easier to run it here"	Running mid-code loses the context, tracing, and interruption safety.

Red Flags

• Effect.runPromise or Effect.runSync inside a loop or helper function.
• Effect.all without a concurrency option on large lists.
• throw keyword inside an Effect.gen block.
• yield without * when calling an Effect.

Cross-References

• REQUIRED BACKGROUND: effect-ts-fundamentals
• REQUIRED SUB-SKILL: effect-ts-concurrency
• REFERENCE: effect-ts-resources