Effect as a Portable Promise Utility

Use Effect as a lightweight utility for running promises robustly. Every effect is self-contained (no services, no layers, no dependency injection) and resolves to a plain Promise at the boundary via Effect.runPromise.

The shape is always: Effect.fn or Effect.gen -> pipe operators -> Effect.runPromise.

Quick Reference

Capability	API	Avoid
Wrap promises	Effect.tryPromise	Effect.promise (swallows errors)
Define functions	Effect.fn("name")	Anonymous generators
Errors	Data.TaggedError with _tag	Plain Error or untagged objects
Catch by tag	catchTag / catchTags	catchAll (loses type narrowing)
Fallbacks	orElse, orElseSucceed	Nested try/catch
Timeouts	Effect.timeout / Effect.timeoutFail	Manual AbortController + setTimeout
Retries	Effect.retry with Schedule	Manual retry loops
Caching	Effect.cachedWithTTL / Effect.cachedFunction	Manual Map-based caches
Concurrency	Effect.all with { concurrency: N }	Manual Promise.all chunking
Pattern matching	Match.value / Match.type with Match.tag	Switch statements on _tag
Tracing	Effect.withSpan / Effect.annotateCurrentSpan	Manual console.time
Run at boundary	Effect.runPromise	Effect.runSync for async work

Core Pattern: Portable Effect Functions

Every effect function follows this structure - build an Effect<Success, Error, never> (no requirements), then run it as a promise at the call site.

import { Effect, Data } from "effect";

class FetchError extends Data.TaggedError("FetchError")<{
  url: string;
  status: number;
  message: string;
}> {}

const fetchUser = Effect.fn("fetchUser")(function* (userId: string) {
  const response = yield* Effect.tryPromise({
    try: () => fetch(`/api/users/${userId}`),
    catch: () =>
      new FetchError({
        url: `/api/users/${userId}`,
        status: 0,
        message: "Network error",
      }),
  });

  if (!response.ok) {
    return yield* Effect.fail(
      new FetchError({
        url: `/api/users/${userId}`,
        status: response.status,
        message: response.statusText,
      }),
    );
  }

  const user = yield* Effect.tryPromise({
    try: () => response.json() as Promise<User>,
    catch: () =>
      new FetchError({
        url: `/api/users/${userId}`,
        status: response.status,
        message: "Invalid JSON",
      }),
  });

  return user;
});

// At the call site - always resolves to a plain Promise
const user: User = await Effect.runPromise(fetchUser("123"));

Tagged Errors

Define errors with Data.TaggedError. The _tag field enables type-safe error matching without services or schemas.

import { Data } from "effect";

class TimeoutError extends Data.TaggedError("TimeoutError")<{
  operation: string;
  durationMs: number;
}> {}

class NotFoundError extends Data.TaggedError("NotFoundError")<{
  resource: string;
  id: string;
}> {}

class ValidationError extends Data.TaggedError("ValidationError")<{
  field: string;
  message: string;
}> {}

Catching Errors by Tag

Use catchTag for single tags, catchTags for multiple. Both preserve type narrowing.

const result =
  yield *
  fetchUser("123").pipe(
    Effect.catchTag("NotFoundError", (error) =>
      Effect.succeed({ id: error.id, name: "Unknown", fallback: true }),
    ),
    Effect.catchTag("TimeoutError", (error) =>
      Effect.fail(
        new ServiceUnavailableError({
          message: `${error.operation} timed out`,
        }),
      ),
    ),
  );

// Or handle multiple tags at once
const result =
  yield *
  fetchUser("123").pipe(
    Effect.catchTags({
      NotFoundError: (error) => Effect.succeed(defaultUser),
      ValidationError: (error) => Effect.fail(new BadRequestError({ message: error.message })),
    }),
  );

Timeouts

Basic Timeout (raises TimeoutException)

const result = yield * fetchUser("123").pipe(Effect.timeout("5 seconds"));

Timeout with Custom Error

const result =
  yield *
  fetchUser("123").pipe(
    Effect.timeoutFail({
      duration: "5 seconds",
      onTimeout: () => new TimeoutError({ operation: "fetchUser", durationMs: 5000 }),
    }),
  );

Timeout with Fallback Value

const result =
  yield *
  fetchUser("123").pipe(
    Effect.timeoutTo({
      duration: "5 seconds",
      onSuccess: (user) => user,
      onTimeout: () => defaultUser,
    }),
  );

Retries

Fixed Retry Count

import { Schedule } from "effect";

const result = yield * fetchUser("123").pipe(Effect.retry({ times: 3 }));

Exponential Backoff

const result =
  yield *
  fetchUser("123").pipe(
    Effect.retry(Schedule.exponential("100 millis").pipe(Schedule.compose(Schedule.recurs(5)))),
  );

Retry Only Specific Errors

const result =
  yield *
  fetchUser("123").pipe(
    Effect.retry({
      times: 3,
      while: (error) => error._tag === "TimeoutError",
    }),
  );

Retry with Fallback on Exhaustion

const result =
  yield *
  Effect.retryOrElse(fetchUser("123"), { times: 3 }, (error, fiberId) =>
    Effect.succeed(defaultUser),
  );

Combining Timeout + Retry

const robustFetch = Effect.fn("robustFetch")(function* (userId: string) {
  const user = yield* fetchUser(userId).pipe(
    Effect.timeoutFail({
      duration: "3 seconds",
      onTimeout: () => new TimeoutError({ operation: "fetchUser", durationMs: 3000 }),
    }),
    Effect.retry(Schedule.exponential("200 millis").pipe(Schedule.compose(Schedule.recurs(3)))),
  );
  return user;
});

Fallbacks

// Try primary, fall back to secondary on any failure
const result = yield * fetchFromPrimary(id).pipe(Effect.orElse(() => fetchFromSecondary(id)));

// Fall back to a default value
const result = yield * fetchUser(id).pipe(Effect.orElseSucceed(() => defaultUser));

// Remap the error type on failure
const result =
  yield *
  fetchUser(id).pipe(
    Effect.orElseFail(() => new ServiceUnavailableError({ message: "All sources failed" })),
  );

// Try multiple sources, use the first that succeeds
const result =
  yield * Effect.firstSuccessOf([fetchFromCache(id), fetchFromPrimary(id), fetchFromSecondary(id)]);

Caching

Cache an Effect with TTL

import { Effect } from "effect";

const cachedConfig = Effect.cachedWithTTL(
  Effect.tryPromise(() => fetch("/api/config").then((r) => r.json())),
  "5 minutes",
);

// Use it - first call fetches, subsequent calls return cached value within TTL
const program = Effect.gen(function* () {
  const getConfig = yield* cachedConfig;
  const config1 = yield* getConfig;
  const config2 = yield* getConfig; // same value, no re-fetch
});

Cache with Manual Invalidation

const [getConfig, invalidate] = yield * Effect.cachedInvalidateWithTTL(fetchConfig, "10 minutes");

const config = yield * getConfig;
yield * invalidate; // force re-fetch on next call

Memoize a Function by Arguments

const memoizedFetchUser =
  yield *
  Effect.cachedFunction((userId: string) =>
    Effect.tryPromise(() => fetch(`/api/users/${userId}`).then((r) => r.json())),
  );

const user1 = yield * memoizedFetchUser("123"); // fetches
const user2 = yield * memoizedFetchUser("123"); // returns cached
const user3 = yield * memoizedFetchUser("456"); // fetches (different key)

Concurrency

Parallel Execution

// Run all effects in parallel (unbounded)
const [users, posts, comments] =
  yield *
  Effect.all([fetchUsers, fetchPosts, fetchComments], {
    concurrency: "unbounded",
  });

// Bounded concurrency (e.g., max 5 at a time)
const results = yield * Effect.all(tasks, { concurrency: 5 });

// Parallel forEach
const enrichedUsers = yield * Effect.forEach(userIds, (id) => fetchUser(id), { concurrency: 10 });

Racing (First to Succeed)

// Race two effects, take the first to complete
const result = yield * Effect.race(fetchFromEast, fetchFromWest);

// Race many effects
const result = yield * Effect.raceAll([fetchFromCache(id), fetchFromDb(id), fetchFromRemote(id)]);

Pattern Matching

Use Match for exhaustive, type-safe branching on tagged unions and error types.

Matching Tagged Errors

import { Match } from "effect";

type ApiError = NotFoundError | TimeoutError | ValidationError;

const describeError = (error: ApiError) =>
  Match.value(error).pipe(
    Match.tag("NotFoundError", (e) => `${e.resource} ${e.id} not found`),
    Match.tag("TimeoutError", (e) => `${e.operation} timed out after ${e.durationMs}ms`),
    Match.tag("ValidationError", (e) => `Invalid ${e.field}: ${e.message}`),
    Match.exhaustive,
  );

Matching by Type (Reusable Matcher)

const handleResult = Match.type<string | number | boolean>().pipe(
  Match.when(Match.string, (s) => `string: ${s}`),
  Match.when(Match.number, (n) => `number: ${n}`),
  Match.when(Match.boolean, (b) => `boolean: ${b}`),
  Match.exhaustive,
);

handleResult("hello"); // "string: hello"
handleResult(42); // "number: 42"

Matching with Predicates

const categorize = Match.type<{ status: number }>().pipe(
  Match.when({ status: (s) => s >= 500 }, () => "server_error"),
  Match.when({ status: (s) => s >= 400 }, () => "client_error"),
  Match.when({ status: (s) => s >= 200 }, () => "success"),
  Match.orElse(() => "unknown"),
);

Tracing

Adding Spans

Effect.fn automatically creates a span with the given name. For additional spans or annotations:

const fetchAndProcess = Effect.fn("fetchAndProcess")(function* (userId: string) {
  yield* Effect.annotateCurrentSpan("userId", userId);

  const user = yield* fetchUser(userId).pipe(Effect.withSpan("fetchUser"));

  const processed = yield* processUser(user).pipe(Effect.withSpan("processUser"));

  yield* Effect.annotateCurrentSpan("processed", true);
  return processed;
});

Logging within Effects

const program = Effect.fn("program")(function* () {
  yield* Effect.log("Starting operation");
  const result = yield* doWork();
  yield* Effect.log("Operation complete", { resultId: result.id });
  return result;
});

Complete Example: Robust API Call

Combining all patterns into a single portable function:

import { Effect, Data, Schedule, Match } from "effect";

class ApiError extends Data.TaggedError("ApiError")<{
  url: string;
  status: number;
  message: string;
}> {}

class ApiTimeoutError extends Data.TaggedError("ApiTimeoutError")<{
  url: string;
  durationMs: number;
}> {}

const fetchApi = Effect.fn("fetchApi")(function* <T>(url: string) {
  yield* Effect.annotateCurrentSpan("url", url);
  yield* Effect.log("Fetching", { url });

  const response = yield* Effect.tryPromise({
    try: () => fetch(url),
    catch: () => new ApiError({ url, status: 0, message: "Network error" }),
  }).pipe(
    Effect.timeoutFail({
      duration: "10 seconds",
      onTimeout: () => new ApiTimeoutError({ url, durationMs: 10_000 }),
    }),
    Effect.retry(Schedule.exponential("500 millis").pipe(Schedule.compose(Schedule.recurs(3)))),
  );

  if (!response.ok) {
    return yield* Effect.fail(
      new ApiError({
        url,
        status: response.status,
        message: response.statusText,
      }),
    );
  }

  return yield* Effect.tryPromise({
    try: () => response.json() as Promise<T>,
    catch: () => new ApiError({ url, status: response.status, message: "Invalid JSON" }),
  });
});

// Usage at the boundary - resolves to a plain Promise
const data = await Effect.runPromise(
  fetchApi<User>("/api/users/123").pipe(
    Effect.catchTag("ApiTimeoutError", () =>
      Effect.succeed({ id: "123", name: "Unknown" } as User),
    ),
  ),
);

Rules

Rule	Guidance
No services or layers	Keep effects self-contained with R = never. Use services only when the effect-best-practices skill applies.
Always Effect.fn	Name every function for automatic tracing spans.
Always Data.TaggedError	Every distinct failure gets its own tagged error class.
Always Effect.tryPromise	Never use Effect.promise - it swallows errors as defects.
Prefer catchTag	Use catchTag / catchTags over catchAll to preserve type narrowing.
Timeout everything	External calls should always have Effect.timeout or Effect.timeoutFail.
Run at the boundary	Call Effect.runPromise at the outermost call site, not inside effect functions.

References

• Creating Effects
• Expected Errors
• Fallback
• Retrying
• Timing Out
• Pattern Matching
• Basic Concurrency
• Caching Effects
• Tracing