Effect TS

Effect-TS Skill

Use this skill to add or modify Effect-TS code. It provides quick patterns for finding Effect usage and for composing, testing, and structuring Effect-based modules.

Find Effect usage fast

• Effect imports or type references.
• Effect.gen for generator-based composition.
• Layer for dependency wiring.
• pipe for functional composition.
• Context.Tag or Context.GenericTag for services.
• Schedule, Stream, Fiber, Ref, Queue for concurrency/streaming.

Snippets

Basic Effect

import { Effect } from "effect";

const program = Effect.succeed("ok");

Generator composition

import { Effect } from "effect";

const program = Effect.gen(function* () {
  const value = yield* Effect.succeed(42);
  return value + 1;
});

Error handling

import { Effect } from "effect";

const program = Effect.fail(new Error("boom"));
const recovered = Effect.catchAll(program, () => Effect.succeed("fallback"));

Resource acquisition

import { Effect } from "effect";

const acquire = Effect.sync(() => ({ close: () => void 0 }));
const use = (resource: { close: () => void }) => Effect.succeed(resource);

const program = Effect.acquireUseRelease(
  acquire,
  use,
  (resource) => Effect.sync(() => resource.close()),
);

Layer and service

import { Context, Effect, Layer } from "effect";

class Logger extends Context.Tag("Logger")<Logger, { log: (msg: string) => Effect.Effect<void> }>() {}

const LoggerLive = Layer.succeed(Logger, {
  log: (msg) => Effect.sync(() => console.log(msg)),
});

const program = Effect.gen(function* () {
  const logger = yield* Logger;
  yield* logger.log("hi");
});

Templates

Example with Contexts

We use Context tags to yield dependencies in our programs

import { Context, Effect, Layer } from "effect";

class Config extends Context.Tag("Config")<Config, { apiUrl: string }>() {}
class Api extends Context.Tag("Api")<Api, { 
  fetchStatus: () => Effect.Effect<string, Error> 
}>() {}

const ConfigLive = Layer.succeed(Config, { apiUrl: "https://api.example.com" });

const ApiLive = Layer.effect(Api, Effect.gen(function* () {
  const config = yield* Config;
  return {
    fetchStatus: () =>
      Effect.tryPromise({
        try: async () => {
          const res = await fetch(`${config.apiUrl}/status`);
          if (!res.ok) throw new Error("Bad status");
          return res.text();
        },
        catch: (cause) => cause as Error,
      }),
  };
}));

const program = Effect.gen(function* () {
  const api = yield* Api;
  return yield* api.fetchStatus();
});

Effect.runPromise(
  program.pipe(
    Effect.provide(ApiLive),
    Effect.provide(ConfigLive),
  ),
);

Always use Effect.fn with Generator Functions

When converting a function to use Effect.fn and the generator function doesn't have any yield statements, add yield* Effect.void with a TODO comment:

export const myFunction = Effect.fn('myFunction')(
  function* (props: {...}): Generator<any, ReturnType, any> {
    // TODO: yield something or remove function*
    yield* Effect.void;
    // ... rest of function
  }
);

Always yield* in Effect.fn

If there is no OTHER yield* in the generator function scope, then use yield* Effect.void

ManagedRuntime

See: https://effect.website/docs/runtime/