Effect

Effect

Use this skill for Effect v4 RC (effect@rc, currently 4.0.0-rc.x). npm latest is still Effect v3. Do not mix v3 and v4 packages.

Workflow

1. Inspect the local Effect surface: effect / @effect/* versions, import style (from "effect" vs effect/Effect), runtime (CLI, HTTP, worker, browser, test), and whether layers/config/platform are already in play.
2. Prefer a local checkout of Effect-TS/effect main under .temp/effect (v4 RC source). Effect-TS/effect-smol is archived; v4 history now lives in Effect-TS/effect. Browse sources, LLMS.md, MIGRATION.md, migration/*, packages/effect/SCHEMA.md, and ai-docs/. Details: source-map.md.
3. Refresh docs when the installed RC differs from the snapshot or the task asks for latest APIs. Resolve library docs, then check installed .d.ts.
4. Route by concern:
   • Install, Effect.fn/gen, errors, forks, run, Config → setup-core.md
   • Services, layers, scopes, ManagedRuntime, Layer.launch → services-layers-runtime.md
   • Full stable module catalog (what each module is for) → modules-stable.md
   • Stream, Queue/PubSub, fibers, STM-like Tx*, caching, requests → concurrency.md
   • Schema v4 → schema-v4.md
   • effect/unstable/* → modules-unstable.md
   • @effect/platform-*, sql-*, ai-*, atom-*, OTel, tools → ecosystem.md
   • @effect/vitest@rc → vitest-testing.md
   • v3 → v4 → migration.md
5. Implement in project style: match the installed RC, keep framework edges thin, compose layers explicitly, treat effect/unstable/* as RC-plus-unstable (may break in minor releases).

Coding style (canonical)

From Effect’s own LLMS.md / ai-docs:

• Named effectful functions use Effect.fn("name") (span + stack). Library internals in Effect’s own repo prefer Effect.fnUntraced. Do not wrap Effect.gen in a plain function; do not .pipe the result of Effect.fn.
• Domain errors: Schema.TaggedError. Always return yield* when failing so TypeScript narrows.
• Services: Context.Service, identifier like "myapp/db/Database", implement with Database.of({ ... }), attach static readonly layer. Prefer yield* Service over Service.use except one-liners.
• Untrusted data: Schema, not ad-hoc predicates. Use Predicate for isString / isObject / composition — do not invent those helpers.
• Dates/time in Effect programs: DateTime + Clock, not raw Date.now().
• Observability: logs/traces/metrics in core; new projects prefer effect/unstable/observability OTLP. Use @effect/opentelemetry when an existing OTel SDK is required.
• Process entry: NodeRuntime.runMain / BunRuntime.runMain (BunRuntime is the shared Node runner) or DenoRuntime / BrowserRuntime, or Layer.launch. Prefer those over Runtime.makeRunMain. Cloudflare: no platform package — Fetch + SQL D1/DO drivers.
• Prefer Stream/Sink over Channel; Schema over SchemaAST; Cache vs ScopedCache vs Pool vs RcMap per concurrency.md.

Judgment

• Align every @effect/* package to the same 4.0.0-rc.N as effect.
• TypeScript 5.9+ (7 recommended for Effect’s TS tooling). strict: true. Node 18+ generally; some SQL drivers need Node 22.16+.
• No v3 APIs in v4 code: Context.Tag / GenericTag, Effect.Tag / Effect.Service, Either, FiberRef, Effect.catchAll, Effect.fork (use forkChild), Effect.async (use callback).
• Layers memoize across Effect.provide unless { local: true } or Layer.fresh.
• Runtime<R> is gone. Run with Effect.run* / run*With(context) or ManagedRuntime.
• Ref / Deferred / Fiber are not yieldable Effects — use Ref.get, Deferred.await, Fiber.join.
• Prefer typed failures over throw/die. Defects are programmer errors.
• Keep Schema Encoded vs Type visible at boundaries.
• Tests: @effect/vitest@rc + vitest@^4.1.0. Prefer it.effect (already scoped + TestClock; do not wrap in Effect.scoped). Import TestClock from effect/testing. it.effect suppresses logs; it.live does not.
• Do not: catchAll/fork/forkDaemon/Context.Tag/Effect.Service/.Default/Either/FiberRef/Date.now/try/catch in gen/Schema.Date for ISO strings/Union(A,B)/it.effect + extra Effect.scoped.

Module routing (quick)

Every stable module: modules-stable.md. Every unstable area: modules-unstable.md.

Verification

• Typecheck Effect<A, E, R>, layers, Schema encoded/type sides, RC names.
• @effect/vitest@rc: it.effect, TestClock, Exit/Result, per-test Effect.provide when isolation matters.
• Schema: valid/invalid input, defaults, excess keys, classes, tagged errors, JSON Schema.
• Migration scan: v3 imports, Either, FiberRef, old catch/fork/runtime/Schema APIs.