Effect 4.0 Reference Skill

Comprehensive reference for building applications with Effect 4.0 (TypeScript). Use this skill when writing, reviewing, or refactoring code that uses the effect package.

When to Load Docs

Use the decision tree below to determine which doc files to read. Always start with the most relevant file for the user's task. Read additional files only as needed.

Doc Files

Decision Tree

What is the user doing?
│
├─ Writing/reading core Effect code (Effect.gen, pipe, map, flatMap)
│  └─ Read: docs/01-core-effect.md
│
├─ Defining services, layers, or dependency injection
│  └─ Read: docs/02-services-layers.md
│
├─ Working with Option, Result, collections, or domain types
│  └─ Read: docs/03-data-types.md
│
├─ Concurrency (fibers, refs, queues, semaphores, scheduling)
│  └─ Read: docs/04-concurrency.md
│
├─ Streaming data (Stream, Sink, Channel)
│  └─ Read: docs/05-streaming.md
│
├─ Validation, parsing, or Schema
│  └─ Read: docs/06-schema.md
│
├─ Transactions or STM
│  └─ Read: docs/07-transactions.md
│
├─ Resource management patterns, pattern matching, "which module?"
│  └─ Read: docs/08-patterns.md
│
├─ Migrating from v3, understanding renames
│  └─ Read: docs/09-migration.md
│
├─ Testing, platform setup, ecosystem packages
│  └─ Read: docs/10-ecosystem.md
│
├─ HTTP server/client, API definitions, RPC, SQL, CLI, AI, Cluster, Workflow
│  └─ Read: docs/10-ecosystem.md (unstable modules section)
│
└─ Not sure / general question
   └─ Read: docs/01-core-effect.md, then others as needed

Critical v4 Rules

When generating Effect 4.0 code, always follow these rules:

1. Services use ServiceMap.Service, not Context.Tag or Effect.Tag
2. Ref, Deferred, Fiber are NOT Effect subtypes -- use Ref.get(), Deferred.await(), Fiber.join()
3. Effect.fork is now Effect.forkChild, Effect.forkDaemon is now Effect.forkDetach
4. Effect.catchAll is now Effect.catch, Effect.catchAllCause is now Effect.catchCause
5. Either is now Result -- constructors: Result.succeed(value), Result.fail(error). Fields: .success, .failure. Tags: "Success", "Failure"
6. FiberRef is removed -- use ServiceMap.Reference / References.*
7. Schema is in core effect package -- no @effect/schema import
8. Layer naming convention: use .layer not .Default or .Live
9. Generator this: use Effect.gen({ self: this }, function*() { ... }) not Effect.gen(this, function*() { ... })
10. Cause is flat: cause.reasons array, not a recursive tree
11. Tagged errors with fields: use Schema.TaggedErrorClass<Self>()("Tag", { field: Schema.String }), NOT Data.TaggedError("Tag")<{ field: string }> (the <{}> generic syntax breaks in v4 types). Errors without fields can still use Data.TaggedError("Tag").
12. Schema.Union takes an array: Schema.Union([A, B]) not Schema.Union(A, B)
13. Schema.Literal is single-value: use Schema.Literals(["a", "b"]) for multiple literals, not Schema.Literal("a", "b")
14. Schema.decodeUnknown → Schema.decodeUnknownEffect (the Effect-returning variant was renamed; decodeUnknownSync is unchanged)
15. Schema.Class.make → Schema.Class.makeUnsafe
16. @effect/platform is removed -- import HttpClient, HttpBody, HttpClientRequest, FetchHttpClient from effect/unstable/http
17. LogLevel is a string union ("Info", "Error", etc.), not an object with .label
18. Logger.replace is removed -- use Logger.layer([myLogger]) to provide a custom logger
19. Effect.zipRight → Effect.andThen, Effect.firstSuccessOf → Effect.raceAll, Schedule.upTo → Schedule.compose(Schedule.recurs(n))
20. Brand.make → callable: Schema.brand("X") produces a schema; Brand.nominal<T>() produces a callable constructor (UserId("123") not UserId.make("123"))
21. Config is NOT an Effect subtype -- Config is Yieldable (works with yield*) but CANNOT be passed to Effect.orDie, Effect.map, etc. Wrap in Effect.gen first.
22. class extends Schema.Struct({...}) breaks -- this pattern (distinct from Schema.Class) is invalid in v4. Convert to const MySchema = Schema.Struct({...}) with a separate type MySchema = Schema.Type<typeof MySchema>.
23. Effect.tryPromise(async () => v) bare overload removed -- must use Effect.tryPromise({ try: ..., catch: ... }) object form.
24. Effect.tap callback must return Effect -- Effect.tap(voidFn) breaks; wrap as Effect.tap((x) => Effect.sync(() => voidFn(x))).
25. Schema.Object is removed -- use Schema.Unknown instead.
26. Schema.transform → Schema.decodeTo -- decode/encode must be wrapped in SchemaGetter.transform(fn).
27. Effect.orDieWith(f) is removed -- use Effect.mapError(f) then Effect.orDie chained.
28. Namespace flattening: Effect.Effect.Error<T> → Effect.Error<T>, Effect.Effect.Success<T> → Effect.Success<T>, Layer.Layer.Success<T> removed, ManagedRuntime.ManagedRuntime.Context<T> removed.
29. Span.parent is no longer yieldable -- it's a plain property that may be undefined. Guard with if (span.parent) before access.

Migration Workflow

When migrating a codebase from v3 to v4, always read docs/09-migration.md first. It contains a phased checklist. The recommended order is:

1. Packages first (update versions, remove @effect/platform and @effect/schema)
2. Services second (these are the foundation; everything else depends on them)
3. Error classes third (Data.TaggedError → Schema.TaggedErrorClass)
4. API renames fourth (bulk find-and-replace for catchAll, decodeUnknown, Union, etc.)
5. Logger/runtime last (these are leaf concerns)
6. Verify: typecheck → tests → bundle size