Idiomatic Effect-TS

Write Effect code that looks like it came from the Effect core team.

Prerequisites (Optional)

Effect source (recommended): git clone https://github.com/Effect-TS/effect for pattern verification
Effect MCP: https://github.com/tim-smart/effect-mcp for API lookups (fallback: https://effect.website/docs)

Research First (ALWAYS)

1. Effect Docs MCP (preferred for concepts)

If you have effect-mcp configured:

// Search documentation
mcp__effect-docs__effect_docs_search({ query: "Layer composition" })

// Then read specific doc
mcp__effect-docs__get_effect_doc({ documentId: 123 })

Otherwise, check https://effect.website/docs for API reference.

2. Effect Source (preferred for real patterns)

# Clone Effect source for pattern verification (if not already done)
# git clone https://github.com/Effect-TS/effect <effect-repo>

# Service patterns (Context.Tag for libraries)
rg "class .* extends Context\.Tag" <effect-repo>/packages/workflow/src
rg "class .* extends Context\.Tag" <effect-repo>/packages/cluster/src

# Effect.Service for concrete impls/tests
rg "Effect\.Service" <effect-repo>/packages/cluster/src

# Error patterns
rg "Schema\.TaggedError" <effect-repo>/packages/cluster/src

# Layer composition
rg "Layer.provideMerge" <effect-repo>/packages/platform/src

Use parallel searches when patterns are unclear.

Quick Reference

Service Patterns (Choose Based on Use Case)

Pattern	Use When	Layer Access
`Context.Tag`	Library interfaces, multiple impls	`ServiceName.Live`
`Effect.Service`	Concrete impls, tests, apps	`ServiceName.Default`

// Library interface (Context.Tag) - canonical for libraries
export class MyService extends Context.Tag("app/MyService")<MyService, {
  readonly doThing: (x: string) => Effect.Effect<Result, MyError>
}>() {
  static readonly Live: Layer.Layer<MyService, never, Deps> = Layer.effect(...)
}

// Concrete impl (Effect.Service) - convenient for apps/tests
export class AppCache extends Effect.Service<AppCache>()("app/Cache", {
  effect: Effect.gen(function* () {
    return { get: (k) => ..., set: (k, v) => ... }
  }),
  dependencies: [SomeDep.Default]  // auto-composed
}) {}

Domain Errors

export class MyError extends Schema.TaggedError<MyError>()("MyError", {
  reason: Schema.Literal("NotFound", "Invalid"),
  cause: Schema.optional(Schema.Defect)
}) {
  static is(u: unknown): u is MyError {
    return hasProperty(u, "_tag") && isTagged(u, "MyError")
  }
}

Domain Types

// Value objects
export class Address extends Schema.Class<Address>("Address")({
  host: Schema.String,
  port: Schema.Number
}) {}

// Branded IDs — always with REAL constraints
export const UserId = Schema.NonEmptyString.pipe(
  Schema.pattern(/^usr_[a-z0-9]+$/),
  Schema.brand("UserId")
)
export type UserId = typeof UserId.Type

Layer Composition — COMMON GOTCHA

This causes most Effect type errors. Know the difference:

Method	Deps Satisfied	Available to Program	Use When
`Layer.provide`	Yes	No	Internal layer building
`Layer.provideMerge`	Yes	Yes	Tests using multiple services
`Layer.mergeAll`	No	Yes	Combining independent layers

// WRONG - test can't access FileSystem
const TestLive = MyService.Live.pipe(Layer.provide(PlatformLive))
// yield* FileSystem.FileSystem -> ERROR

// RIGHT - both services available
const TestLive = MyService.Live.pipe(Layer.provideMerge(PlatformLive))
// yield* MyService -> works
// yield* FileSystem.FileSystem -> also works!

Error pattern to recognize:

Effect<A, E, SomeService> is not assignable to Effect<A, E, never>

or diagnostic: Missing 'SomeService' in the expected Effect context

-> Means SomeService still required. Use provideMerge instead of provide.

Test Pattern

it.effect("works", () =>
  Effect.gen(function* () {
    const svc = yield* MyService
    expect(yield* svc.doThing("x")).toBe(expected)
  }).pipe(Effect.provide(TestLive))  // <- MUST provide at boundary
)

References (Read When Needed)

Topic	File	When to Read
Service & Layer patterns	references/services.md	Creating services, Context.Tag vs Effect.Service, layer composition
Layer dependencies	references/layer-dependencies.md	How services access deps (yield + closure), why NOT factory functions
Schema decision matrix	references/schema-decision.md	Schema.Class vs Struct vs TaggedClass, branded types, migration patterns
Error handling	references/errors.md	Schema.TaggedError, TypeId, refail patterns
Domain types	references/domain-types.md	Schema.Class, branded types, TaggedRequest
Testing	references/testing.md	@effect/vitest setup, TDD workflow, test helpers
Collections & State	references/data.md	Chunk, Option, Ref patterns
Process management	references/processes.md	Scope, Command, background processes

Anti-Patterns

Anti-Pattern	Fix
`Data.TaggedError` for domain errors	Use `Schema.TaggedError`
`Layer.mergeAll` with dependent layers	Use `Layer.provideMerge`
`try/catch` in Effect.gen	Use `Effect.try` or `mapError`
Missing `Effect.provide(...)` in tests	Always provide at test boundary
Plain interfaces for domain types	Use `Schema.Struct` (or `Schema.Class` if needs behavior)
String IDs without branding	Use `Schema.brand()`
Guessing patterns	Grep Effect source first
`Effect.Service` for library interfaces	Use `Context.Tag`
Factory function with service param	Yield deps in Effect.gen, close over (see layer-dependencies.md)
`Schema.Class` for simple DTOs	Use `Schema.Struct` unless needs Equal/Hash
String literal union in struct	Use `Schema.TaggedClass` for variants
Phantom type `& { _tag }`	Use `Schema.brand()` for runtime validation
Bare `Schema.String.pipe(Schema.brand(...))`	Add real constraints: NonEmptyString, pattern(), etc.

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