# Effectts

> Idiomatic Effect-TS patterns for TypeScript functional programming. USE THIS SKILL WHEN: Writing new Effect services or layers, debugging Effect type errors (layer composition, error types), choosing between Effect patterns, setting up Effect testing. TRIGGERS ON: 'effect pattern', 'Layer.provide', 'Context.Tag', 'effect service', 'effect error', 'effect test', 'idiomatic effect'. NOT FOR: general TypeScript without Effect.

- Skill: `akarachen/effectts` (Agent Skill, multi-file: 9 files)
- Install (CLI): `npx skillmds@latest add akarachen/effectts`
- Raw SKILL.md: https://api.skillmd.com/api/skills/akarachen/effectts/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: akarachen (https://skillmd.com/u/akarachen)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/akarachen/effectts

---


# 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:

```typescript
// 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)

```bash
# 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` |

```typescript
// 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

```typescript
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

```typescript
// 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      |

```typescript
// 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

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