# Effect

> Effect v4 implementation and code-style guidance. Use for Effect workflows, composition and control flow, Schema models, services and Layers, Config, Schedule or retry, Cache, Stream, unstable HTTP, reactivity or Atom, and Effect tests. Resolve the project-pinned version before applying a branch.

- Skill: `endalk200/effect` (Agent Skill, multi-file: 13 files)
- Install (CLI): `npx skillmds@latest add endalk200/effect`
- Raw SKILL.md: https://api.skillmd.com/api/skills/endalk200/effect/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: endalk200 (https://skillmd.com/u/endalk200)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/endalk200/effect

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# Effect

Work source-verified: project conventions and the project-pinned Effect source take precedence over this reference.

## Version Gate

Complete these in order:

1. Read the nearest `AGENTS.md` and any project-local Effect practices document; finish with the applicable conventions identified.
2. Resolve the installed `effect` version and inspect its exported source; finish with the definitions for every API the task will touch located.
3. Compare that version with `references/UPSTREAM.md`; finish with one authoritative snapshot selected, using `source-context` to resolve mismatches or missing source.
4. Read every matching branch below. Editing begins after all applicable references are loaded.

## Branch Chooser

- Application entrypoints, dual APIs, pipelines, nested composition, branching, pattern matching, loops, traversal, or combining effects: read `references/CODE_STYLE.md`.
- Data models, schemas, brands, variants, optional keys, or decoders: read `references/SCHEMA.md`.
- Services, layers, runtime wiring, errors, `Effect.fn`, or test services: read `references/SERVICES_LAYERS.md`.
- Runtime config, env variables, `ConfigProvider`, or `layerConfig`: read `references/CONFIG.md`.
- Retry, repeat, polling, backoff, jitter, rate-limit-aware policies, or pass loops: read `references/SCHEDULING.md`.
- Memoization, per-key TTL caches, deduplicating concurrent lookups, or request batching: read `references/CACHING.md`.
- Streams, event sources, async iterables, queues/pubsubs, pagination, backpressure, or stream consumers: read `references/STREAMS.md`.
- `effect/unstable/reactivity`, `@effect/atom-*`, `AsyncResult`, API-backed UI, frontend queries or mutations, atom runtimes, reactivity keys, optimistic updates, or stream-backed UI state: read `references/EFFECT_ATOM.md`.
- Outgoing HTTP calls, Effect HttpClient, status handling, or HTTP rate limiting: read `references/HTTP_CLIENTS.md`.
- Effect tests, time, sleeps, concurrency synchronization, or fakes: read `references/TESTING.md`.

## Cross-Cutting Defaults

- Define reusable traced operation boundaries with `Effect.fn("Domain.operation")`.
- Use `Effect.fnUntraced` for reusable helpers whose stack-frame and span metadata are intentionally unnecessary.
- Keep expected failures typed and preserve defects and interruption for the owning supervision boundary.
- Decode unknown boundary input with `Schema.decodeUnknownEffect`; use `schema.makeEffect` for typed constructor input whose type-side validation can fail.
- Keep long-lived resources and background work scoped to the Layer or component that owns their lifetime.
- Retry only idempotent operations, at the narrowest boundary that can classify the failure truthfully.
- Preserve type information and solve Effect typing problems without unchecked casts or non-null assertions.

## Boundary Rules

- Keep HTTP handlers thin: decode input, read context, call services, map typed errors to transport responses.
- Place business rules in services or domain functions.
- Wrap HTTP clients, SDKs, CLIs, and external integrations in named effects at adapter boundaries.
- Decode persisted rows with Schema or SQL-specific helpers when values are not trivially trusted.
- Keep provider/network calls outside authoritative database transactions.
- Catch or retry only when the current boundary has a truthful response.
- Let exhausted failures remain visible unless the boundary has a real fallback.

## Completion Criterion

Before finishing:

- account for every Effect API introduced or changed against the project-pinned source
- apply every branch selected by the task
- run the narrowest relevant typecheck and tests
- leave no unchecked cast, swallowed interruption, unscoped long-lived work, or unbounded retry introduced by the change

The work is complete only when every item above is checkable and satisfied.