Effect v4

Overview

Use this skill to produce Effect code and reviews that match the current v4 APIs in effect-smol, not legacy v3 patterns.

Treat migration docs as intent and packages/effect/src + tests as ground truth.

Workflow

1. Classify the task.

• New implementation
• v3-to-v4 migration
• Code review / compatibility audit

2. Load only the needed reference file.

• Migration mapping: references/migration-map.md
• Current idioms with evidence: references/idioms-and-evidence.md
• Deterministic audit procedure: references/review-checklist.md

3. Apply v4-first rules.

• Prefer ServiceMap.Service / ServiceMap.Reference over Context.*, Effect.Tag, Effect.Service, FiberRef.
• Prefer Effect.catch, catchCause, catchFilter, catchReason, catchReasons.
• Prefer Effect.forkChild / forkDetach and explicit Fiber.join / Fiber.await.
• Prefer Scope.provide over Scope.extend.
• Use Yieldable correctly: yield* is fine; non-Effect combinator inputs need .asEffect().

4. Validate with local evidence.

• Confirm symbols exist in packages/effect/src/*.ts.
• Confirm behavior with nearby tests in packages/effect/test.
• Do not claim semantics not evidenced by code/tests.

Response Contract

When this skill is used, produce:

1. Exact API replacements (old -> new) used.
2. Behavior-impact notes (memoization, keep-alive, Cause flattening, equality).
3. Concrete file references used as evidence.
4. Known limitations or unresolved migrations.

Guardrails

• Do not reintroduce v3 APIs in generated code.
• Do not claim stable semantics for effect/unstable/* modules.
• Do not infer runtime guarantees without source/test confirmation.