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Learn-skills.dev effect-v4

Effect v4 implementation and migration guidance for current Effect APIs. Use when writing or reviewing code that depends on Effect v4, migrating v3-style APIs (Context.Tag, Effect.Service, FiberRef, catchAll/catchSome, fork/forkDaemon), debugging service/layer wiring, or validating compatibility with effect-smol/effect@4.0.0-beta.x.

install
source · Clone the upstream repo
git clone https://github.com/NeverSight/learn-skills.dev
Claude Code · Install into ~/.claude/skills/
T=$(mktemp -d) && git clone --depth=1 https://github.com/NeverSight/learn-skills.dev "$T" && mkdir -p ~/.claude/skills && cp -r "$T/data/skills-md/agustif/effect-v4-skill/effect-v4" ~/.claude/skills/neversight-learn-skills-dev-effect-v4 && rm -rf "$T"
manifest: data/skills-md/agustif/effect-v4-skill/effect-v4/SKILL.md
source content

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
  1. 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
  1. 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()
    .
  1. 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.