effect-ts

# Effect-TS Effect is a TypeScript library for building production-grade software with typed errors, structured concurrency, dependency injection, and built-in observability. ## Version Detection Before writing Effect code, detect which version the user is on: ```bash # Check installed version cat package.json | grep '"effect"' ``` - **v4.x** (recommended, the direction Effect is heading): `Context.Service`, `Effect.catch`, `Effect.forkChild`, `Schema.TaggedErrorClass` - **v3.x** (stable, still common in production): `Context.Tag`, `Effect.catchAll`, `Effect.fork`, `Data.TaggedError` > Note: v4 beta briefly used a `ServiceMap` module, renamed back to `Context` on 2026-04-07 (PR #1961). If you see `ServiceMap.*` in any doc or older beta code, it is the current `Context.*`. Both v3 and v4 import `Context` from `"effect"`; the exports inside differ (`Context.Service` in v4 vs `Context.Tag` in v3). **Prefer v4 for new projects** - it's where Effect is going. In an existing codebase, match the installed version: don't rewrite v3 code in v4 syntax unless asked. If the version is genuinely unclear, default to v4 and say so. v4 is still in beta, so pin an exact version (`4.0.0-beta.x`) and expect occasional API churn. ## Primary Documentation Sources **v4 (primary):** - https://github.com/Effect-TS/effect-smol (v4 source + migration guides) - https://github.com/Effect-TS/effect-smol/blob/main/LLMS.md (v4 LLM guide) **v3 (for existing codebases):** - https://effect.website/d...