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:

# Check installed version
cat package.json | grep '"effect"'

• v3.x (stable, most production codebases): Context.Tag, Effect.catchAll, Effect.fork, Data.TaggedError
• v4.x (beta, Feb 2026+): Context.Service, Effect.catch, Effect.forkChild, Schema.TaggedErrorClass

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.Tag in v3 vs Context.Service in v4).

If the version is unclear, ask the user. Default to v3 patterns for existing codebases, v4 for new projects.