effect-best-practices

Use Effect.Service with accessors set to true and declare dependencies in the dependencies array so layers wire automatically. Define methods with Effect.fn for automatic tracing, and compose everything at the app root with Layer.mergeAll.

Define errors with Schema.TaggedError including a message field and rich context like entity IDs, then handle them with Effect.catchTag or catchTags. Avoid catchAll and mapError because they discard type information needed for specific handling.

The Effect Language Server is a TypeScript plugin (@effect/language-service) that detects Effect-specific issues like floating Effects and missing requirements at edit time. Install it as a devDependency and register it in tsconfig.json compilerOptions plugins.

Define atoms outside components with Atom.make, add Atom.keepAlive for persistent global state, and use useAtomValue and useAtomSet hooks in components. Render async results with Result.builder using onErrorTag for typed error handling.

Layer.mergeAll produces flat compositions with simpler types, keeping the TypeScript language server fast, while deeply nested Layer.provide chains create recursive types that slow the LSP. Layers also memoize construction so shared services instantiate only once.

Context.Tag is acceptable for infrastructure with runtime injection, such as Cloudflare KV or worker bindings, and for factory patterns where resources are provided externally. Business logic services should always use Effect.Service for built-in accessors and Default layers.