Effectful Conventions

Dispatch Choice

• •Default: Static (Effectful.Reader.Static, Effectful.State.Static.Local)
• •Only use Dynamic when MonadReader/MonadState instances are needed

Effect Stack Order

Always consistent: Reader -> State -> Error -> IOE

runApp :: Config -> AppState -> App a -> IO (Either (CallStack, AppError) (a, AppState))
runApp cfg st = runEff . runError . runStateLocal st . runReader cfg

Custom Effects

• •First-order (no m parameter): use interpret_
• •Higher-order (uses m parameter): use interpret + localSeqUnlift

Error Handling

• •Use constrained effect signatures so caller decides error handling
• •Error.Static lacks MonadError -- define local liftEither helper
• •Use runErrorNoCallStack or runErrorWith at boundaries

Concurrency

ALWAYS use Effectful.Concurrent.*, NEVER Control.Concurrent.* with liftIO.

Anti-patterns

• •Adding Error constraint to interpreter instead of effect definition
• •Returning raw Either when errors should propagate via effect
• •Mixing Error.Static and Error.Dynamic
• •Over-constraining effect signatures
• •Using Control.Concurrent.* with liftIO instead of Effectful.Concurrent.*

Reference

For detailed code examples (dependency injection, testing, MTL migration, concurrency):

• •references/effectful-examples.md