Haskell Conventions

Type Design

• •Newtype wrappers for domain primitives (UserId, Email) -- never raw Int/Text
• •Smart constructors to enforce invariants at construction time
• •Sum types for states -- never booleans for mutually exclusive states

Error Handling

• •Either / ExceptT for expected, recoverable errors
• •throwIO for unexpected, unrecoverable errors
• •Never use partial functions: head, tail, fromJust, read

Application Structure

• •ReaderT pattern (ReaderT AppEnv IO) over deep transformer stacks
• •MTL-style typeclasses for effect abstraction when needed
• •Avoid stacking more than 2-3 transformers

Strictness

• •Use Text, never String (unless legacy interop)
• •Strict IO (Data.Text.IO) over lazy IO
• •Use streaming (conduit/pipes) for large data

Concurrency

• •STM / TVar for shared mutable state
• •async / mapConcurrently for concurrent tasks
• •race for timeouts

Anti-patterns

• •String for text data
• •Lazy IO (Prelude.readFile)
• •Deep monad transformer stacks
• •Boolean parameters instead of sum types
• •Partial functions