haskell-patterns

name	haskell-patterns
description	Idiomatic Haskell conventions and decision rules. Use when writing Haskell code, designing data types, or handling errors.

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