name: preferences-haskell-development description: Haskell development conventions covering type classes, monadic composition, and GHC extensions. Load when working with .hs files or Haskell projects.

Haskell Development

Architectural patterns alignment

See @~/.claude/skills/preferences-architectural-patterns/SKILL.md for overarching principles.

Haskell embodies the theoretical ideal of functional programming with native support for monad transformers, lawful abstractions, and referential transparency. It serves as the reference model that other languages approximate with their respective libraries (Effect-TS, ZIO, etc.).

Core patterns and libraries

• Effect composition: Use monad transformers from transformers and mtl for composing effects
• Error handling: Either and Maybe for composable error handling, no exceptions
• State management: StateT for explicit state threading through computations
• Reader pattern: ReaderT for dependency injection and configuration
• IO isolation: Isolate IO effects to boundaries, keep pure functions pure
• Type-level programming: Leverage type classes, GADTs, and type families for compile-time guarantees

Monad transformer stacks

• Structure applications as monad transformer stacks (e.g., ReaderT Config (StateT AppState (ExceptT Error IO)))
• Ensure all monad instances are lawful (identity, associativity for bind)
• Use lift and liftIO to traverse effect transformer stacks
• Apply newtype wrappers for domain-specific effect stacks

Type safety and purity

• Leverage Haskell's type system to make illegal states unrepresentable
• Use newtype for domain-specific types to prevent mixing incompatible values
• Encode effects explicitly in type signatures (IO, State, Reader, etc.)
• Prefer total functions; use Maybe/Either instead of partial functions
• Use smart constructors to enforce invariants at construction time

Recommended libraries

• Monad transformers: transformers, mtl
• Effect systems: polysemy, eff, freer-simple for more flexible effect composition
• Parsing: megaparsec, attoparsec for parser combinators
• Error handling: either, validation, errors for enhanced error composition
• Testing: QuickCheck for property-based testing, hspec for BDD-style tests
• Lenses: lens or optics for functional record updates

Code quality and tooling

• Use hlint for linting and style suggestions
• Use ormolu, fourmolu, or stylish-haskell for consistent formatting
• Enable -Wall and -Wcompat for comprehensive warnings
• Use ghcid for fast feedback during development
• Run stack test or cabal test before committing

Project structure

• Use Stack or Cabal for package management
• Organize code into modules with clear public APIs
• Separate pure business logic from IO-heavy code
• Use src/ for library code, app/ for executables, test/ for tests
• Document public functions with Haddock comments

Best practices

• Write point-free style judiciously (when it improves clarity)
• Use pattern matching exhaustively
• Leverage lazy evaluation but be aware of space leaks
• Profile with -prof and +RTS flags before optimizing
• Use strict fields in data types when appropriate (! bang patterns)
• Consider strictness annotations for performance-critical code