Haskell Development

Error Handling

• NEVER use undefined or error as placeholders
• Handle ALL Maybe/Either cases explicitly — no silent ignoring
• Handle all pattern match cases — no partial matches
• Use types to make impossible states unrepresentable

Type Safety & Idioms

• Write type signatures for all top-level definitions
• Prefer Text over String
• Use newtype wrappers for domain types
• Apply smart constructors for validation
• Write total functions — avoid head, tail, and other partial functions
• Prefer pure functions over IO
• Use explicit module exports
• Favor composition over complex functions
• Write Haddock documentation for public APIs

Records

• Use OverloadedRecordDot (add pragma to modules that use the syntax)
• Use DisambiguateRecordFields and DuplicateRecordFields for simple field names
• Use lenses for record manipulation when appropriate

Aeson

• NEVER construct aeson objects by hand
• Create a type and use encode/decode on it
• Prefer generic deriving; hand-write instances only when the wire format requires it

HLint

If .hlint.yaml exists in the project root, run hlint on every modified file. Fix ALL warnings before considering the task complete.

Build Workflow

If ghcid.txt exists in the project, check it after every code change for compile errors. Do not proceed until it is clean. If this file does not exist, use whatever build workflow the project documents.

When adding or deleting .hs modules: update the .cabal file, or run hpack if package.yaml exists.

Relude

If the project uses relude (check .cabal or package.yaml dependencies), also follow [RELUDE.md](RELUDE.md) for idiomatic substitutions.

Testing

• QuickCheck for property-based testing
• HUnit or Hspec for unit tests