Haskell Development

Expert assistance for Haskell programming.

Guidelines

CRITICAL - Error Handling in Code: NEVER write code that silently ignores errors:

• Do NOT use undefined or error as placeholders
• Do NOT skip handling error cases in pattern matches
• Do NOT ignore Maybe/Either failure cases
• Handle all possible cases explicitly
• Use types to make impossible states unrepresentable

Every error case in generated code must be handled properly.

Code Quality:

• Write type signatures for all top-level definitions
• Write total functions (avoid head, tail)
• Prefer pure functions over IO when possible
• Use explicit exports in modules
• Leverage type system for safety
• Favor composition over complex functions
• Write Haddock documentation for public APIs

Idiomatic Patterns:

• Prefer Text over String
• Use newtype wrappers for domain types
• Apply smart constructors for validation
• Use lenses for record manipulation when appropriate
• Use Applicative and Monad appropriately

Testing

• Use QuickCheck for property-based testing
• Use HUnit or Hspec for unit tests
• Provide good examples in documentation

Build instructions

As you make code changes, start a subagent in parallel to resolve any compile errors in ghcid.log.

IMPORTANT: Do not run build commands yourself. The human runs ghcid on the terminal, which then updates ghcid.log with any compile error or warning (if this file does not exist, or if ghcid has stopped, remind the human to address it). You should read ghcid.log (in entirety) after making code changes; this file updates near-instantly. Don't rely on VSCode diagnostics.

Adding/Deleting modules: When a new .hs file is added or deleted, the .cabal file must be updated accordingly. However, if package.yaml exists in the project, run hpack instead to regenerate the .cabal file with the updated module list. This will trigger ghcid to restart automatically.

HLint warnings: Once all code changes are made and ghcid.log shows success, check if the project has a .hlint.yaml file. If it does, run hlint to ensure there are no warnings and address any that appear.