haskell-type-designlisted

# Haskell Type Design Write the types first, then the function falls out. This is the central discipline: the type signature constrains the implementation so tightly that wrong code usually doesn't type-check. ## Type-Driven Design 1. **Domain first, functions second.** Sketch the algebraic data types for the problem before any logic. Sum types (`data Status = Pending | Approved | Rejected Reason`) replace state-tracking booleans and string-typed enums. 2. **Newtypes for every primitive that has meaning.** `newtype UserId = UserId Int64` and `newtype Email = Email Text` cost nothing at runtime and prevent entire classes of bugs at the type level. Use `deriving newtype` for the underlying instances you actually want exposed: ```haskell newtype UserId = UserId Int64 deriving stock (Show, Eq, Ord) deriving newtype (FromJSON, ToJSON) ``` 3. **Make functions total.** No `head`, no `fromJust`, no `error` in business logic. Return `Maybe a` or `Either MyError a`. The `safe` package provides `headMay` etc. if you want it. 4. **Parse, don't validate.** A validation function returns `Bool`; a parser returns a *narrower type* that carries proof of validity. - Wrong: `validateEmail :: Text -> Bool` - Right: `parseEmail :: Text -> Either EmailError Email` Once you hold an `Email`, you never need to check it again — the type carries the proof. "Make illegal states unrepresentable." 5. **Phantom types for state machines.** When state transitions must b