Main.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TypeSynonymInstances #-}
{-# LANGUAGE TypeFamilies #-}

module Main where
  
import Data.Coerce

untag :: Coercible a b => (b -> a) -> a -> b
untag _ = coerce

data Validation = Validated | Unvalidated

data MyError = MyError String deriving (Show)

newtype Name (a :: Validation) = Name String deriving (Show)
mkName :: String -> Name Unvalidated
mkName = Name

class Validatable a where
  type ValidationError a :: *
  validate :: a Unvalidated -> Either (ValidationError a) (a Validated)

instance Validatable Name where
  type ValidationError Name = MyError
  validate n
    | length (untag Name n) >= 3 = Right (coerce n)
    | otherwise                  = Left (MyError "too short")

main :: IO ()
main = do
  print $ validate (mkName "abcd")
  print $ validate (mkName "a")