Main.hs

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

module Main where
  
import Data.Coerce
import Data.Tagged

data Validated
data Unvalidated

data MyError = MyError String deriving (Show)

newtype Name a = Name { unName :: String } deriving (Show)
-- data NameTag
-- type Name a = Tagged NameTag String
-- mkName :: String -> Name Unvalidated
-- mkName = Tagged

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 (unName n) >= 3 = Right (coerce n)
    | otherwise              = Left (MyError "too short")

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