Type level tagging

Tag.hs

{-# LANGUAGE DerivingVia #-}
{-# LANGUAGE RecordWildCards #-}
{-# LANGUAGE TypeApplications #-}

module Tag where

newtype Tag t a = MkTag { unTag :: a } deriving (Eq, Ord, Show) via a

----

data Dirty
data Clean

data Foo = MkFoo
  { a :: Int
  , b :: String
  } deriving (Eq, Ord, Show)

validate :: Tag Dirty Foo -> Maybe (Tag Clean Foo)
validate (MkTag MkFoo{..})
  | a > 10 && b == "yes" = Just $ MkTag @Clean MkFoo{..}
  | otherwise            = Nothing

----

badFoo :: Tag Dirty Foo
badFoo = MkTag @Dirty MkFoo {a = 5, b = "hello"}

goodFoo :: Tag Dirty Foo
goodFoo = MkTag @Dirty MkFoo {a = 50, b = "yes"}