가연 빈자를 받는 함수

{-# LANGUAGE DataKinds #-} 
{-# LANGUAGE FlexibleInstances #-} 
{-# LANGUAGE KindSignatures #-} 
{-# LANGUAGE PolyKinds #-} 
{-# LANGUAGE ScopedTypeVariables #-} 
{-# LANGUAGE TypeApplications #-} 
{-# LANGUAGE TypeFamilies #-} 
{-# LANGUAGE TypeOperators #-} 
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE AllowAmbiguousTypes #-}

module Printf () where
  
import Data.Kind (Type) 
import Data.Monoid ((<>)) 
import Data.Proxy (Proxy(..))
import GHC.TypeLits
import Data.Coerce

symVal :: forall sym. KnownSymbol sym => String
symVal = symbolVal (Proxy @sym)

data Nil = Nil
data (a :: Either Type Symbol) :<< (b :: Either Nil k1)
infixr 5 :<<

class HasPrintf a where
  type Printf a :: Type
  
  printf :: String -> Printf a

instance HasPrintf (Left 'Nil) where
  type Printf (Left 'Nil) = String
  
  printf = id

instance HasPrintf tail => HasPrintf (Right tail) where
  type Printf (Right tail) = Printf tail
  
  printf = printf @tail
  
instance (KnownSymbol sym, HasPrintf tail) => HasPrintf (Right sym :<< tail) where
  type Printf (Right sym :<< tail) = Printf tail
  
  printf string = printf @tail (string <> symVal @sym)
  
instance (Show param, HasPrintf tail) => HasPrintf (Left param :<< tail) where
  type Printf (Left param :<< tail) = param -> Printf tail
  
  printf string param = printf @tail (string <> show param)
  

safePrintf :: forall a. HasPrintf a => Printf a
safePrintf = printf @a mempty