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{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
import Data.Kind (Type)
import Data.Proxy (Proxy (Proxy))
import GHC.TypeLits (KnownSymbol, Symbol, symbolVal)
data (a :: k1) :<< (b :: k2)
infixr 5 :<<
class HasPrintf a where
type Printf a :: Type
format :: String -> Proxy a -> Printf a
instance KnownSymbol text => HasPrintf (text :: Symbol) where
type Printf text = String
format s _ = s <> symbolVal (Proxy @text)
instance (HasPrintf a, KnownSymbol text) => HasPrintf ((text :: Symbol) :<< a) where
type Printf (text :<< a) = Printf a
format s _ = format (s <> symbolVal (Proxy @text)) (Proxy @a)
instance (HasPrintf a, Show param) => HasPrintf ((param :: Type) :<< a) where
type Printf (param :<< a) = param -> Printf a
format s _ param = format (s <> show param) (Proxy @a)
instance {-# OVERLAPPING #-} HasPrintf a => HasPrintf (String :<< a) where
type Printf (String :<< a) = String -> Printf a
format s _ param = format (s <> param) (Proxy @a)
printf :: HasPrintf a => Proxy a -> Printf a
printf = format ""
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