There is an instance for any type class

instance_of_typeclass.hs

{-# LANGUAGE GADTs #-}
-- take `Monad` for example

class Monad m where
    return :: a -> m a
    (>>=)  :: m a -> (a -> m b) -> m b

-- we can define an instance mechanically:

data IsMonad a where
    Return :: a -> IsMonad a
    Bind   :: IsMonad a -> (a -> IsMonad b) -> IsMonad b

instance Monad IsMonad where
    return = Return
    (>>=)  = Bind