i can't see the solution

it's an .lhs file

data Alumno = A { dni :: Integer, nombre :: String,nota::Float }

shf :: Alumno -> String shf alum = show (dni alum) ++ nombre alum ++ show (nota alum )

i don't wanna use this sentence: deriving Show i wanna use "my show function" but ... Here is the problem !!

instance Show a => Show (Alumno a) where show x = shf x

Thanks .