Example

{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ExistentialQuantification #-}

import Data.Generics
import Data.List

class Shape a where
    name :: a -> String
    
    -- The function that actually does the work
    listArgs :: Data a => a -> [String]
    listArgs = map showConstr . gmapQ toConstr

    render :: Data a => a -> String
    render s = intercalate "," (name s : listArgs s) ++ ";"

data Rectangle = Rectangle Float Float Float Float Float Float deriving (Show, Typeable, Data)
instance Shape RPP where
    name = const "RPP"

...Which gives:

render $ Rectangle 1.0 2.0 1.0 2.0 1.0 2.0
>> "RPP,1.0,2.0,1.0,2.0,1.0,2.0;"