Haskell Typeclasses

types.hs

import Text.Printf(printf)

data Message = 
    Message {msgType  :: String,
             msgTo    :: String,
             msgValue :: String}
    deriving (Eq, Read)
    
data FooMessage =
    FooMessage {msg       :: Message,
                extraData :: String}
    deriving (Eq, Read)

instance Show Message where
    show m = printf "%s:%s:%s"
               (msgType m) (msgTo m) (msgValue m)

instance Show FooMessage where
    show m = printf "%s#%s"
               (show (msg m)) (extraData m)