phantom.hs

data Message = PlainText String | Encrypted String

send :: Message -> Recipient -> IO ()
send (Encrypted m) recipient = some magic with m
send (PlainText _) _ = undefined

data Maybe a = Just a | Nothing

-- movin' to phantom types
data Message a = Message String
data Encrypted
data PlainText

-- change send now
send :: Message Encrypted -> Recipient -> IO ()
encrypt :: Message PlainText -> Message Encrypted
decrypt :: Message Encrypted -> Message PlainText

newMessage :: String -> Message PlainText
newMessage s = Message s

-- this will get rejected by the type-checker
-- send (newMessage "hello!") "abc@xyz.com"