{-# LANGUAGE TypeData, TypeFamilies #-}

type PlaintextPass = String 
type EncodedPass   = Int
type data UserType
  = Full
  | Patch
  | Create


type family XUserId      (x::UserType)
type family XUserName    (x::UserType)
type family XUserPass    (x::UserType)
type family XUserBirth   (x::UserType)
type family XUserFriends (x::UserType)

data User' (x :: UserType) = MkUser 
  { userId :: XUserId x
  , userUserName :: XUserName x 
  , userPassword :: XUserPass x
  , userBirthday :: XUserBirth x
  , userFriends :: XUserFriends x
  }

type instance XUserId      Full = Int
type instance XUserName    Full = String
type instance XUserPass    Full = EncodedPass 
type instance XUserBirth   Full = String
type instance XUserFriends Full = [Int]

type User = User' Full 
deriving instance Show User

type instance XUserId      Create = ()
type instance XUserName    Create = String
type instance XUserPass    Create = PlaintextPass
type instance XUserBirth   Create = String
type instance XUserFriends Create = [Int]

--etc... 

example :: User
example = MkUser 0 "Frank" 1 "01-01-0001" []
main = print example