How can I create Answers of type Gender, Race, or Age? These should be possible: λ> Answer Male λ> Answer White λ> Answer Black λ> Answer 28 Others such as using a string should not be possible: λ> Answer "a string" -- should throw type error {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE StandaloneDeriving #-} module Tutorial where data Gender = Male | Female deriving (Show) data Race = White | Black deriving (Show) type Age = Int data Answer a where Answer :: Gender -> Answer Gender deriving instance Show (Answer w) -- λ> -- This is pretty useful and accepts Answers with the Gender -- λ> Answer Male -- Answer Male -- λ> :t Answer Male -- Answer Male :: Answer Gender -- λ> -- how do I also accept Race and Age? -- below I try to sort out how to use type families (unsuccessfully) before bed -- type family Answer :: * -- type instance Answer Gender = Answer Gender