I've retitled this post just in case (-: Having spent some more time on the prior post (questionnaire data design patterns), I've managed to produce a version that allows for heterogeneous lists but that seems to necessarily involve existential quantification. Three question remain: (1) figure out how to show 'questions' in version3.hs (I'm stumped so far) (2) figure out how to make Question and its subtypes polymorphic in the context of existential quantification:     such that you can have a Choice a where a is an Int and another Choice Double, etc... yet still allow for     heterogeneous lists so that questions = [open ...., test ...., choice ...] would still be possible. (3) translate version3.hs to use records (if possible) <version3.hs> {-# LANGUAGE ExistentialQuantification #-} {-# LANGUAGE TypeSynonymInstances #-} module Main where import Text.Show.Functions -- dummy funcs for now class Question_ a where     ask        :: a -> String     view       :: a -> String     parse      :: a -> String     data Question = forall a. Question_ a => Question a type Name          = String type QString       = String type AnswerType    = String type CorrectAnswer = String type Option        = (String, String) type Options       = [Option] -- question types data Open      = Open   Name QString AnswerType data Test      = Test   Name QString AnswerType CorrectAnswer data Choice    = Choice Name QString AnswerType CorrectAnswer Options instance Question_ Open where     ask   (Open n q a) = n     view  (Open n q a) = q     parse (Open n q a) = a instance Question_ Test where     ask   (Test n q a c) = n ++ c     view  (Test n q a c) = q ++ c     parse (Test n q a c) = a ++ c instance Question_ Choice where     ask   (Choice n q a c os) = n ++ c     view  (Choice n q a c os) = q ++ c     parse (Choice n q a c os) = a ++ c instance Question_ Question where     ask   (Question q) = ask   q     view  (Question q) = view  q     parse (Question q) = parse q -- -- Smart constructor -- open :: Name -> QString -> AnswerType -> Question open n q a = Question (Open n q a) test :: Name -> QString -> AnswerType -> CorrectAnswer -> Question test n q a c = Question (Test n q a c) choice :: Name -> QString -> AnswerType -> CorrectAnswer -> Options -> Question choice n q a c os = Question (Choice n q a c os) questions :: [Question] questions = [ open   "q1" "what is your name" "str"             , test   "q2" "what is 1+1?"      "int" "2"             , choice "q3" "what is 2+2?"      "int" "4" [("a", "4")]             ] </version.hs> Alia             ]