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)
{-# 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")]
]
Alia
]
