extract :: Question' -> Question a extract q = case q of QuestionS x -> extractQString q QuestionI x -> extractQInt q QuestionD x -> extractQDouble q
Antoine wrote:
What is a caller supposed to do with this function? How were you imagining it would be called?
The sole purpose of the above function would be to unbox the inner parametrized (Question a) type from its (Question') wrapper. If it were possible I could write this: testQ' :: Question' -> Answer -> Bool testQ' q a = testQ (extract q) a given: testQ :: (Eq a) => Question a -> Answer -> Bool testQ q ans = case (correctAnswer q) of Nothing -> False Just x -> x == (answerFunc q $ ans) instead of this testQ' :: Question' -> Answer -> Bool testQ' q a = case q of QuestionS x -> testQS q a QuestionI x -> testQI q a QuestionD x -> testQD q a where testQS q a = testQ (extractQString q) a testQI q a = testQ (extractQInt q) a testQD q a = testQ (extractQDouble q) a In fact this seemingly redundant pattern emerges in all wrapper handling code, for instance: askQ' :: Question' -> IO Answer askQ' q = case q of QuestionS x -> askQS q QuestionI x -> askQI q QuestionD x -> askQD q where askQS q = askQ (extractQString q) askQI q = askQ (extractQInt q) askQD q = askQ (extractQDouble q) Hope this clarifies things somewhat. Alia Alia