> 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