Hi. Another way is to take advantage of Haskell's laziness:

class Foo a where foo :: a -> Int

data Oof1 = Oof1 Int instance Foo Oof1 where foo (Oof1 i) = i

data Oof2 = Oof2 Int instance Foo Oof2 where foo (Oof2 i) = i

list1 = [foo (Oof1 10), foo (Oof2 20)]

That's all it takes. The applications of foo in list1 don't get evaluated until list1 is used, e.g. by sum. Regards, Tom