[Haskell-cafe] how to Enumerate without lists?

Dear Cafe (again), I was trying to write sum $ map (^ 2) $ [ 1 :: Int .. 10^8 ] in a list-free style getSum $ foldMap (Sum . (^ 2)) $ [ 1 :: Int .. 10^8 ] This avoids building an intermediate list (of squares) but it will allocate, since foldMap uses foldr (by default, and it's not overridden for lists) The conclusion would be: not to use lists at all. Which will be the point of my talk anyway. But here, we get a list from enumFromTo. Can we avoid that? Let's try: we just reify enumFromTo data Enumerator a = Enumerator {from :: a, to :: a} we have this for decades, it is called (a,a) in Data.Ix, and then instance Foldable Enumerator where ... Oh no, foldMap (and others) would need an Enum constraint! - J