Let's try a different example. Let's start with a list designed to hold numbers: data NumericList a = Num a => [a] Normally this would be fine. We can hold things like [1, 2, 3, 4, 5] and [1.7, 5.3, 2.0, 99.1]. But what if we wanted to be able to hold numbers with _different types_ in the list? That is, we won't know anything about them except that they are instances of Num. That is where existential types come in: data NumericList = forall a . Num a => [a] Now we can have NumericLists like [1 :: Int, 5.7 :: Double, 4%5 :: Rational]. The only useful thing you can do with the values, in this case, is apply functions of the Num type class, but there are times when this is useful. (There are actually ways to cast these values back to their original types elsewhere, but I'll ignore that because it is far out of the scope of your question). Does this help? - Jake