[Haskell-beginners] type constructors

I was playing around with type constructors and I wrote this: data Foo a b = Foo1 [a] | Foo2 (a -> b) t3 = Foo1 [1, 2, 3] I wanted to see what ghci thought the type of t3 was. Essentially, it's data that doesn't use all of the type variables. So this ran fine, and *Main> :t t3 t3 :: Foo Integer b Wow! The data exists but it doesn't have a "complete type" so to speak. This worked, too: f3 (Foo1 xs) = length xs *Main> f3 t3 3 This is surprising to a conventional programmer. But does this naturally relate to other features of Haskell. Perhaps laziness? (I.e. data of type Foo doesn't always need a type b so it just doesn't have one until it needs one.) Thanks, Mike