Re: [Haskell-cafe] beginner's problem about lists

On Wed, 2006-10-11 at 11:40 +0100, Malcolm Wallace wrote:

ihope wrote:

...

It's possible to make both "infinite list" and "finite list" datatypes:

data Inf a = InfCons a (Inf a) data Fin a = FinCons a !(Fin a) | FinNil

At least, I think the Fin type there has to be finite...

No, your Fin type can also hold infinite values. The strictness annotation on the (Fin a) component merely ensures that the tail exists to one constructor's depth (Head Normal Form). It does not force strictness all the way down (full Normal Form).

Are you sure? Since it does it recursively, if each cons ensures that it has a tail. Then that tail must be Nil or Cons (since we know it can't be _|_), and if it's Cons... It's not full normal form of course because it's not strict in the list elements, but it is spine strict all the way down. longList 0 = undefined longList n = n : longList (n-1) case longList 3 of n : _ -> n 3 longFin 0 = undefined longFin n = n `FinCons` longFin (n-1) case longFin 3 of FinCons n _ -> n *** Exception: Prelude.undefined Or am I really confused? Duncan