Not sure if this is haskell-beginners material, but it's possible in a lazy
language like Haskell to define an inhabitant of NotVoid recursively:
let x = NotVoid x in x
Or, alternatively, using fix from Data.Function:
fix NotVoid
This gives NotVoid (NotVoid (NotVoid (...)))
Nick
On Fri, Oct 25, 2013 at 5:54 AM, Brent Yorgey
On Fri, Oct 25, 2013 at 12:44:06PM +0900, KwangYul Seo wrote:
Hello,
It seems there are three different ways to declare an empty type in Haskell.
http://www.haskell.org/haskellwiki/Empty_type
1) data E0 = E0
This one is not empty, as others have pointed out. It is inhabited by _|_ and E0.
2) newtype Void = Void Void
This one is in fact empty (that is, only inhabited by _|_), but it depends on the fact that newtype constructors do not add any laziness. The same thing done with 'data',
data NotVoid = NotVoid NotVoid
is not empty, because it is inhabited by
_|_, NotVoid _|_, NotVoid (NotVoid _|_), ...
With the data declaration, these are all different. With the newtype, they are all equal to _|_. This is a bit of a technical point, however; if I were you I wouldn't worry about it at this point. It sounds like the most important thing for you to understand is below:
I'd like to know how the second trick works. Is it possible to create a new type from itself? How should I interpret this?
Yes, it is possible to create a new type from itself! This is called a "recursive data type", and they are the bread and butter of Haskell programming. For some other less silly/trivial examples, consider
data IntList = Nil | Cons Int IntList
data BTree a = Empty | Node a (BTree a) (BTree a)
both of which are recursive types.
-Brent _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners