On 1 Jun 2005, at 15:54, Henning Thielemann wrote:
On Wed, 1 Jun 2005, Thomas Davie wrote:
Hi, I was wondering if I hat missed something and it was possible to do this within the Haskell type system or not...
Essentially I would like some sort of inderritance property for Haskell types, I often find myself wanting to for example extend a tree with black/white colouring, or later extend the tree with some sort of ID, etc.
So I would eno up with three data types
data MyTree = Branch MyTree MyTree | Leaf
type BwTag = Bool data MyBwTree = Branch BwTag MyBwTree MyBwTree | Node BwTag
What about
data MyTree a = Branch a (MyTree a) (MyTree a) | Node a
and the types MyTree () MyTree Bool MyTree (Bool, Int) ?
That's exactly what I would normally do, but my data type is in a library and is not parameterised. Tom Davie