[Haskell-cafe] Type extensions

[Thomas Davie]

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

data MyBwTaggedTree = Branch BwTag Int MyBwTaggedTree MyBwTaggedTree  
| Node BwTag Int

and several functions to move from one to another.  (Or define the  
most complex and not always use all the attrdbutes). What I would  
prefer is to be able to spocify something like:

data MyTree = Branch MyTree MyTree | Leaf

type BwTag = Bool
data MyBwTree extends MyTree with BwTag=True

data MyBwTaggedTree extends MyBwTree with Int=0

Specifying that myBwTree is the same as MyTree but with an extra  
argument on each node, and to generate functions to translate between  
them that fill in True as a default value for the extra tag.

Is this possible?

Thanks

Tom Davie