[Haskell-cafe] Type extensions

[Thomas Davie]

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