Am Montag, 4. August 2008 07:12 schrieb Levi Stephen:

Hi,

I was wondering what the best way to represent a 'subset' type relationship would be. The use case I'm looking at is where the subset would be a user selection, so a filter would not be appropriate.

For example, say I have something like:

type Order = [OrderItem] type OrderItem = OrderItem { qty :: Int, product: Product }

Now I'd like the user to be able to select a number of items (e.g., priority shipping, gift wrapping)

One possible way I can see is to add extra fields to the OrderItem record, but this seems like it would not be easy to extend.

It doesn't seem like using [Product] would be appropriate as if the Product is updated it will need to be updated in both the OrderItem list and in the subset.

Another approach would be to use a ProductIdType in place of Product, then look up the actual Product as needed.

Are there other (and better) ways to do this?

Thanks, Levi

Perhaps something like data Extra = PriorityShipping | GiftWrapping ... data OrderItem = OrderItem { qty :: Int, product :: Product, extras :: [Extra] } ? Or make it type Quantity = Int type Order = [(Product, Quantity, [Extras])] ? Cheers, Daniel