How to get a list from a ListTable

[remeike]

So I've been trying to figure out how to use ListTables but can't seem to figure out how to work with the resulting data. I've been doing something similar to this example from the documentation:

ordersWithItems :: Query (Order Expr, ListTable Expr (Item Expr))
ordersWithItems = do
   order <- each orderSchema
   items <- aggregate $ listAgg <$> itemsFromOrder order
   return (order, items)

My understanding is that if I run a select statement on this I should get back the following:

resultingOrderWithItems :: Statement () [(Order Result, ListTable Result (Item Result))]
resultingOrderWithItems = select ordersWithItems

The trouble is I'm not sure what to do with ListTable Result (Item Result). I was assuming I would be able to go from there to [Item Result], but I haven't been able to figure it out. I suspect there's something more fundamental I'm missing here.

[ocharles]

Is that actually what you get when you select? I'm pretty sure a ListTable Expr t should turn into [t'], where t' is whatever t would transpose to in the Result context.

[remeike]

Ok, I see where I had messed up here. I was kind of assuming that the result of the select would be [(Order Result, ListTable Result (Item Result))] so I hade a different function that was trying to handle that type but really it should have been written out as [FromExprs (Order Result, ListTable Result (Item Result))] or just [(Order Result, [Item Result])].

I think what really tripped me up is that originally I was trying to do this with a product type that wasn't a tuple. So really it looked more like this:

data OrderItems f =
  OrderItems
    { order :: Order f
    , items :: ListTable f (Item f)
    }
  deriving stock Generic
  deriving anyclass Rel8able

ordersWithItems :: Query (OrderItems Expr)
ordersWithItems = do
   order <- each orderSchema
   items <- aggregate $ listAgg <$> itemsFromOrder order
   return (OrderItem order items)

resultingOrderWithItems :: Statement () [FromExprs (OrderItems Result)]
resultingOrderWithItems = select ordersWithItems

In this case the items field on OrderItems can't magically be turned into [Item Result] in the Result context, which I think is why I was trying to figure out if there was anything I could do with ListTable Result (Item Result). So I suppose my question is really if there's a clever way to make this sort of thing work with records or if you're just better off sticking with tuples.

[ocharles]

So I suppose my question is really if there's a clever way to make this sort of thing work with records or if you're just better off sticking with tuples.

Good news, there is! Try this:

data OrderItems f =
  OrderItems
    { order :: Order f
    , items :: HList f (Item f)
    }
  deriving stock Generic
  deriving anyclass Rel8able

HList is a type family that will also change when f changes.

[remeike]

Brilliant! That works. Thank you!