"S.D.Mechveliani" wrote:
> With existentials, i hoped to achieve the effect of simplification,
> something like ...
> Just d -> (dn , d)
> _ -> (s:dn, s)
> where
> s = DMeat {name = "shashlyk"...}
> Is this possible?
> If there is no chance, i would rather forget of existentials, so far.
>
> Thanks a lot for the help.
I'm not sure if this is a real world problem, or just a for-instance.
(You never know! :) Anyway, I can see that fundamental issue that you
want to resolve is how to store various types of data into a single
list. The issue of figuring out the type of each element is is resolved
by the `Name' stored with each element. I don't know much yet about
existential types, but I would think that you would still have to have
some function or something that relates `Name's to particular types.
But my real point in replying to this message is to see if you actually
need a multi-type list. Do you really *need* to have separate types for
DMeat, DVine, etc.? For example, instead of having:
data DMeat = DMeat {mName :: String -- ,...several fields
}
data DVegetable = DVegetable { different fields }
data DVine = DVine { different fields }
...
-- 50 kinds of dishes, each described in individual manner
data Dish = Meat' DMeat | Vegetable' DVegetable | ... -- 50 items
would it be possible to use:
data Dish = DMeat {mName :: String, ...}
| DVegetable {...}
| DVine {...}
| ...
This would eliminate the need to have a multi-type list, for this
particular example anyway.
- Michael Hobbs
