I am also not an expert on existential types but I was yesterday playing
with them.
I think that you want to have a Finite Map with items
of different type. It is not possible in Haskell (you cannot define
a list with elements of different types). But you could do a little
trick using type classes.
To simplify, I define a FiniteMap as a list:
> type FiniteMap a b = [(a,b)]
> addToFM :: FiniteMap a b -> a -> b -> FiniteMap a b
> addToFM xs x y = (x,y):xs
> data K = K1 | K2 | K3
> deriving (Eq, Show)
The trick is to define a type class which relates the elements that you
can insert in your finite map
> class Item i
<You could add some common methods if you want>
Now, you can define some instances:
> instance Item Char
> instance Item (Char,Char)
And define an ITEM as:
> data ITEM = forall i . Item i => MkItem i
Finally, your KTab will store values of type ITEM
> type KTab = FiniteMap K ITEM
And now your example works:
> f :: KTab -> KTab
> f t = (addToFM (addToFM t K1 (MkItem 'a')) K2 (MkItem ('a','b')))
There is a paper by Koen Claessen about "Type Classes and Existential
Types" which describes this trick and has a first part very readable. At this
moment, I have not the reference, but I can try to look for it, if you want.
Best Regards, Jose E. Labra
http://lsi.uniovi.es/~labra
