[Haskell-cafe] abstract extensible types?

[Ryan Ingram]

http://www.cs.nott.ac.uk/~wss/Publications/DataTypesALaCarte.pdf

This lets you create sets of types to store in a container, with the
static guarantee that only members of the set of types are included.
The types can contain other elements of the set within them
recursively.

To extract value from the container, you can provide observation
algebras that work on subsets of these values.

It's a great solution to this problem and avoids the "anything with
Typeable" issue that you get using Dynamic, which adds lots of
run-time failure cases if something you didn't expect makes it into
your data.

  -- ryan

2008/11/12 Alberto G. Corona <agocorona at gmail.com>:
> Is there any abstract container  that permits the addition of  new types of
> data? I know how to simulate the extension of Algebraic datatypes, but this
> does not permit the addition of data with new types in the same container
> and recover them in a  type-safe way.
>
> Did I reinvent the Weel? I found something, that permits this for any
> Typeable datatype. For example
>
>
>  x=5
>  list= [put x,  put "hello"]
>
>  [t1,t2 ]= list
>
>  x' = get t1 :: Int
>  s = get t2 :: String
>  print (x' +1)     -- 2
>  print s            -- "hello"
>
>  x''= get t2 :: Int            --"get: casting from String  to type Int"
>
>
>
> The code:
>
> data Abstract= forall a. Typeable a => T !String  a
>
>
> class FromToAbstract x where
>  put :: x -> Abstract
>  get ::  Abstract -> x
>  unsafeGet :: Abstract -> x
>
>  -- get(put x)== x
>
> instance Typeable x => FromToAbstract x where
>  put x= T (typeString x) x
>
>  get (Abstract type1 a)= if type2 == type1 then v
>                         else error ("get: casting "++ "from type "++type1++"
> to type "++type2)
>            where
>            v=  unsafeCoerce a :: x
>            type2= typeString v
>
>  unsafeGet (Abstract type1 a)= unsafeCoerce a
>
>
> typeString !x= tyConString $ typeRepTyCon $ typeOf x
>
> instance Typeable T where
>   typeOf _= mkTyConApp (mkTyCon "Abstract") []
>
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