[Haskell-cafe] Polymorphic (typeclass) values in a list?

TJ tjay.dreaming at gmail.com
Fri Oct 19 13:20:47 EDT 2007

Dan Licata: Thanks for explaining the mechanics behind it. Knowing how
it (could) be implemented always helps me understand things.

On 10/20/07, Jules Bean <jules at jellybean.co.uk> wrote:
> Quite often an explicit ADT is much nicer. But they represent two
> opposing patterns of code-writing. Explicit ADT allows you to write case
> statements handling 'all the logic in one place'; a class forces you to
> separate the differences into 'separate instances'.

Nice ADT example. Indeed that would be how I'd do it in SML. Use a
record type holding closures referencing an object of unknown type.
The nice thing I've found about doing it in SML this way is that I can
extract the object back out, using exceptions. e.g.

(* Start Standard ML *)

datatype Renderable = Renderable { render : unit -> RenderedImage,
extract : unit -> unit, tag : exn }

  datatype Point = Point Something
  exception ExtractMe Point
  exception Tag
  fun mkPoint Something =
    let val p = Point Something
    in { render = fn () => ... ,
         extract = fn () => raise ExtractMe p,
         tag = Tag }
  (* extractPoint would return the Point hidden away in a Renderable. *)
  fun extractPoint (Renderable { tag = Tag, extract, ... }) =
    (extract (); Point SomethingPointless)
    handle ExtractMe p => p

(* End SML *)

I don't know if this would work in Haskell, as I'm not familiar with
Haskell exceptions. Anyway I see that Haskell has a Dynamic type...

I've got a good grip on this now, I think. Thanks everyone.


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