Re: [Caml-list] Subtyping

[Goswin von Brederlow <goswin-v-b@web.de>]

David MENTRE <dmentre@linux-france.org> writes:

> Hello,
>
> On Tue, Apr 7, 2009 at 07:48, Goswin von Brederlow <goswin-v-b@web.de> wrote:
>> In the last 2 weeks I've been playing around with lots of different
>> ways to do the same thing to get a feel for what style suites me
>> best. If you have improvements or alternative ways of doing the two
>> things below let me know.
>
> Well, if you are learning OCaml, I would advise you to read regular
> OCaml code, e.g. the standard library. You'll learn The Right OCaml
> Style(tm).

I've been using ocaml on and off for years now. Just trying out new
things.

>> Lets look another thing going in a similar direction. I want to
>> have a structure where I can store key value pairs. But just for fun
>> lets say I want to store values of different types and the key knows
>> the type of value. In short a heterogeneous associative container:
>
> Well, why don't you just do:
>
> # type k = Int_k of int | Float_k of int;;
> type k = Int_k of int | Float_k of int
> # type v = Int_v of int | Float_v of float;;
> type v = Int_v of int | Float_v of float
> # let h = Hashtbl.create 3;;
> val h : ('_a, '_b) Hashtbl.t = <abstr>
> # Hashtbl.add h (Int_k 3) (Int_v 4);;
> - : unit = ()
> # Hashtbl.add h (Float_k 5) (Float_v 0.6);;
> - : unit = ()

That would allow storing a float value under an int key or vice
versa. Something that would completly corrupt the on-disk format of my
data. One could add runtime checks that prevent this but I would
really like to have this ensured by the type system at compile time.

And I would like to avoid having to write a private insert functions
and public

let insert_int k v = insert (Int_k k) (Int_v v)
let insert_float k v = insert (Float_k k) (Float_v v)

For that the storing data structure would have to know that there are
int and float values and I would rather have the storing structure
polymorphic.

MfG
        Goswin