Re: [Caml-list] How much optimized is the 'a option type ?

[Yotam Barnoy <yotambarnoy@gmail.com>]

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Just to clarify, by value types you mean stuff allocated on the stack,
right?

-Yotam


On Thu, Jan 23, 2014 at 6:20 PM, Jon Harrop <jon@ffconsultancy.com> wrote:

> Goswin wrote:
> > So Some x is the value x unless x is an option type?
>
> No, Some x is always a reference to a heap-allocated object that contains
> the value "x".
>
> In fact, I think it is essentially the same as OCaml's representation for
> the 'a option type (except for the run-time "type" information required by
> the GC).
>
> > You can't encode 'a option differently depending on 'a unless you have a
> flag for which encoding it used as well.
>
> Yes. I think it is a bad data representation though. Option types should
> never allocate anything at all. They should be a value type containing a
> boolean "HasValue" and the value itself which has a default in the event
> that it is None. So None=(false, _), Some 3=(true, 3), Some None=(true,
> (false, _)) and Some(Some 3)=(true, (true, 3)). You could even store the
> Booleans and bytes as pack them so Some(Some 3) would take the same amount
> of space (16 bytes) as Some 3.
>
> Cheers,
> Jon.
>
> -----Original Message-----
> From: caml-list-request@inria.fr [mailto:caml-list-request@inria.fr] On
> Behalf Of Goswin von Brederlow
> Sent: 23 January 2014 09:29
> To: caml-list@inria.fr
> Subject: Re: [Caml-list] How much optimized is the 'a option type ?
>
> On Wed, Jan 22, 2014 at 09:26:09PM -0000, Jon Harrop wrote:
> > Goswin wrote:
> > > In F#, with nil pointer, that will be a problem. But I guess nobody
> > > ever
> > has 'a option option types there
> >
> > I believe the representation of Some None in F# is a heap allocated
> > block containing a NULL pointer.
> >
> > Cheers,
> > Jon.
>
> So Some x is the value x unless x is an option type? That wouldn't work for
> polymorphic functions, those taking 'a option. You can't encode 'a option
> differently depending on 'a unless you have a flag for which encoding it
> used as well.
>
> I think you can only have one: option types using nil or 'a option option.
>
> Which doesn't mean you can't have nil too, just not to represent the None
> part of 'a option. In ocaml you would need a new type syntax like
>
> type 'a ptr_option = NIL | PTR of 'a constraint type b . 'a != b ptr_option
>
> MfG
>         Goswin
>
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