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

["Jon Harrop" <jon@ffconsultancy.com>]

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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