RE: [Caml-list] Semantics of physical equality

RE: [Caml-list] Semantics of physical equality

[Kevin S. Millikin]

On Friday, February 27, 2004 3:32 PM, Basile STARYNKEVITCH 
[SMTP:basile@starynkevitch.net] wrote:
    >> On Fri, Feb 27, 2004 at 02:29:50PM -0600, Kevin S. Millikin
    >> wrote:
    >>
    >> # V1(0) == V1(0);; - : bool = false
    >>
    >> V1's are different.  Is this guaranteed?

    >> What do you mean by guaranteed?

I mean ``guaranteed'' in the sense that separate calls to cons in 
Scheme
are guaranteed to produce objects that are distinct (according to eqv?
and eq?), or that a call to malloc in C is guaranteed to never return a
pointer that is the same as any other currently valid pointer in the
program (according to ==).

    >> Why would you want a guarantee that V1 0 is not physically equal
    >> to V1 0? I tend to think that making such an hypothesis is
    >> dangerous and wrong, even if the current implementation
    >> demonstrate it.

For the same reason that I occasionally rely on separate cons cells not
being eq? in Scheme ;).  In the current problem, it can save me a great
deal of effort involved in generating temporary identifiers that are
guaranteed to be unique, and in wrapping library data structures that I
am unwilling to change in order merely to tag them.

I realize that it would be dangerous and wrong to rely on them being ==
if the semantics of value constructors and == did not guarantee they
would be.  So I guess that's my question: "is it dangerous and wrong to
rely on separately constructed values being different according to 
==?".

-- Kevin

PS: Anyway, it looks like Michal Moskal has answered in the negative.
Neither behavior is guaranteed, which is an acceptable answer (though
not the one I was looking for).

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Re: [Caml-list] Semantics of physical equality

[Michal Moskal]

On Fri, Feb 27, 2004 at 05:16:06PM -0600, Kevin S. Millikin wrote:
> In the current problem, it can save me a great
> deal of effort involved in generating temporary identifiers that are
> guaranteed to be unique, and in wrapping library data structures that I
> am unwilling to change in order merely to tag them.

type t = V of int ref

let t1 = V (ref 0)
let t2 = V (ref 0)

In this case you can assume t1 != t2. If it is not a sum type, but
record type, you can use record with at least one mutable field.

-- 
: Michal Moskal :: http://www.kernel.pl/~malekith :: GCS !tv h e>+++ b++
: When in doubt, use brute force. -- Ken Thompson :: UL++++$ C++ E--- a?

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[Caml-list] Re: Semantics of physical equality

[Jed Davis]

Michal Moskal <malekith@pld-linux.org> writes:

> On Fri, Feb 27, 2004 at 05:16:06PM -0600, Kevin S. Millikin wrote:
>> In the current problem, it can save me a great
>> deal of effort involved in generating temporary identifiers that are
>> guaranteed to be unique, and in wrapping library data structures that I
>> am unwilling to change in order merely to tag them.
>
> type t = V of int ref
>
> let t1 = V (ref 0)
> let t2 = V (ref 0)
>
> In this case you can assume t1 != t2. If it is not a sum type, but
> record type, you can use record with at least one mutable field.

Hm:

type t = unit ref
let t1 : t = ref ()
let t2 : t = ref ()
let mt : _ -> t = ref


-- 
[Jed Davis]              A098 903E 9B9A DEF4 168F  AA09 BF07 807E F336 59F9
 <jdev@panix.com>                                               0xF33659F9
  "But life wasn't yes-no, on-off.  Life was shades of gray, and rainbows
   not in the order of the spectrum." -- L. E. Modesitt, Jr., _Adiamante_

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