* Why stack can hold indefinite objects but records cannot?
@ 2017-11-28 20:31 Victor Porton
2017-11-28 20:46 ` Dmitry A. Kazakov
2017-11-28 22:51 ` Randy Brukardt
0 siblings, 2 replies; 5+ messages in thread
From: Victor Porton @ 2017-11-28 20:31 UTC (permalink / raw)
Why is it OK to create an indefinite object as a local variable ("on stack")
but not OK to put an indefinite object into a record?
How records are less powerful than the stack?
By the way, how indefinite local variables can be implemented? (just
curious)
Moreover, wouldn't it to be a good idea to allow indefinite objects in
records (thus making the record itself indefinite, even if it has no
discriminants)? I realize that then it would be difficult to track which
records are definite and which indefinite. But what's about the idea?
--
Victor Porton - http://portonvictor.org
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: Why stack can hold indefinite objects but records cannot?
2017-11-28 20:31 Why stack can hold indefinite objects but records cannot? Victor Porton
@ 2017-11-28 20:46 ` Dmitry A. Kazakov
2017-11-28 21:31 ` Victor Porton
2017-11-28 22:51 ` Randy Brukardt
1 sibling, 1 reply; 5+ messages in thread
From: Dmitry A. Kazakov @ 2017-11-28 20:46 UTC (permalink / raw)
On 2017-11-28 21:31, Victor Porton wrote:
> Why is it OK to create an indefinite object as a local variable ("on stack")
> but not OK to put an indefinite object into a record?
It is OK in both cases. You must provide the constraint that is.
> By the way, how indefinite local variables can be implemented? (just
> curious)
They cannot, there is no such thing.
A variable can hold a value which constraints vary. Each value is
constrained. These either deploy a maximum allocation length like
bounded strings or hidden referential semantics like unbounded strings.
> Moreover, wouldn't it to be a good idea to allow indefinite objects in
> records (thus making the record itself indefinite, even if it has no
> discriminants)?
It is a bad idea because there is no straightforward implementation of.
The proper solution is to have an abstract record interface which would
allow anything appear a record member whereas the implementation would
be left to the user. The compiler should provide simple implementation
of that out of the box, maybe even less than it does already.
--
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: Why stack can hold indefinite objects but records cannot?
2017-11-28 20:46 ` Dmitry A. Kazakov
@ 2017-11-28 21:31 ` Victor Porton
2017-11-28 21:50 ` Dmitry A. Kazakov
0 siblings, 1 reply; 5+ messages in thread
From: Victor Porton @ 2017-11-28 21:31 UTC (permalink / raw)
Dmitry A. Kazakov wrote:
> On 2017-11-28 21:31, Victor Porton wrote:
>> Why is it OK to create an indefinite object as a local variable ("on
>> stack") but not OK to put an indefinite object into a record?
>
> It is OK in both cases. You must provide the constraint that is.
>
>> By the way, how indefinite local variables can be implemented? (just
>> curious)
>
> They cannot, there is no such thing.
>
> A variable can hold a value which constraints vary. Each value is
> constrained. These either deploy a maximum allocation length like
> bounded strings or hidden referential semantics like unbounded strings.
>
>> Moreover, wouldn't it to be a good idea to allow indefinite objects in
>> records (thus making the record itself indefinite, even if it has no
>> discriminants)?
>
> It is a bad idea because there is no straightforward implementation of.
>
> The proper solution is to have an abstract record interface which would
> allow anything appear a record member whereas the implementation would
> be left to the user. The compiler should provide simple implementation
> of that out of the box, maybe even less than it does already.
But for the stack this is implemented. I don't understand why the same
cannot be done for records.
--
Victor Porton - http://portonvictor.org
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: Why stack can hold indefinite objects but records cannot?
2017-11-28 21:31 ` Victor Porton
@ 2017-11-28 21:50 ` Dmitry A. Kazakov
0 siblings, 0 replies; 5+ messages in thread
From: Dmitry A. Kazakov @ 2017-11-28 21:50 UTC (permalink / raw)
On 2017-11-28 22:31, Victor Porton wrote:
> But for the stack this is implemented.
No it is not. You must provide constraint. E.g.:
X : String (1..80);
or
X : String := "1234";
> I don't understand why the same cannot be done for records.
Records are just same:
type Foo is record
X : String (1..80);
end record;
You cannot have unconstrained variables, members, objects. As I said,
there is no such thing, and there cannot be. It is incomputable.
--
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: Why stack can hold indefinite objects but records cannot?
2017-11-28 20:31 Why stack can hold indefinite objects but records cannot? Victor Porton
2017-11-28 20:46 ` Dmitry A. Kazakov
@ 2017-11-28 22:51 ` Randy Brukardt
1 sibling, 0 replies; 5+ messages in thread
From: Randy Brukardt @ 2017-11-28 22:51 UTC (permalink / raw)
"Victor Porton" <porton@narod.ru> wrote in message
news:ovkh24$1bh5$1@gioia.aioe.org...
> Why is it OK to create an indefinite object as a local variable ("on
> stack")
> but not OK to put an indefinite object into a record?
>
> How records are less powerful than the stack?
Because records can be composed into other data structures, while a local
variable cannot be composed into something else.
> By the way, how indefinite local variables can be implemented? (just
> curious)
Depends on the compiler. Janus/Ada allocates them from a special storage
pool (which really just wraps the normal heap), then frees them when the
scope is left. Some other compilers change the size of the stack frame
instead.
> Moreover, wouldn't it to be a good idea to allow indefinite objects in
> records (thus making the record itself indefinite, even if it has no
> discriminants)? I realize that then it would be difficult to track which
> records are definite and which indefinite. But what's about the idea?
It would be very hard to implement in composite objects: imagine an
multi-dimensional array of such records. That would be especially true for
implementations that try to avoid using storage pools at all costs (which is
necessary for some safety-critical customers).
Randy.
^ permalink raw reply [flat|nested] 5+ messages in thread
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2017-11-28 20:31 Why stack can hold indefinite objects but records cannot? Victor Porton
2017-11-28 20:46 ` Dmitry A. Kazakov
2017-11-28 21:31 ` Victor Porton
2017-11-28 21:50 ` Dmitry A. Kazakov
2017-11-28 22:51 ` Randy Brukardt
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