* null exclusion and generics
@ 2006-10-19 12:51 Georg Bauhaus
2006-10-19 13:06 ` Dmitry A. Kazakov
2006-10-19 20:08 ` Randy Brukardt
0 siblings, 2 replies; 4+ messages in thread
From: Georg Bauhaus @ 2006-10-19 12:51 UTC (permalink / raw)
If a generic formal type is private, I can supply a
null excluding subtype for the type parameter.
A generic formal private type means assignment is possible.
Declarations of objects of the null excluding
actual inside the generic need explicit initialization,
though.
generic
type T is private;
procedure Nop(source: in T);
procedure Nop(source: in T) is
tmp: T; -- no known way to initialize
begin
tmp := source;
end Nop;
Can I excpect, then, that Ada.Containers.* will typically
not be usable with null excluding subtypes? (Because an
implementation will likely declare local variables without
knowing how to initialize them.)
Is it a possible enhancement to add something like
generic
type T is private;
Default_Value: ...; -- or a subprogram, or ...
...
-- Georg
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: null exclusion and generics
2006-10-19 12:51 null exclusion and generics Georg Bauhaus
@ 2006-10-19 13:06 ` Dmitry A. Kazakov
2006-10-19 20:08 ` Randy Brukardt
1 sibling, 0 replies; 4+ messages in thread
From: Dmitry A. Kazakov @ 2006-10-19 13:06 UTC (permalink / raw)
On Thu, 19 Oct 2006 14:51:59 +0200, Georg Bauhaus wrote:
> If a generic formal type is private, I can supply a
> null excluding subtype for the type parameter.
> A generic formal private type means assignment is possible.
> Declarations of objects of the null excluding
> actual inside the generic need explicit initialization,
> though.
>
> generic
> type T is private;
> procedure Nop(source: in T);
Is it really so? I would expect them to require:
generic
type T (<>) is private;
procedure Nop(source: in T);
for "not null" is a constraint.
What would a language layer say?
--
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: null exclusion and generics
2006-10-19 12:51 null exclusion and generics Georg Bauhaus
2006-10-19 13:06 ` Dmitry A. Kazakov
@ 2006-10-19 20:08 ` Randy Brukardt
2006-10-20 7:57 ` Dmitry A. Kazakov
1 sibling, 1 reply; 4+ messages in thread
From: Randy Brukardt @ 2006-10-19 20:08 UTC (permalink / raw)
"Georg Bauhaus" <bauhaus@futureapps.de> wrote in message
news:1161262319.24061.19.camel@localhost.localdomain...
...
> Can I excpect, then, that Ada.Containers.* will typically
> not be usable with null excluding subtypes? (Because an
> implementation will likely declare local variables without
> knowing how to initialize them.)
Yes, that's probably true. And it will be true for pretty much any use you
might put a null-excluding subtype to.
Null-excluding subtypes were defined for object cases, like components or
parameters. They're not very useful as named subtypes, because they're
unlikely to work in many contexts (as you've noted here); they especially
won't match many generics (and if they do, the generic probably won't work).
A null-excluding *type* is even worse: it is virtually useless. Perhaps we
got a bit too orthogonal with them.
Dmitry A. Kazakov <mailbox@dmitry-kazakov.de> writes:
>Is it really so? I would expect them to require:
>
> generic
> type T (<>) is private;
> procedure Nop(source: in T);
>
>for "not null" is a constraint.
No, a null-excluding type is definite, so (<>) isn't required. We didn't
want to add indefinite elementary types (just as we are unwilling to add
limited elementary types), because it would be a significant new concept - a
large burden on compilers without much benefit.
So all access types are initialized with null, and if the subtype is
null-excluding, that will automatically raise Constraint_Error. A null
exclusion is essentially a kind of constraint (it is not technically a
constraint, but that has to do with independence from other forms of
constraint). Think of it being like:
subtype A is Integer range 1 .. 10;
Obj : A := Integer'First;
which also will raise C_E. (If we had allowed a way to declare automatic
initializations for scalar types -- something I wish we had done -- it would
have had a similar effect as the explicit initialization here).
Randy.
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: null exclusion and generics
2006-10-19 20:08 ` Randy Brukardt
@ 2006-10-20 7:57 ` Dmitry A. Kazakov
0 siblings, 0 replies; 4+ messages in thread
From: Dmitry A. Kazakov @ 2006-10-20 7:57 UTC (permalink / raw)
On Thu, 19 Oct 2006 15:08:17 -0500, Randy Brukardt wrote:
> So all access types are initialized with null, and if the subtype is
> null-excluding, that will automatically raise Constraint_Error. A null
> exclusion is essentially a kind of constraint (it is not technically a
> constraint, but that has to do with independence from other forms of
> constraint). Think of it being like:
>
> subtype A is Integer range 1 .. 10;
>
> Obj : A := Integer'First;
>
> which also will raise C_E. (If we had allowed a way to declare automatic
> initializations for scalar types -- something I wish we had done -- it would
> have had a similar effect as the explicit initialization here).
This is one more reason to finally introduce a sane
construction/destruction model in Ada. Compromises never buy off.
--
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de
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2006-10-19 12:51 null exclusion and generics Georg Bauhaus
2006-10-19 13:06 ` Dmitry A. Kazakov
2006-10-19 20:08 ` Randy Brukardt
2006-10-20 7:57 ` Dmitry A. Kazakov
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