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!standard 3.10 (06)          01-10-01 AI95-00231/02
!standard 3.10 (12)
!standard 4.6 (49)
!standard 8.6 (25)
!class amendment 00-04-13
!status work item 00-04-13
!status received 00-04-13
!priority Medium
!difficulty Hard
!subject Access-to-constant parameters and null-excluding subtypes
!summary
Parameters and discriminants of an anonymous access-to-constant type are introduced. Access parameters and access discriminants are generalized to have a variant that allows a null value. Null-excluding subtypes of a named access type are introduced.
!problem
(See discussion.)
!proposal
Introduce a not-null constraint, and an access_constraint syntactic category.
Modify 3.2.2(5) to:
constraint ::= scalar_constraint | access_constraint | composite_constraint
access_constraint ::= not_null_constraint | composite_constraint [not_null_constraint]
Modify 3.10(6) to:
not_null_constraint ::= not null
access_definition ::= access all subtype_mark [not_null_constraint] | access constant subtype_mark [not_null_constraint] | access subtype_mark
Modify 3.10(12) to:
An access_definition defines an anonymous general access type; the subtype mark denotes its @i<designated subtype>. If the word @b<all> appears, the type is an access-to-variable type. If the word @b<constant> appears, the type is an access-to-constant type. If the words @b<not null> appear, the access_definition defines an access subtype which excludes the null value. If neither @b<all> nor @b<constant> appear, the access definition is equivalent to having both @b<all> and @b<not null> present, and hence defines an access-to-variable subtype that excludes the null value. An access_definition is used in the specification of an access discriminant (see 3.7) or an access parameter (see 6.1).
Drop the parenthetical "(named)" from the first sentence of 3.10(13), since in our new model, anonymous access types have a null value, even though a subtype might exclude the null value.
Modify 3.10(15), to:
An access_constraint includes a not_null_constraint, a composite_constraint, or both. An access_constraint with a composite_constraint is compatible with an unconstrained access subtype if the composite_constraint is compatible with the designated subtype. An access_constraint with a not_null_constraint is compatible with an access subtype if the subtype includes a null value. An access value satisfies a composite_constraint imposed on an access subtype if it equals the null value of its type or if it designates an object whose value satisfies the constraint. An access value satisifes a not_null_constraint imposed on an access subtype if it does not equal the null value of its type.
Delete paragraph 4.1.4(7) which says that anonymous access types don't have a null value.
Modify 4.6(49) to:
If the target subtype excludes the null value, then a check is made that the value of the operand is not null; if the target subtype includes null, then the result of the conversion is null if the operand value is null.
Modify 8.6(25) so that implicit conversion to any anonymous access type is permitted, with the expected rules (i.e. it is illegal to convert an access-to-constant to access-to-variable).
!wording
(See proposal.)
!example
!discussion
It is surprising that an access-to-constant parameter/discriminant is not available. There are several circumstances where such a parameter/discriminant would be appropriate:
As a controlling parameter of an operation that doesn't modify the
designated object.
As a way to force pass-by-reference when interfacing with a foreign
language, when the external operation does not update the designated object,
As a way to provide read-only access via a discriminant.
The rule disallowing "null" for access parameters and access discriminants has turned out to be confusing, and not what is wanted in all cases when interfacing with a foreign language. Therefore, we propose to define an explicit way to exclude nulls from an access subtype, make "access all T" and "access constant T" includes nulls by default, but for backward compatibility make "access T" a shorthand for "access all T not null."
The general ability to specify an access subtype that excludes null for both named and anonymous access types can provide useful documentation and higher efficiency. This is especially true for parameters, by allowing the nullness check to be "pushed" to the caller, where it can be more likely removed.
What should be the default initialization of an object of a subtype that excludes null? It seems clear that the default is still null, and the initialization will raise Constraint_Error. Hence, objects of such a subtype will require explicit initialization. Perhaps a NOTE to this effect should be included in 3.10.
!appendix

Randy Brukardt  00-04-13

This proposal was split out of the "with type" proposal (AI-00217) in
April 2000. Some early conversation on this feature can be found in that
AI's appendix.

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!topic Missing change in AI95-00231
!reference AI95-00231, RM95 6.1(24)
!from Adam Beneschan 09-30-02
!discussion

Nitpick du jour:

I just noticed AI95-00231, which proposes that, among other things,
ACCESS CONSTANT be allowed as a subprogram access parameter.  It
appears that in the list of RM changes for this AI, 6.1(24) was
missed.  This paragraph includes the sentence:

   "An access parameter is of an anonymous general access-to-variable
   type (see 3.10)."

which will no longer be true since the access parameter could now be
an access-to-constant type.

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From: Dan Eilers
Sent: Friday, January 17, 2003  2:41 PM

AI-231 proposes the syntax:

    not_null_constraint ::= NOT NULL

for null-excluding subtypes of a named access type.

I propose generalizing this to allow value-excluding subtypes
of other named types, serving the same purpose:

> The general ability to specify an access subtype that excludes null for both
> named and anonymous access types can provide useful documentation and higher
> efficiency. This is especially true for parameters, by allowing the nullness
> check to be "pushed" to the caller, where it can be more likely removed.

The same principle applies to the division-by-zero check.

   not_constraint ::= NOT NULL
   not_constraint ::= NOT 0
   not_constraint ::= NOT 0.0
   not_constraint ::= NOT Friday

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From: Tucker Taft
Sent: Tuesday, May 6, 2003  2:49 PM

During the AdaUK Ada 200Y workshop, Franco Gasperoni made a good
suggestion.  He felt it will be too confusing to make "access T" mean "access
all T not null" (I think others have made this point as well).

Why not just change the semantics of "access T" to allow null (though
when used as a controlling operand, there will be a run-time
check for non-null), so that "access all T" and "access T" are
synonyms.  This will be upward compatible for programs that
don't raise Constraint_Error, and the slight potential
performance degradation when the non-null aspect of access T was
important can be made up for by using "not null" explicitly.

This seems like a reasonable suggestion, and is probably one
we have debated in the past, and just were perhaps worried
about the compatibility issue.  But in fact, I suspect this
is the kind of thing that may fix more bugs that it creates.
Most uses of anon access types are for controlling operands,
and those will see no change.  The other uses often discover
a need to pass "null" eventually, and then have to switch
to a named access type, or play some other game.

Certainly when interfacing with C or Java, an "access T" that allows
null will be more useful.  And altough we could say that
"access T" is just a convenient shorthand for "access all T not null"
it does seem like a recipe for confusion when teaching Ada 200Y.

It seems like if the non-null aspect of an access parameter is
important, users will welcome the chance to say "not null" explicitly.

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From: John Barnes
Sent: Thursday, May 8, 2003  1:28 AM

Good idea..  I always hated the potential confusion as it was.

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From: Gary Dismukes
Sent: Tuesday, May 13, 2003  6:33 AM

I agree that this seems like a sensible change with minimal disadvantages.

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From: Robert A Duff
Sent: Thursday, May 8, 2003  7:34 AM

Me, three.

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