8.4 Use Clauses
[A
use_package_clause
achieves direct visibility of declarations that appear in the visible
part of a package; a
use_type_clause
achieves direct visibility of the primitive operators of a type.]
Language Design Principles
If and only if the visibility
rules allow P.A, "
use P;" should make A directly visible
(barring name conflicts). This means, for example, that child library
units, and generic formals of a formal package whose
formal_package_actual_part
is (<>), should be made visible by a
use_clause
for the appropriate package.
The rules for
use_clauses
were carefully constructed to avoid so-called
Beaujolais effects,
where the addition or removal of a single
use_clause,
or a single declaration in a "use"d package, would change the
meaning of a program from one legal interpretation to another.
Syntax
use_package_clause ::= use package_name {,
package_name};
Legality Rules
Ramification: This includes formal packages.
Static Semantics
For a
use_clause
immediately within a declarative region, the scope is the portion of
the declarative region starting just after the
use_clause
and extending to the end of the declarative region. However, the scope
of a
use_clause
in the private part of a library unit does not include the visible part
of any public descendant of that library unit.
Reason: The
exception echoes the similar exception for “immediate scope (of
a declaration)” (see
8.2). It makes
use_clauses
work like this:
package P is
type T is range 1..10;
end P;
with P;
package Parent is
private
use P;
X : T;
end Parent;
package Parent.Child is
Y : T; -- Illegal!
Z : P.T;
private
W : T;
end Parent.Child;
The declaration of Y is illegal because the
scope of the “use P” does not include that place,
so T is not directly visible there. The declarations of X, Z, and W are
legal.
{
AI95-00217-06}
{
AI05-0150-1}
For each package named in a
use_package_clause
whose scope encloses a place, each declaration that occurs immediately
within the declarative region of the package is
potentially use-visible
at this place if the declaration is visible at this place. For each type
T or
T'Class named in a
use_type_clause
whose scope encloses a place, the declaration of each primitive operator
of type
T is potentially use-visible at this place if its declaration
is visible at this place. If a
use_type_clause
whose scope encloses a place includes the reserved word
all, then
the following entities are also potentially use-visible at this place
if the declaration of the entity is visible at this place:
{
AI05-0150-1}
Each primitive subprogram of
T including each enumeration literal
(if any);
{
AI05-0150-1}
Each subprogram that is declared immediately within the declarative region
in which an ancestor type of
T is declared and that operates on
a class-wide type that covers
T.
The semantics described here should be similar
to the semantics for expanded names given in
4.1.3,
“
Selected Components” so as to
achieve the effect requested by the “principle of equivalence of
use_clauses
and
selected_components.”
Thus, child library units and generic formal parameters of a formal package
are potentially use-visible when their enclosing package is use'd.
The "visible at that place" part implies
that applying a
use_clause
to a parent unit does not make all of its children use-visible —
only those that have been made visible by a
with_clause.
It also implies that we don't have to worry about hiding in the definition
of "directly visible" — a declaration cannot be use-visible
unless it is visible.
Note that "use type T'Class;"
is equivalent to "use type T;", which helps avoid breaking
the generic contract model.
{
AI05-0131-1}
Certain implicit declarations may become potentially use-visible in certain
contexts as described in
12.6.
A
declaration is
use-visible if it is potentially use-visible, except
in these naming-conflict cases:
A potentially use-visible declaration is not use-visible
if the place considered is within the immediate scope of a homograph
of the declaration.
Potentially use-visible declarations that have
the same
identifier
are not use-visible unless each of them is an overloadable declaration.
Ramification: Overloadable declarations
don't cancel each other out, even if they are homographs, though if they
are not distinguishable by formal parameter names or the presence or
absence of
default_expressions,
any use will be ambiguous. We only mention
identifiers
here, because declarations named by
operator_symbols
are always overloadable, and hence never cancel each other. Direct visibility
is irrelevant for
character_literals.
Dynamic Semantics
Examples
Example of a use
clause in a context clause:
with Ada.Calendar; use Ada;
Example of a
use type clause:
use type Rational_Numbers.Rational; --
see 7.1
Two_Thirds: Rational_Numbers.Rational := 2/3;
Ramification: In “use X,
Y;”, Y cannot refer to something made visible by the “use”
of X. Thus, it's not (quite) equivalent to “use X; use
Y;”.
If a given declaration is already immediately
visible, then a
use_clause
that makes it potentially use-visible has no effect. Therefore, a
use_type_clause
for a type whose declaration appears in a place other than the visible
part of a package has no effect; it cannot make a declaration use-visible
unless that declaration is already immediately visible.
"Use type S1;" and "use
type S2;" are equivalent if S1 and S2 are both subtypes of
the same type. In particular, "use type S;" and
"use type S'Base;" are equivalent.
Reason: We considered adding a rule that
prevented several declarations of views of the same entity that all have
the same semantics from cancelling each other out. For example, if a
(possibly implicit)
subprogram_declaration
for "+" is potentially use-visible, and a fully conformant
renaming of it is also potentially use-visible, then they (annoyingly)
cancel each other out; neither one is use-visible. The considered rule
would have made just one of them use-visible. We gave up on this idea
due to the complexity of the rule. It would have had to account for both
overloadable and nonoverloadable
renaming_declarations,
the case where the rule should apply only to some subset of the declarations
with the same defining name, and the case of
subtype_declarations
(since they are claimed to be sufficient for renaming of subtypes).
Extensions to Ada 83
Wording Changes from Ada 83
The phrase “omitting from this set any
packages that enclose this place” is no longer necessary to avoid
making something visible outside its scope, because we explicitly state
that the declaration has to be visible in order to be potentially use-visible.
Wording Changes from Ada 95
{
AI95-00217-06}
Limited views of packages are not allowed in use clauses. Defined
named
in a use clause for use in other limited view rules (see
10.1.2).
Extensions to Ada 2005
Wording Changes from Ada 2005
{
AI05-0131-1}
Correction: Added wording to allow other declarations to be potentially
use-visible, to support corrections to formal subprograms.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
