8.4 Use Clauses
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_clause
were carefully constructed to avoid so-called Beaujolais
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.
use_package_clause ::= use package_name
Ramification: This includes formal packages.
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.
exception echoes the similar exception for “immediate scope (of
a declaration)” (see 8.2
). It makes use_clause
work like this:
package P is
type T is range 1..10;
package Parent is
X : T;
package Parent.Child is
Y : T; -- Illegal!
Z : P.T;
W : T;
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
For each package named
in denoted by a package_name
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
'Class named in determined
by a subtype_mark
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:
Each primitive subprogram of T including
each enumeration literal (if any);
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
” so as to
achieve the effect requested by the “principle of equivalence of
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.
Certain implicit declarations may become potentially
use-visible in certain contexts as described in 12.6.
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.
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_expression
any use will be ambiguous. We only mention identifier
here, because declarations named by operator_symbol
are always overloadable, and hence never cancel each other. Direct visibility
is irrelevant for character_literal
Example of a use
clause in a context clause:
with Ada.Calendar; use Ada;
Example of a
use type clause:
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
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.
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_declaration
the case where the rule should apply only to some subset of the declarations
with the same defining name, and the case of subtype_declaration
(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
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
Correction: Added wording to allow other
declarations to be potentially use-visible, to support corrections to
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe