4.4 Expressions
Syntax
Name Resolution Rules
A
name
used as a
primary
shall resolve to denote an object or a value.
Discussion: This replaces RM83-4.4(3).
We don't need to mention named numbers explicitly, because the name of
a named number denotes a value. We don't need to mention attributes explicitly,
because attributes now denote (rather than yield) values in general.
Also, the new wording allows attributes that denote objects, which should
always have been allowed (in case the implementation chose to have such
a thing).
Reason: It might seem odd that this is
an overload resolution rule, but it is relevant during overload resolution.
For example, it helps ensure that a
primary
that consists of only the identifier of a parameterless function is interpreted
as a
function_call
rather than directly as a
direct_name.
Static Semantics
Each expression has a type; it specifies the computation
or retrieval of a value of that type.
Dynamic Semantics
The value of a
primary
that is a
name
denoting an object is the value of the object.
{
AI12-0227-1}
An expression of a numeric universal type is evaluated
as if it has type root_integer (for universal_integer)
or root_real (otherwise) unless the context identifies a specific
type (in which case that type is used).
Ramification: This
has no effect for a static expression; its value may be arbitrarily small
or large since no specific type is expected for any expression for which
this rule specifies one of the root types. The only effect of this rule
is to allow Constraint_Error to be raised if the value is outside of
the base range of root_integer or root_real when the expression
is not static.
Reason: This rule
means that implementations don't have to support unlimited range math
at runtime for universal expressions. Note that universal expressions
for which the context doesn't specify a specific type are quite rare;
attribute prefixes and results are the only known cases. (For operators,
8.6 already specifies that the operator of
a root type get used, which provides a specific type.)
Implementation Permissions
For
the evaluation of a
primary
that is a
name
denoting an object of an unconstrained numeric subtype, if the value
of the object is outside the base range of its type, the implementation
may either raise Constraint_Error or return the value of the object.
Ramification: {
AI05-0299-1}
This means that if extra-range intermediates are used to hold the value
of an object of an unconstrained numeric subtype, a Constraint_Error
can be raised on a read of the object, rather than only on an assignment
to it. Similarly, it means that computing the value of an object of such
a subtype can be deferred until the first read of the object (presuming
no side effects other than failing an Overflow_Check are possible). This
permission is over and above that provided by subclause
11.6,
since this allows the Constraint_Error to move to a different handler.
Reason: This permission is intended to
allow extra-range registers to be used efficiently to hold parameters
and local variables, even if they might need to be transferred into smaller
registers for performing certain predefined operations.
Discussion: There is no need to mention
other kinds of
primarys,
since any Constraint_Error to be raised can be “charged”
to the evaluation of the particular kind of
primary.
Examples
Examples of primaries:
4.0 -- real literal
Pi -- named number
(1 .. 10 => 0) -- array aggregate
Sum -- variable
Integer'Last -- attribute
Sine(X) -- function call
Color'(Blue) -- qualified expression
Real(M*N) -- conversion
(Line_Count + 10) -- parenthesized expression
Examples of expressions:
{
AI95-00433-01}
Volume --
primary
not Destroyed --
factor
2*Line_Count --
term
-4.0 --
simple expression
-4.0 + A --
simple expression
B**2 - 4.0*A*C --
simple expression
R*Sin(θ)*Cos(φ) --
simple expression
Password(1 .. 3) = "Bwv" --
relation
Count
in Small_Int --
relation
Count
not in Small_Int --
relation
Index = 0
or Item_Hit --
expression
(Cold
and Sunny)
or Warm --
expression (parentheses are required)
A**(B**C) --
expression (parentheses are required)
Extensions to Ada 83
In Ada 83,
out parameters
and their nondiscriminant subcomponents are not allowed as
primaries.
These restrictions are eliminated in Ada 95.
In various contexts throughout the language
where Ada 83 syntax rules had
simple_expression,
the corresponding Ada 95 syntax rule has
expression
instead. This reflects the inclusion of modular integer types, which
makes the logical operators "
and", "
or",
and "
xor" more useful in expressions of an integer type.
Requiring parentheses to use these operators in such contexts seemed
unnecessary and potentially confusing. Note that the bounds of a
range
still have to be specified by
simple_expressions,
since otherwise
expressions
involving membership tests might be ambiguous. Essentially, the operation
".." is of higher precedence than the logical operators, and
hence uses of logical operators still have to be parenthesized when used
in a bound of a range.
Wording Changes from Ada 2005
Inconsistencies With Ada 2012
{
AI12-0039-1}
Corrigendum: Revised
membership syntax to eliminate ambiguities. In some cases, previously
ambiguous membership expressions will now have an unambiguous meaning.
If an Ada 2012 implementation chose the "wrong" meaning, the
expression could silently change meaning. Virtually all such expressions
will become illegal because of type mismatches (and thus be incompatible,
not inconsistent). However, if the choices are all of a Boolean type,
resolution might succeed. For instance, A in B | C and
D now always means (A in B | C) and D, but
the original Ada 2012 syntax would have allowed it to mean A in
B | (C and D). If a compiler allowed the expression and interpreted
it as the latter, the meaning of the expression would silently change.
We expect this to be extremely rare as membership operations on Boolean
types are unlikely (and this can happen only in code written for Ada
2012).
Incompatibilities With Ada 2012
{
AI12-0039-1}
Corrigendum: The revised
membership syntax will require parentheses in membership_choice_lists
in some cases where the Ada 2012 grammar did not require them. For instance,
A in B in C | D is now illegal. However, such
expressions can be interpreted in multiple ways (either A in
(B in C) | D or A in (B in C | D)
for this example), so using such expressions is likely to be dangerous
(another compiler might interpret the expression differently). In addition,
all such expressions occur only in Ada 2012 syntax; so they should be
rare.
Wording Changes from Ada 2012
{
AI12-0227-1}
Correction: Added wording so that universal
expressions evaluated at runtime can raise Constraint_Error if the value
is outside of the range of root_integer or root_real. We
don't document this as an inconsistency as the rule requires no implementation
to change (as Constraint_Error is not required); it just allows implementations
that already raise Constraint_Error (which is all of them surveyed) to
be considered correct.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe