6.8 Expression Functions
Syntax
Name Resolution Rules
Static Semantics
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A
potentially static expression is defined in the same way as
a static expression except that
a name denoting a formal parameter of an expression
function is a potentially static expression; and
each use of “static expression” in
the definition of “static expression” is replaced with a
corresponding use of “potentially static expression” in the
definition of “potentially static expression”.
Discussion: These uses occur in the definition
of “static expression” in the cases of function calls, type
conversions, qualified expressions, membership tests, short circuit control
forms, conditional expressions, and parenthesized expressions.
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The following language-defined representation aspect may be specified
for an expression function:
Static
The type of aspect Static is Boolean. When aspect Static is True for
an expression function, the function is a
static expression function.
If directly specified, the
aspect_definition
shall be a static
expression.
Aspect Description for Static: Specifies
that an associated expression function can be used in static expressions.
The Static value for an inherited function is True if some corresponding
primitive function of the parent or progenitor type is a static expression
function; otherwise, if not directly specified, the aspect is False.
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[A static expression function is a static function; see
4.9.]
Legality Rules
Ramification: This can only fail if the
discriminant is an access to a part of a nonaliased parameter, as there
can be no local declarations here.
Discussion: {
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We don't need to repeat any of the other Legality Rules for return statements
since none of them can fail here: the implicit return statement has to
apply to this function (and isn't nested in something), there clearly
is a return statement in this function, and the static class-wide accessibility
check cannot fail as a tagged type cannot be declared locally in an expression
function.
is not a completion;
has an
expression
that is a potentially static expression;
contains no calls to itself;
each parameter (if any) is of mode in and
is of a static subtype;
has a result subtype that is a static subtype;
has no applicable precondition or postcondition
expression; and
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for result type
R, if the function is a boundary entity for type
R (see
7.3.2), no type invariant applies
to type
R; if
R has a component type
C, a similar
rule applies to
C.
Ramification: Since a string subtype
can be static, this allows an expression function of a string type to
be static.
Paragraph 6 was deleted.
Dynamic Semantics
Discussion: The last sentence effectively
means that all of the dynamic wording in
6.5
applies as needed, and we don't have to repeat it here.
Examples
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function Is_Origin (P :
in Point)
return Boolean
is --
see 3.9
(P.X = 0.0
and P.Y = 0.0);
Extensions to Ada 2005
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Expression functions are new in Ada 2012.
Extensions to Ada 2012
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Corrigendum: A
aggregate
can directly be the return expression of an expression function. This
eliminates the double parentheses that otherwise would be necessary.
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Aspect Static is new; it allows using suitable expression functions in
static expressions.
Wording Changes from Ada 2012
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Clarified the term “expression function” so it matches the
meaning expected in
6.1.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
