Syntax:
pragma Predicate ([Entity =>] type_LOCAL_NAME, [Check =>] EXPRESSION);
This pragma (available in all versions of Ada in GNAT) encompasses both
the Static_Predicate and Dynamic_Predicate aspects in
Ada 2012. A predicate is regarded as static if it has an allowed form
for Static_Predicate and is otherwise treated as a
Dynamic_Predicate. Otherwise, predicates specified by this
pragma behave exactly as described in the Ada 2012 reference manual.
For example, if we have
type R is range 1 .. 10; subtype S is R; pragma Predicate (Entity => S, Check => S not in 4 .. 6); subtype Q is R pragma Predicate (Entity => Q, Check => F(Q) or G(Q));
the effect is identical to the following Ada 2012 code:
type R is range 1 .. 10; subtype S is R with Static_Predicate => S not in 4 .. 6; subtype Q is R with Dynamic_Predicate => F(Q) or G(Q);
Note that there are no pragmas Dynamic_Predicate
or Static_Predicate. That is
because these pragmas would affect legality and semantics of
the program and thus do not have a neutral effect if ignored.
The motivation behind providing pragmas equivalent to
corresponding aspects is to allow a program to be written
using the pragmas, and then compiled with a compiler that
will ignore the pragmas. That doesn’t work in the case of
static and dynamic predicates, since if the corresponding
pragmas are ignored, then the behavior of the program is
fundamentally changed (for example a membership test
A in B would not take into account a predicate
defined for subtype B). When following this approach, the
use of predicates should be avoided.