J.3 Reduced Accuracy Subtypes
1
A digits_constraint may
be used to define a floating point subtype with a new value for its requested
decimal precision, as reflected by its Digits attribute. Similarly, a
delta_constraint may be used to define an
ordinary fixed point subtype with a new value for its delta, as
reflected by its Delta attribute.
1.a
Discussion: It might be more direct to
make these attributes specifiable via an attribute_definition_clause,
and eliminate the syntax for these _constraints.
Syntax
2
delta_constraint ::= delta static_expression [
range_constraint]
Name Resolution Rules
3
{expected type (delta_constraint
expression) [partial]} The
expression
of a
delta_constraint is expected to be of
any real type.
Legality Rules
4
The expression of a delta_constraint
shall be static.
5
For a subtype_indication
with a delta_constraint, the subtype_mark
shall denote an ordinary fixed point subtype.
6
{notwithstanding}
For a
subtype_indication
with a
digits_constraint, the
subtype_mark
shall denote either a decimal fixed point subtype or a floating point
subtype (notwithstanding the rule given in
3.5.9
that only allows a decimal fixed point subtype).
6.a/2
This paragraph
was deleted.Discussion: {
AI-00114-01}
We may need a better way to deal with obsolescent
features with rules that contradict those of the non-obsolescent parts
of the standard.
Static Semantics
7
A subtype_indication with
a subtype_mark that denotes an ordinary fixed
point subtype and a delta_constraint defines
an ordinary fixed point subtype with a delta given by the value
of the expression of the delta_constraint.
If the delta_constraint includes a range_constraint,
then the ordinary fixed point subtype is constrained by the range_constraint.
8
A subtype_indication with
a subtype_mark that denotes a floating point
subtype and a digits_constraint defines a
floating point subtype with a requested decimal precision (as reflected
by its Digits attribute) given by the value of the expression
of the digits_constraint. If the digits_constraint
includes a range_constraint, then the floating
point subtype is constrained by the range_constraint.
Dynamic Semantics
9
{compatibility (delta_constraint
with an ordinary fixed point subtype) [partial]} A
delta_constraint is
compatible with
an ordinary fixed point subtype if the value of the
expression
is no less than the
delta of the subtype, and the
range_constraint,
if any, is compatible with the subtype.
10
{compatibility (digits_constraint
with a floating point subtype) [partial]} A
digits_constraint is
compatible with
a floating point subtype if the value of the
expression
is no greater than the requested decimal precision of the subtype, and
the
range_constraint, if any, is compatible
with the subtype.
11
{elaboration (delta_constraint)
[partial]} The elaboration of a
delta_constraint
consists of the elaboration of the
range_constraint,
if any.
11.a
Reason: A numeric subtype is considered
“constrained” only if a range constraint applies to it. The
only effect of a digits_constraint or a delta_constraint
without a range_constraint is to specify the
value of the corresponding Digits or Delta attribute in the new subtype.
The set of values of the subtype is not “constrained” in
any way by such _constraints.
Wording Changes from Ada 83
11.b
In Ada 83, a delta_constraint
is called a fixed_point_constraint, and a digits_constraint
is called a floating_point_constraint. We have adopted other terms because
digits_constraints apply primarily to decimal
fixed point types now (they apply to floating point types only as an
obsolescent feature).
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