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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. 

Syntax

2/4
delta_constraint ::= delta static_simple_expression [range_constraint]

Name Resolution Rules

3/4
The simple_expression of a delta_constraint is expected to be of any real type. 

Legality Rules

4/4
The simple_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
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). 

Static Semantics

7/4
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 simple_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.
8/4
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 simple_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

9/4
A delta_constraint is compatible with an ordinary fixed point subtype if the value of the simple_expression is no less than the delta of the subtype, and the range_constraint, if any, is compatible with the subtype.
10/4
 A digits_constraint is compatible with a floating point subtype if the value of the simple_expression is no greater than the requested decimal precision of the subtype, and the range_constraint, if any, is compatible with the subtype.
11
The elaboration of a delta_constraint consists of the elaboration of the range_constraint, if any. 

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