Re: unsigned integers in Ada

[stt@ada-uts]

Ada requires that all predefined integer types be
symmetric about zero, excepting an extra negative value
(LRM 3.5.4:7), and that all integer types be
derived directly or indirectly from one of these.
However, it would be permissible to define a LONG_INTEGER
which is signed 64 bits, and then define a subtype
of that which is unsigned 32 bits, and implement it
using unsigned 32 bit representation in memory
(though the arithmetic would still have to allow
64-bit intermediate results without NUMERIC_ERROR).

Representation clauses are only allowed for "first named subtypes"
(defined in LRM 13.1:3).  The first named subtype is the
subtype identified in a "type definition."  The type
is generally anonymous and unconstrained; the identifier specified designates
a subtype.  For example
    type My_Int is range 0..15;
This is formally equivalent to (see LRM 3.5.4:5,6):
    type <anon> is new <predefined-int-type>;
    subtype My_Int is <anon> range 0..15;
"My_Int" is therefore the "first named subtype."

Allowing unsigned types in general has been proposed as
a possible addition for the next revision of Ada, but
beware that it would imply NUMERIC_ERROR if the arithmetic
result goes negative, rather than just wrapping around, as
most "C" programmers expect.  Furthermore, there would
have to be a rule for deciding which predefined
type is selected for any given integer type definition with
a non-negative range (the signed one or the unsigned one), since
this would determine the legality of negative intermediate
arithmetic results.

Tucker Taft
Intermetrics, Inc.
Cambridge, MA  02138