Ada Enums to Values - enums

Question

I could handle this in a not as clean way, but I was hoping to take advantage of the following:
type Prefix is (Yocto, Zepto, Atto, Femto, Pico, Nano,
Micro, Milli, Centi, Deci, None, Deca, Hecto, Kilo,
Mega, Giga, Tera, Peta, Exa, Zetta, Yotta);
for Prefix use (
Yocto => -24,
Zepto => -21,
Atto => -18,
Femto => -15,
Pico => -12,
Nano => -9,
Micro => -6,
Milli => -3,
Centi => -2,
Deci => -1,
None => 0,
Deca => 1,
Hecto => 2,
Kilo => 3,
Mega => 6,
Giga => 9,
Tera => 12,
Peta => 15,
Exa => 18,
Zetta => 21,
Yotta => 24);
GNAT doesn't complain about the representation clause. However, I can't seem to actually get the value out, since the only attributes related to this, that I am aware of, have to do with position, not assigned value.

Answer 1

Enumerations were never intended for this sort of purpose, and you shouldn't try to use them for that. (I think the main purpose was to define enumerations that had values other than 0, 1, ... in some external representation or a hardware register or something.) However, you can actually fix this while keeping your code almost the same:
type Prefix is (Yocto, Zepto, Atto, Femto, Pico, Nano,
Micro, Milli, Centi, Deci, None, Deca, Hecto, Kilo,
Mega, Giga, Tera, Peta, Exa, Zetta, Yotta);
type Prefix_To_Integer_Map is array (Prefix) of Integer;
Power_of_Ten : constant Prefix_To_Integer_Map := (
Yocto => -24,
Zepto => -21,
Atto => -18,
Femto => -15,
Pico => -12,
Nano => -9,
Micro => -6,
Milli => -3,
Centi => -2,
Deci => -1,
None => 0,
Deca => 1,
Hecto => 2,
Kilo => 3,
Mega => 6,
Giga => 9,
Tera => 12,
Peta => 15,
Exa => 18,
Zetta => 21,
Yotta => 24);
Should be about as clean as what you had. And saying Power_Of_Ten (My_Prefix) is more descriptive than My_Prefix'Enum_Rep or Prefix'Enum_Rep(My_Prefix) or whatever it is.

Answer 2

If an implementation defined attribute is acceptable, GNAT provides two attributes that are useful in this context:
Enum_Rep, which "returns the representation value for the given enumeration value."
Enum_Val, which "returns the enumeration value whose representation matches the argument."

Answer 3

The representation clause is used to set the internal value that is stored in memory for the type. If you want to get this value as an integer in a standard way, you should do an instantiation of the Unchecked_Conversion function.
with Ada.Unchecked_Conversion;
{...}
for Prefix'Size use Integer'Size;
function Value_of is new
Ada.Unchecked_Conversion (Prefix, Integer);