guideline

Use enumeration types instead of numeric codes.

Only if absolutely necessary, use representation clauses to match requirements of external devices.

Use:

type Color is (Blue, Red, Green, Yellow);

rather than:

Blue   : constant := 1;
Red    : constant := 2;
Green  : constant := 3;
Yellow : constant := 4;

and add the following if necessary:

for Color use (Blue   => 1,
               Red    => 2,
               Green  => 3,
               Yellow => 4);

rationale

Enumerations are more robust than numeric codes; they leave less potential for errors resulting from incorrect interpretation and from additions to and deletions from the set of values during maintenance. Numeric codes are holdovers from languages that have no user-defined types.

In addition, Ada provides a number of attributes ('Pos, 'Val, 'Succ, 'Pred, 'Image, and 'Value) for enumeration types that, when used, are more reliable than user-written operations on encodings.