Re: Error-names.

[tmoran@acm.org]

>   function Get_Error_Code (E : Exception_Occurrence) return C_Integer is
>   begin
>       if Exception_Identity (E) = Program_Error'Identity then
>           return Program_Error_Code;
>       elsif Exception_Identity (E) = Constraint_Error'Identity then
>           return Constraint_Error_Code;
>       elsif ...
  "Use the Force, Luke", or more specifically, Let the Ada compiler
help you find and eliminate errors.

  It would not be hard to leave out, or have multiple uses of, an error
code, as in:
    Constraint_Error_Code : constant Exception_Codes := -10;
    Program_Error_Code    : constant Exception_Codes := -9;
    Storage_Error_Code    : constant Exception_Codes := -9;
    Status_Error_Code     : constant Exception_Codes := -7;
    My_Special_Error_Code : constant Exception_Codes := -5;
    ...
Similarly, a new exception might be added, but someone forgets to
add it to the if-elsif list.

With the if-elsif style you can only hope that neither of those happens.
But if you do

    Coded_Exception_List : constant array(Exception_Codes)
    of Ada.Exceptions.Exception_ID
      := (Constraint_Error_Code => Constraint_Error'Identity,
          Program_Error_Code    => Program_Error'Identity,
          Storage_Error_Code    => Storage_Error'Identity,
          IO_Status_Error_Code  => Ada.IO_Exceptions.Status_Error'Identity,
          My_Special_Error_Code => My_Special_Exception'Identity,
          ...
then the compiler will warn you if two of your error codes are the
same, or if you've left any out in the Exception_Codes range.
  It also has the (possible) advantage that you can trivially translate
a code back into an exception:
  Ada.Exceptions.Raise_Exception(Coded_Exception_List(Code), Message);