Re: A question re meaning/use of the "for ... use ..."

[Keith Thompson <kst-u@mib.org>]

"David C. Hoos" <david.c.hoos.sr@ada95.com> writes:
> Is there some reason the following code is not portable"
>
> package Enums
> is
>    type Sparse_Enum is (One, Ten, One_Hundred, One_Thousand);
>    for Sparse_Enum use
>       (One => 1, Ten => 10, One_Hundred => 100, One_Thousand => 1000);
> end Enums;
>
> with Enums;
> with Ada.Exceptions;
> with Ada.Text_IO;
> with Ada.Unchecked_Conversion;
> procedure Test_Enums
> is
>    type Sparse_enum_Rep is range 0 .. 2**Enums.Sparse_Enum'Size - 1;
>
>    function To_Rep is new Ada.Unchecked_Conversion
>      (Source => Enums.Sparse_Enum, Target => Sparse_enum_Rep);
>
>    function To_Enum is new Ada.Unchecked_Conversion
>      (Target => Enums.Sparse_Enum, Source => Sparse_enum_Rep);
>
>    function From_Rep (Rep : Sparse_enum_Rep) return Enums.Sparse_Enum
>    is
>       Enum : Enums.Sparse_Enum := To_Enum (Rep);
>    begin
>       if Enum'Valid then
>          return Enum;
>       end if;
>       Ada.Exceptions.Raise_Exception
>         (E => Constraint_Error'Identity,
>          Message => Sparse_enum_Rep'Image (Rep) &
>            " is an invalid representation of type Enums.Sparse_Enum.");
>    end From_Rep;
>
> begin
>    Ada.Text_IO.Put_Line ("Sparse_Enum representations:");
>    Ada.Text_IO.New_Line;
>
>    for N in Enums.Sparse_Enum loop
>       Ada.Text_IO.Put_Line
>         (Enums.Sparse_Enum'Image (N) & " => " &
>          Sparse_enum_Rep'Image (To_Rep (N)));
>    end loop;
>
>    Ada.Text_IO.New_Line (2);
>
>    Ada.Text_IO.Put_Line
>      ("Attempts to convert possible representations to Sparse_Enum.");
>    Ada.Text_IO.New_Line;
>
>    for I in Sparse_enum_Rep'First .. Sparse_enum_Rep'First + 12 loop
>       begin
>          Ada.Text_IO.Put_Line
>            (Sparse_enum_Rep'Image (I) & " => " &
>             Enums.Sparse_Enum'Image (From_Rep (I)));
>          Ada.Text_IO.New_Line;
>       exception
>          when E: others =>
>             Ada.Text_IO.Put_Line (Ada.Exceptions.Exception_Information (E));
>       end;
>    end loop;
>
> end Test_Enums;

Yes.  Try changing the declaration of Sparse_Enum to:

   type Sparse_Enum is (Minus_One, One, Ten, One_Hundred, One_Thousand);
   for Sparse_Enum use
      (Minus_One => -1, One => 1, Ten => 10, One_Hundred => 100,
       One_Thousand => 1000);

It shows the representation of Minus_One as 2047 rather than -1,
and the loop runs from 0 to 12, skipping Minus_One.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
San Diego Supercomputer Center             <*>  <http://users.sdsc.edu/~kst>
We must do something.  This is something.  Therefore, we must do this.