Re: Question about Ada.Unchecked_Conversion

[Nick Roberts <nick.roberts@acm.org>]

skidmarks wrote:

> I don't have the full picture of what you're trying to do so my answer
> may be off the mark.

I think you're dead right in principle, and slightly off only in a few
minor details.

> When I build a custom lexor I include a (usually) 256 cell array. The
> cell is mapped one-to-one with (any) 256 representation of the
> character set in use, and 256 has the property of guaranteeing nor
> constraint errors. The contents of each cell is a enumeration
> representing the purpose of the mapped character. My lexer is built
> around this.

Exactly how I do it. Which is not to say that it is necessarily the best
possible way, but I have used it with success in the past.

> For example:
> 
>   type enum { ign, sep, op, sym, num, hex, ... );

   type Character_Category is
      ( Ignore, Separator, Operator, Symbolic, [and so on] );

>   type enum_Map is array ( Integer range 1 .. 256 ) of enum;
> 
>   Map : constant enum_Map := ( ign, ... sep, ... );

   Cat_Of: constant array (Character) of Character_Category :=
      ( ',' | ';' | ':' | '(' | ')' => Separator,
        '+' | '-' | '*' | '/'       => Operator,
        [and so on],
        others                      => Ignore );

> begin -- 
> 
>    if ( enum_Map( Character'Pos(char) ) = <> ) then <> end if;

   if Cat_Of(Char) = Separator then
      ...

> or
> 
>    case enum_Map( Character'Pos(char) ) is
>    when ign => <>
>    when sep => <>
>    when op  => <>
>    when sym => <>
>    when num => <>

   case Cat_Of(Char) is
      when Ignore =>
         null;
      when Separator => 
         if Char = ')' then
            End_of_List := True;
         elsif Char /= ',' then
            raise Syntax_Error;
         end if;
         if Id = "" then
            raise Syntax_Error;
         end if;
         Append( Id_List, Id );
         Clear( Id );
         Get_Next_Character;
      [and so on]

>  and so on.
> 
> I use a Moore Machine for my finite state machine, the arcs
> representing the transition states (ign, sep, ...) and actions
> associated with taking the transition.

-- 
Nick Roberts