Re: Building an encapsulated library that uses GNAT sockets under Windows

["Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org>]

On 04/28/2016 01:23 PM, Randy Brukardt wrote:
>
> A typical case would be a lexer for numeric literals, where one wants to
> discard the underscores and keep the other characters. That would look
> something like:
>
>     while not End_of_File loop
>          case Next_Char is
>                when '0'..'9' =>
>                      null;
>                when '+' | '-' =>
>                      {some code}
>                when 'E' | 'e' =>
>                      {some code}
>                when '_' =>
>                      if Last_Char = '_' then
>                            Error (...);
>                      else
>                            goto Continue; -- Skip further processing.
>                      end if;
>               when others =>
>                      exit; -- We've reached the end of the number.
>          end case;
>          {Store the Next_Char into the literal token}
>       <<Continue>>
>     end loop;
>
> To avoid the goto/continue, you'd have to introduce a Boolean, or duplicate
> the "Store" code, or make the "Store" code into a subprogram, and make
> duplicate calls on that. None of which is clearer, or faster, than the above
> code. (Faster matters here as this loop is the third most used of the inner
> lexer loops, behind comments and identifiers. And a lexer is one of the top
> CPU users in a typical compiler.)

Or do

               when '_' =>
                  if Last_Char = '_' then
                     Error (...);
                  end if;
               when others =>
                  exit; -- We've reached the end of the number.
          end case;
          if Next_Char /= '_' then
             {Store Next_Char into the literal token}
          end if;
     end loop;

which is still clearer than the goto.

Maybe you have some more complex, more deeply nested, real-world example where 
this kind of alternative isn't possible. I've never encountered one.

-- 
Jeff Carter
"[T]he Musgroves had had the ill fortune
of a very troublesome, hopeless son, and
the good fortune to lose him before he
reached his twentieth year ..."
Persuasion
154