Re: Order of logical operations

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

James Van Buskirk wrote:

> "Arjen Markus" <arjen.markus@wldelft.nl> wrote in message
> news:40C0208A.80DBE53A@wldelft.nl...
> 
>> Short-circuiting does not necessarily lead to quicker programs.
>> And if you really need it, you might invent a new operator,
>> like ".cand." (the implementation ought to be very simple).
> 
> As others have pointed out such operators have to be defined
> at the language level.

Not necessarily.

The language might support a more general concept: lazy arguments. Consider:

-- This is not Ada!
function "and then" (Left : Boolean; Right : lazy Boolean)
   return Boolean is
begin
   if Left then
      return True;
   else
      return Right;
         -- Right is evaluated only at this point on the
         -- caller's context.
   end if;
end "and then";

It is a very powerful mechanism that allows to enforce the evaluation order
of subroutine arguments:

-- This is not Ada!
function "&" (A : String, B : lazy String) return String is
   Value_A : String renames A; -- This evaluates A
begin
   return Value_A & B; -- B is evaluated after A
end "&";

Now it is safe to write:

... := Read_String & Read_String & Read_String;

Though, that could be tricky to implement, especially if we consider lazy
arguments of recursive calls, protected operations, side effects of lazy
evaluations on alien contexts and all that stuff.

-- 
Regards,
Dmitry A. Kazakov
www.dmitry-kazakov.de