Using closures was:Re: Lua5.0 new "For" question

[Eero Pajarre <epajarre@...>]

Peter Hill wrote:
> I'm having trouble trying to imagine a way to get the same functionality
> (which doesn't mean it can't be done of course!). As an example, let's try
> to implement complex numbers as an abstract type.

This is a quick (late night) implementation of an idea I got,
I think it has reasonable overhead. It is heavyweight for
something as simple as a complex number, but maybe reasonable
for something more complex. (I think that the cost of a small
function is ok)

Here I am giving full access to the cadd function on purpose,
it would have been easier to provide accessor functions, but
I thought this is a more intresting excercise.

Btw, this is still not safe against someone who is actively
trying to cheat, but as I use Lua embedded in my application
for me it is more important how Lua secures datatypes implemented
with userdata. I think that is something which appears to be built
nicely into the language + implementation.

(The cheating could be blocked with extra code, and some overhead,
I think)

		Eero

-------------

function cpack()
   local key={};
   complex=function(r,i)
	      local v={
		 real=r;
		 imag=i;
	      }
	      return (
		      function(k)
			 if k==key then
			    return v;
			 else
			    error("bad access");
			 end
		      end
		   );
	   end
   cadd=function(x,y)
	   local tx=x(key);
	   local ty=y(key);
	   return complex(tx.real+ty.real,tx.imag+ty.imag);
	end
   real=function(x)
	   return x(key).real;
	end
   imag=function(x)
	   return x(key).imag;
	end
end

cpack()

x=complex(1,2);
y=complex(3,7);
z=cadd(x,y);
print(real(z)," ",imag(z));
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