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

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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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