comparison operators for user defined types?

[Chris Marrin <chris@...>]

I have an object system that looks to Lua like userdata objects. These 
objects encapsulate many different types, some strings, some numbers, 
some objects that can be placed in a hierarchy to do some rendering. I 
place simple types like floating point numbers into objects because they 
have special behavior: specifically firing events into the node objects 
that contain them.

I want to be able to do simple operations on these objects, specifically 
on the numeric or string ones. So I added metamethods for things like 
__lt and __mul. For the arithmetic functions like __mul, everything 
works fine. If either of the operands is not a number it will call 
__mul, first trying the left op and then the right. So I can go:

    local a = SFFloat(5.2) -- a contains a floating pt. object
                           -- with the number 5.2 in it
    local c = a * 4.7

and my __mul function gets called. I get my object from the passed 
userdata, convert it to a number, do the mul and return a number. Great!

But this does not work for __lt. If you look at luaV_lessthan() you can 
see that it is because both operands must be of the same type and have 
the same metamethod. It seems like the same logic could be used (call 
either metamethod, preferring the one for the left operand). Is there a 
reason it was done this way? I looked at both 5.02 and work6 and they 
were both the same.

--
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